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A signal is a software interrupt delivered to a process. The operating system uses signals to report exceptional situations to an executing program. Some signals report errors such as references to invalid memory addresses; others report asynchronous events, such as disconnection of a phone line.
The GNU C library defines a variety of signal types, each for a particular kind of event. Some kinds of events make it inadvisable or impossible for the program to proceed as usual, and the corresponding signals normally abort the program. Other kinds of signals that report harmless events are ignored by default.
If you anticipate an event that causes signals, you can define a handler function and tell the operating system to run it when that particular type of signal arrives.
Finally, one process can send a signal to another process; this allows a parent process to abort a child, or two related processes to communicate and synchronize.
This section explains basic concepts of how signals are generated, what happens after a signal is delivered, and how programs can handle signals.
A signal reports the occurrence of an exceptional event. These are some of the events that can cause (or generate, or raise) a signal:
kill or raise by the same process.
kill from another process. Signals are a limited but
useful form of interprocess communication.
Each of these kinds of events (excepting explicit calls to kill
and raise) generates its own particular kind of signal. The
various kinds of signals are listed and described in detail in
section Standard Signals.
In general, the events that generate signals fall into three major categories: errors, external events, and explicit requests.
An error means that a program has done something invalid and cannot
continue execution. But not all kinds of errors generate signals--in
fact, most do not. For example, opening a nonexistant file is an error,
but it does not raise a signal; instead, open returns -1.
In general, errors that are necessarily associated with certain library
functions are reported by returning a value that indicates an error.
The errors which raise signals are those which can happen anywhere in
the program, not just in library calls. These include division by zero
and invalid memory addresses.
An external event generally has to do with I/O or other processes. These include the arrival of input, the expiration of a timer, and the termination of a child process.
An explicit request means the use of a library function such as
kill whose purpose is specifically to generate a signal.
Signals may be generated synchronously or asynchronously. A synchronous signal pertains to a specific action in the program, and is delivered (unless blocked) during that action. Errors generate signals synchronously, and so do explicit requests by a process to generate a signal for that same process.
Asynchronous signals are generated by events outside the control of the process that receives them. These signals arrive at unpredictable times during execution. External events generate signals asynchronously, and so do explicit requests that apply to some other process.
A given type of signal is either typically synchrous or typically asynchronous. For example, signals for errors are typically synchronous because errors generate signals synchronously. But any type of signal can be generated synchronously or asynchronously with an explicit request.
When a signal is generated, it becomes pending. Normally it remains pending for just a short period of time and then is delivered to the process that was signaled. However, if that kind of signal is currently blocked, it may remain pending indefinitely--until signals of that kind are unblocked. Once unblocked, it will be delivered immediately. See section Blocking Signals.
When the signal is delivered, whether right away or after a long delay,
the specified action for that signal is taken. For certain
signals, such as SIGKILL and SIGSTOP, the action is fixed,
but for most signals, the program has a choice: ignore the signal,
specify a handler function, or accept the default action for
that kind of signal. The program specifies its choice using functions
such as signal or sigaction (see section Specifying Signal Actions). We
sometimes say that a handler catches the signal. While the
handler is running, that particular signal is normally blocked.
If the specified action for a kind of signal is to ignore it, then any such signal which is generated is discarded immediately. This happens even if the signal is also blocked at the time. A signal discarded in this way will never be delivered, not even if the program subsequently specifies a different action for that kind of signal and then unblocks it.
If a signal arrives which the program has neither handled nor ignored, its default action takes place. Each kind of signal has its own default action, documented below (see section Standard Signals). For most kinds of signals, the default action is to terminate the process. For certain kinds of signals that represent "harmless" events, the default action is to do nothing.
When a signal terminates a process, its parent process can determine the
cause of termination by examining the termination status code reported
by the wait or waitpid functions. (This is discussed in
more detail in section Process Completion.) The information it can get
includes the fact that termination was due to a signal, and the kind of
signal involved. If a program you run from a shell is terminated by a
signal, the shell typically prints some kind of error message.
The signals that normally represent program errors have a special property: when one of these signals terminates the process, it also writes a core dump file which records the state of the process at the time of termination. You can examine the core dump with a debugger to investigate what caused the error.
If you raise a "program error" signal by explicit request, and this terminates the process, it makes a core dump file just as if the signal had been due directly to an error.
This section lists the names for various standard kinds of signals and describes what kind of event they mean. Each signal name is a macro which stands for a positive integer--the signal number for that kind of signal. Your programs should never make assumptions about the numeric code for a particular kind of signal, but rather refer to them always by the names defined here. This is because the number for a given kind of signal can vary from system to system, but the meanings of the names are standardized and fairly uniform.
The signal names are defined in the header file `signal.h'.
The value of this symbolic constant is the total number of signals
defined. Since the signal numbers are allocated consecutively,
NSIG is also one greater than the largest defined signal number.
The following signals are generated when a serious program error is detected by the operating system or the computer itself. In general, all of these signals are indications that your program is seriously broken in some way, and there's usually no way to continue the computation which encountered the error.
Some programs handle program error signals in order to tidy up before terminating; for example, programs that turn off echoing of terminal input should handle program error signals in order to turn echoing back on. The handler should end by specifying the default action for the signal that happened and then reraising it; this will cause the program to terminate with that signal, as if it had not had a handler. (See section Handlers That Terminate the Process.)
Termination is the sensible ultimate outcome from a program error in
most programs. However, programming systems such as Lisp that can load
compiled user programs might need to keep executing even if a user
program incurs an error. These programs have handlers which use
longjmp to return control to the command level.
The default action for all of these signals is to cause the process to
terminate. If you block or ignore these signals or establish handlers
for them that return normally, your program will probably break horribly
when such signals happen, unless they are generated by raise or
kill instead of a real error.
When one of these program error signals terminates a process, it also
writes a core dump file which records the state of the process at
the time of termination. The core dump file is named `core' and is
written in whichever directory is current in the process at the time.
(On the GNU system, you can specify the file name for core dumps with
the environment variable COREFILE.) The purpose of core dump
files is so that you can examine them with a debugger to investigate
what caused the error.
The SIGFPE signal reports a fatal arithmetic error. Although the
name is derived from "floating-point exception", this signal actually
covers all arithmetic errors, including division by zero and overflow.
If a program stores integer data in a location which is then used in a
floating-point operation, this often causes an "invalid operation"
exception, because the processor cannot recognize the data as a
floating-point number.
Actual floating-point exceptions are a complicated subject because there
are many types of exceptions with subtly different meanings, and the
SIGFPE signal doesn't distinguish between them. The IEEE
Standard for Binary Floating-Point Arithmetic (ANSI/IEEE Std 754-1985)
defines various floating-point exceptions and requires conforming
computer systems to report their occurrences. However, this standard
does not specify how the exceptions are reported, or what kinds of
handling and control the operating system can offer to the programmer.
BSD systems provide the SIGFPE handler with an extra argument
that distinguishes various causes of the exception. In order to access
this argument, you must define the handler to accept two arguments,
which means you must cast it to a one-argument function type in order to
establish the handler. The GNU library does provide this extra
argument, but the value is meaningful only on operating systems that
provide the information (BSD systems and GNU systems).
FPE_INTOVF_TRAP
FPE_INTDIV_TRAP
FPE_SUBRNG_TRAP
FPE_FLTOVF_TRAP
FPE_FLTDIV_TRAP
FPE_FLTUND_TRAP
FPE_DECOVF_TRAP
The name of this signal is derived from "illegal instruction"; it
means your program is trying to execute garbage or a privileged
instruction. Since the C compiler generates only valid instructions,
SIGILL typically indicates that the executable file is corrupted,
or that you are trying to execute data. Some common ways of getting
into the latter situation are by passing an invalid object where a
pointer to a function was expected, or by writing past the end of an
automatic array (or similar problems with pointers to automatic
variables) and corrupting other data on the stack such as the return
address of a stack frame.
This signal is generated when a program tries to read or write outside the memory that is allocated for it. (Actually, the signals only occur when the program goes far enough outside to be detected by the system's memory protection mechanism.) The name is an abbreviation for "segmentation violation".
The most common way of getting a SIGSEGV condition is by
dereferencing a null or uninitialized pointer. A null pointer refers to
the address 0, and most operating systems make sure this address is
always invalid precisely so that dereferencing a null pointer will cause
SIGSEGV. (Some operating systems place valid memory at address
0, and dereferencing a null pointer does not cause a signal on these
systems.) As for uninitialized pointer variables, they contain random
addresses which may or may not be valid.
Another common way of getting into a SIGSEGV situation is when
you use a pointer to step through an array, but fail to check for the
end of the array.
This signal is generated when an invalid pointer is dereferenced. Like
SIGSEGV, this signal is typically the result of dereferencing an
uninitialized pointer. The difference between the two is that
SIGSEGV indicates an invalid access to valid memory, while
SIGBUS indicates an access to an invalid address. In particular,
SIGBUS signals often result from dereferencing a misaligned
pointer, such as referring to a four-word integer at an address not
divisible by four. (Each kind of computer has its own requirements for
address alignment.)
The name of this signal is an abbreviation for "bus error".
This signal indicates an error detected by the program itself and
reported by calling abort. See section Aborting a Program.
These signals are all used to tell a process to terminate, in one way or another. They have different names because they're used for slightly different purposes, and programs might want to handle them differently.
The reason for handling these signals is usually so your program can tidy up as appropriate before actually terminating. For example, you might want to save state information, delete temporary files, or restore the previous terminal modes. Such a handler should end by specifying the default action for the signal that happened and then reraising it; this will cause the program to terminate with that signal, as if it had not had a handler. (See section Handlers That Terminate the Process.)
The (obvious) default action for all of these signals is to cause the process to terminate.
The SIGHUP ("hang-up") signal is used to report that the user's
terminal is disconnected, perhaps because a network or telephone
connection was broken. For more information about this, see section Control Modes.
This signal is also used to report the termination of the controlling process on a terminal to jobs associated with that session; this termination effectively disconnects all processes in the session from the controlling terminal. For more information, see section Termination Internals.
The SIGINT ("program interrupt") signal is sent when the user
types the INTR character (normally C-c). See section Special Characters, for information about terminal driver support for
C-c.
The SIGQUIT signal is similar to SIGINT, except that it's
controlled by a different key--the QUIT character, usually
C-\---and produces a core dump when it terminates the process,
just like a program error signal. You can think of this as a
program error condition "detected" by the user.
See section Program Error Signals, for information about core dumps. See section Special Characters, for information about terminal driver support.
Certain kinds of cleanups are best omitted in handling SIGQUIT.
For example, if the program creates temporary files, it should handle
the other termination requests by deleting the temporary files. But it
is better for SIGQUIT not to delete them, so that the user can
examine them in conjunction with the core dump.
The SIGTERM signal is a generic signal used to cause program
termination. Unlike SIGKILL, this signal can be blocked,
handled, and ignored.
The shell command kill generates SIGTERM by default.
The SIGKILL signal is used to cause immediate program termination.
It cannot be handled or ignored, and is therefore always fatal. It is
also not possible to block this signal.
This signal is generated only by explicit request. Since it cannot be
handled, you should generate it only as a last resort, after first
trying a less drastic method such as C-c or SIGTERM. If a
process does not respond to any other termination signals, sending it a
SIGKILL signal will almost always cause it to go away.
In fact, if SIGKILL fails to terminate a process, that by itself
constitutes an operating system bug which you should report.
These signals are used to indicate the expiration of timers. See section Setting an Alarm, for information about functions that cause these signals to be sent.
The default behavior for these signals is to cause program termination. This default is rarely useful, but no other default would be useful; most of the ways of using these signals would require handler functions in any case.
This signal typically indicates expiration of a timer that measures real
or clock time. It is used by the alarm function, for example.
This signal typically indicates expiration of a timer that measures CPU time used by the current process. The name is an abbreviation for "virtual time alarm".
This signal is typically indicates expiration of a timer that measures both CPU time used by the current process, and CPU time expended on behalf of the process by the system. Such a timer is used to implement code profiling facilities, hence the name of this signal.
The signals listed in this section are used in conjunction with
asynchronous I/O facilities. You have to take explicit action by
calling fcntl to enable a particular file descriptior to generate
these signals (see section Interrupt-Driven Input). The default action for these
signals is to ignore them.
This signal is sent when a file descriptor is ready to perform input or output.
On most operating systems, terminals and sockets are the only kinds of
files that can generate SIGIO; other kinds, including ordinary
files, never generate SIGIO even if you ask them to.
This signal is sent when "urgent" or out-of-band data arrives on a socket. See section Out-of-Band Data.
These signals are used to support job control. If your system doesn't support job control, then these macros are defined but the signals themselves can't be raised or handled.
You should generally leave these signals alone unless you really understand how job control works. See section Job Control.
This signal is sent to a parent process whenever one of its child processes terminates or stops.
The default action for this signal is to ignore it. If you establish a
handler for this signal while there are child processes that have
terminated but not reported their status via wait or
waitpid (see section Process Completion), whether your new handler
applies to those processes or not depends on the particular operating
system.
You can send a SIGCONT signal to a process to make it continue.
The default behavior for this signal is to make the process continue if
it is stopped, and to ignore it otherwise.
Most programs have no reason to handle SIGCONT; they simply
resume execution without realizing they were ever stopped. You can use
a handler for SIGCONT to make a program do something special when
it is stopped and continued--for example, to reprint a prompt when it
is suspended while waiting for input.
The SIGSTOP signal stops the process. It cannot be handled,
ignored, or blocked.
The SIGTSTP signal is an interactive stop signal. Unlike
SIGSTOP, this signal can be handled and ignored.
Your program should handle this signal if you have a special need to
leave files or system tables in a secure state when a process is
stopped. For example, programs that turn off echoing should handle
SIGTSTP so they can turn echoing back on before stopping.
This signal is generated when the user types the SUSP character (normally C-z). For more information about terminal driver support, see section Special Characters.
A process cannot read from the the user's terminal while it is running
as a background job. When any process in a background job tries to
read from the terminal, all of the processes in the job are sent a
SIGTTIN signal. The default action for this signal is to
stop the process. For more information about how this interacts with
the terminal driver, see section Access to the Controlling Terminal.
This is similar to SIGTTIN, but is generated when a process in a
background job attempts to write to the terminal or set its modes.
Again, the default action is to stop the process.
While a process is stopped, no more signals can be delivered to it until
it is continued, except SIGKILL signals and (obviously)
SIGCONT signals. The SIGKILL signal always causes
termination of the process and can't be blocked or ignored. You can
block or ignore SIGCONT, but it always causes the process to
be continued anyway if it is stopped. Sending a SIGCONT signal
to a process causes any pending stop signals for that process to be
discarded. Likewise, any pending SIGCONT signals for a process
are discarded when it receives a stop signal.
When a process in an orphaned process group (see section Orphaned Process Groups) receives a SIGTSTP, SIGTTIN, or SIGTTOU
signal and does not handle it, the process does not stop. Stopping the
process would be unreasonable since there would be no way to continue
it. What happens instead depends on the operating system you are
using. Some systems may do nothing; others may deliver another signal
instead, such as SIGKILL or SIGHUP.
These signals are used to report various other conditions. The default action for all of them is to cause the process to terminate.
If you use pipes or FIFOs, you have to design your application so that
one process opens the pipe for reading before another starts writing.
If the reading process never starts, or terminates unexpectedly, writing
to the pipe or FIFO raises a SIGPIPE signal. If SIGPIPE
is blocked, handled or ignored, the offending call fails with
EPIPE instead.
Pipes and FIFO special files are discussed in more detail in section Pipes and FIFOs.
Another cause of SIGPIPE is when you try to output to a socket
that isn't connected. See section Sending Data.
The SIGUSR1 and SIGUSR2 signals are set aside for you to
use any way you want. They're useful for interprocess communication.
Since these signals are normally fatal, you should write a signal handler
for them in the program that receives the signal.
There is an example showing the use of SIGUSR1 and SIGUSR2
in section Signaling Another Process.
Particular operating systems support additional signals not listed above. The ANSI C standard reserves all identifiers beginning with `SIG' followed by an uppercase letter for the names of signals. You should consult the documentation or header files for your particular operating system and processor type to find out about the specific signals it supports.
For example, some systems support extra signals which correspond to hardware traps. Some other kinds of signals commonly supported are used to implement limits on CPU time or file system usage, asynchronous changes to terminal configuration, and the like. Systems may also define signal names that are aliases for standard signal names.
You can generally assume that the default action (or the action set up by the shell) for implementation-defined signals is reasonable, and not worry about them yourself. In fact, it's usually a bad idea to ignore or block signals you don't know anything about, or try to establish a handler for signals whose meanings you don't know.
Here are some of the other signals found on commonly used operating systems:
SIGCLD
SIGCHLD.
SIGTRAP
SIGIOT
SIGABRT.
Default action is to dump core.
SIGEMT
SIGSYS
SIGPOLL
SIGIO.
SIGXCPU
SIGXFSZ
SIGWINCH
We mentioned above that the shell prints a message describing the signal
that terminated a child process. The clean way to print a message
describing a signal is to use the functions strsignal and
psignal. These functions use a signal number to specify which
kind of signal to describe. The signal number may come from the
termination status of a child process (see section Process Completion) or it
may come from a signal handler in the same process.
Function: char * strsignal (int signum)
This function returns a pointer to a statically-allocated string containing a message describing the signal signum. You should not modify the contents of this string; and, since it can be rewritten on subsequent calls, you should save a copy of it if you need to reference it later.
This function is a GNU extension, declared in the header file `string.h'.
Function: void psignal (int signum, const char *message)
This function prints a message describing the signal signum to the
standard error output stream stderr; see section Standard Streams.
If you call psignal with a message that is either a null
pointer or an empty string, psignal just prints the message
corresponding to signum, adding a trailing newline.
If you supply a non-null message argument, then psignal
prefixes its output with this string. It adds a colon and a space
character to separate the message from the string corresponding
to signum.
This function is a BSD feature, declared in the header file `stdio.h'.
There is also an array sys_siglist which contains the messages
for the various signal codes. This array exists on BSD systems, unlike
strsignal.
The simplest way to change the action for a signal is to use the
signal function. You can specify a built-in action (such as to
ignore the signal), or you can establish a handler.
The GNU library also implements the more versatile sigaction
facility. This section describes both facilities and gives suggestions
on which to use when.
The signal function provides a simple interface for establishing
an action for a particular signal. The function and associated macros
are declared in the header file `signal.h'.
This is the type of signal handler functions. Signal handlers take one
integer argument specifying the signal number, and have return type
void. So, you should define handler functions like this:
void handler (int signum) { ... }
The name sighandler_t for this data type is a GNU extension.
Function: sighandler_t signal (int signum, sighandler_t action)
The signal function establishes action as the action for
the signal signum.
The first argument, signum, identifies the signal whose behavior you want to control, and should be a signal number. The proper way to specify a signal number is with one of the symbolic signal names described in section Standard Signals---don't use an explicit number, because the numerical code for a given kind of signal may vary from operating system to operating system.
The second argument, action, specifies the action to use for the signal signum. This can be one of the following:
SIG_DFL
SIG_DFL specifies the default action for the particular signal.
The default actions for various kinds of signals are stated in
section Standard Signals.
SIG_IGN
SIG_IGN specifies that the signal should be ignored.
Your program generally should not ignore signals that represent serious
events or that are normally used to request termination. You cannot
ignore the SIGKILL or SIGSTOP signals at all. You can
ignore program error signals like SIGSEGV, but ignoring the error
won't enable the program to continue executing meaningfully. Ignoring
user requests such as SIGINT, SIGQUIT, and SIGTSTP
is unfriendly.
When you do not wish signals to be delivered during a certain part of the program, the thing to do is to block them, not ignore them. See section Blocking Signals.
handler
For more information about defining signal handler functions, see section Defining Signal Handlers.
If you set the action for a signal to SIG_IGN, or if you set it
to SIG_DFL and the default action is to ignore that signal, then
any pending signals of that type are discarded (even if they are
blocked). Discarding the pending signals means that they will never be
delivered, not even if you subsequently specify another action and
unblock this kind of signal.
The signal function returns the action that was previously in
effect for the specified signum. You can save this value and
restore it later by calling signal again.
If signal can't honor the request, it returns SIG_ERR
instead. The following errno error conditions are defined for
this function:
EINVAL
SIGKILL or SIGSTOP.
Here is a simple example of setting up a handler to delete temporary files when certain fatal signals happen:
#include <signal.h>
void
termination_handler (int signum)
{
struct temp_file *p;
for (p = temp_file_list; p; p = p->next)
unlink (p->name);
}
int
main (void)
{
...
if (signal (SIGINT, termination_handler) == SIG_IGN)
signal (SIGINT, SIG_IGN);
if (signal (SIGHUP, termination_handler) == SIG_IGN)
signal (SIGHUP, SIG_IGN);
if (signal (SIGTERM, termination_handler) == SIG_IGN)
signal (SIGTERM, SIG_IGN);
...
}
Note how if a given signal was previously set to be ignored, this code avoids altering that setting. This is because non-job-control shells often ignore certain signals when starting children, and it is important for the children to respect this.
We do not handle SIGQUIT or the program error signals in this
example because these are designed to provide information for debugging
(a core dump), and the temporary files may give useful information.
Function: sighandler_t ssignal (int signum, sighandler_t action)
The ssignal function does the same thing as signal; it is
provided only for compatibility with SVID.
The value of this macro is used as the return value from signal
to indicate an error.
The sigaction function has the same basic effect as
signal: to specify how a signal should be handled by the process.
However, sigaction offers more control, at the expense of more
complexity. In particular, sigaction allows you to specify
additional flags to control when the signal is generated and how the
handler is invoked.
The sigaction function is declared in `signal.h'.
Structures of type struct sigaction are used in the
sigaction function to specify all the information about how to
handle a particular signal. This structure contains at least the
following members:
sighandler_t sa_handler
signal function. The value can be SIG_DFL,
SIG_IGN, or a function pointer. See section Basic Signal Handling.
sigset_t sa_mask
sa_mask. If you want that signal not to be blocked within its
handler, you must write code in the handler to unblock it.
int sa_flags
sigaction.
Function: int sigaction (int signum, const struct sigaction *action, struct sigaction *old_action)
The action argument is used to set up a new action for the signal
signum, while the old_action argument is used to return
information about the action previously associated with this symbol.
(In other words, old_action has the same purpose as the
signal function's return value--you can check to see what the
old action in effect for the signal was, and restore it later if you
want.)
Either action or old_action can be a null pointer. If old_action is a null pointer, this simply suppresses the return of information about the old action. If action is a null pointer, the action associated with the signal signum is unchanged; this allows you to inquire about how a signal is being handled without changing that handling.
The return value from sigaction is zero if it succeeds, and
-1 on failure. The following errno error conditions are
defined for this function:
EINVAL
SIGKILL or SIGSTOP.
signal and sigaction
It's possible to use both the signal and sigaction
functions within a single program, but you have to be careful because
they can interact in slightly strange ways.
The sigaction function specifies more information than the
signal function, so the return value from signal cannot
express the full range of sigaction possibilities. Therefore, if
you use signal to save and later reestablish an action, it may
not be able to reestablish properly a handler that was established with
sigaction.
To avoid having problems as a result, always use sigaction to
save and restore a handler if your program uses sigaction at all.
Since sigaction is more general, it can properly save and
reestablish any action, regardless of whether it was established
originally with signal or sigaction.
If you establish an action with signal and then examine it with
sigaction, the handler address that you get may not be the same
as what you specified with signal. It may not even be suitable
for use as an action argument with signal. But you can rely on
using it as an argument to sigaction.
So, you're better off using one or the other of the mechanisms consistently within a single program.
Portability Note: The basic signal function is a feature
of ANSI C, while sigaction is part of the POSIX.1 standard. If
you are concerned about portability to non-POSIX systems, then you
should use the signal function instead.
sigaction Function Example
In section Basic Signal Handling, we gave an example of establishing a
simple handler for termination signals using signal. Here is an
equivalent example using sigaction:
#include <signal.h>
void
termination_handler (int signum)
{
struct temp_file *p;
for (p = temp_file_list; p; p = p->next)
unlink (p->name);
}
int
main (void)
{
...
struct sigaction new_action, old_action;
/* Set up the structure to specify the new action. */
new_action.sa_handler = termination_handler;
sigemptyset (&new_action.sa_mask);
new_action.sa_flags = 0;
sigaction (SIGINT, NULL, &old_action);
if (old_action.sa_handler != SIG_IGN)
sigaction (SIGINT, &new_action, NULL);
sigaction (SIGHUP, NULL, &old_action);
if (old_action.sa_handler != SIG_IGN)
sigaction (SIGHUP, &new_action, NULL);
sigaction (SIGTERM, NULL, &old_action);
if (old_action.sa_handler != SIG_IGN)
sigaction (SIGTERM, &new_action, NULL);
...
}
The program just loads the new_action structure with the desired
parameters and passes it in the sigaction call. The usage of
sigemptyset is described later; see section Blocking Signals.
As in the example using signal, we avoid handling signals
previously set to be ignored. Here we can avoid altering the signal
handler even momentarily, by using the feature of sigaction that
lets us examine the current action without specifying a new one.
Here is another example. It retrieves information about the current
action for SIGINT without changing that action.
struct sigaction query_action; if (sigaction (SIGINT, NULL, &query_action) < 0) /*sigactionreturns -1 in case of error. */ else if (query_action.sa_handler == SIG_DFL) /*SIGINTis handled in the default, fatal manner. */ else if (query_action.sa_handler == SIG_IGN) /*SIGINTis ignored. */ else /* A programmer-defined signal handler is in effect. */
sigaction
The sa_flags member of the sigaction structure is a
catch-all for special features. Most of the time, SA_RESTART is
a good value to use for this field.
The value of sa_flags is interpreted as a bit mask. Thus, you
should choose the flags you want to set, OR those flags together,
and store the result in the sa_flags member of your
sigaction structure.
Each signal number has its own set of flags. Each call to
sigaction affects one particular signal number, and the flags
that you specify apply only to that particular signal.
In the GNU C library, establishing a handler with signal sets all
the flags to zero except for SA_RESTART, whose value depends on
the settings you have made with siginterrupt. See section Primitives Interrupted by Signals, to see what this is about.
These macros are defined in the header file `signal.h'.
This flag is meaningful only for the SIGCHLD signal. When the
flag is set, the system delivers the signal for a terminated child
process but not for one that is stopped. By default, SIGCHLD is
delivered for both terminated children and stopped children.
Setting this flag for a signal other than SIGCHLD has no effect.
If this flag is set for a particular signal number, the system uses the signal stack when delivering that kind of signal. See section BSD Signal Handling.
This flag controls what happens when a signal is delivered during
certain primitives (such as open, read or write),
and the signal handler returns normally. There are two alternatives:
the library function can resume, or it can return failure with error
code EINTR.
The choice is controlled by the SA_RESTART flag for the
particular kind of signal that was delivered. If the flag is set,
returning from a handler resumes the library function. If the flag is
clear, returning from a handler makes the function fail.
See section Primitives Interrupted by Signals.
When a new process is created (see section Creating a Process), it inherits
handling of signals from its parent process. However, when you load a
new process image using the exec function (see section Executing a File), any signals that you've defined your own handlers for revert to
their SIG_DFL handling. (If you think about it a little, this
makes sense; the handler functions from the old program are specific to
that program, and aren't even present in the address space of the new
program image.) Of course, the new program can establish its own
handlers.
When a program is run by a shell, the shell normally sets the initial
actions for the child process to SIG_DFL or SIG_IGN, as
appropriate. It's a good idea to check to make sure that the shell has
not set up an initial action of SIG_IGN before you establish your
own signal handlers.
Here is an example of how to establish a handler for SIGHUP, but
not if SIGHUP is currently ignored:
...
struct sigaction temp;
sigaction (SIGHUP, NULL, &temp);
if (temp.sa_handler != SIG_IGN)
{
temp.sa_handler = handle_sighup;
sigemptyset (&temp.sa_mask);
sigaction (SIGHUP, &temp, NULL);
}
This section describes how to write a signal handler function that can
be established with the signal or sigaction functions.
A signal handler is just a function that you compile together with the
rest of the program. Instead of directly invoking the function, you use
signal or sigaction to tell the operating system to call
it when a signal arrives. This is known as establishing the
handler. See section Specifying Signal Actions.
There are two basic strategies you can use in signal handler functions:
You need to take special care in writing handler functions because they can be called asynchronously. That is, a handler might be called at any point in the program, unpredictably. If two signals arrive during a very short interval, one handler can run within another. This section describes what your handler should do, and what you should avoid.
Handlers which return normally are usually used for signals such as
SIGALRM and the I/O and interprocess communication signals. But
a handler for SIGINT might also return normally after setting a
flag that tells the program to exit at a convenient time.
It is not safe to return normally from the handler for a program error signal, because the behavior of the program when the handler function returns is not defined after a program error. See section Program Error Signals.
Handlers that return normally must modify some global variable in order
to have any effect. Typically, the variable is one that is examined
periodically by the program during normal operation. Its data type
should be sig_atomic_t for reasons described in section Atomic Data Access and Signal Handling.
Here is a simple example of such a program. It executes the body of
the loop until it has noticed that a SIGALRM signal has arrived.
This technique is useful because it allows the iteration in progress
when the signal arrives to complete before the loop exits.
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
/* This flag controls termination of the main loop. */
volatile sig_atomic_t keep_going = 1;
/* The signal handler just clears the flag and re-enables itself. */
void
catch_alarm (int sig)
{
keep_going = 0;
signal (sig, catch_alarm);
}
void
do_stuff (void)
{
puts ("Doing stuff while waiting for alarm....");
}
int
main (void)
{
/* Establish a handler for SIGALRM signals. */
signal (SIGALRM, catch_alarm);
/* Set an alarm to go off in a little while. */
alarm (2);
/* Check the flag once in a while to see when to quit. */
while (keep_going)
do_stuff ();
return EXIT_SUCCESS;
}
Handler functions that terminate the program are typically used to cause orderly cleanup or recovery from program error signals and interactive interrupts.
The cleanest way for a handler to terminate the process is to raise the same signal that ran the handler in the first place. Here is how to do this:
volatile sig_atomic_t fatal_error_in_progress = 0;
void
fatal_error_signal (int sig)
{
/* Since this handler is established for more than one kind of signal,
it might still get invoked recursively by delivery of some other kind
of signal. Use a static variable to keep track of that. */
if (fatal_error_in_progress)
raise (sig);
fatal_error_in_progress = 1;
/* Now do the clean up actions:
- reset terminal modes
- kill child processes
- remove lock files */
...
/* Now reraise the signal. Since the signal is blocked,
it will receive its default handling, which is
to terminate the process. We could just call
exit or abort, but reraising the signal
sets the return status from the process correctly. */
raise (sig);
}
You can do a nonlocal transfer of control out of a signal handler using
the setjmp and longjmp facilities (see section Non-Local Exits).
When the handler does a nonlocal control transfer, the part of the program that was running will not continue. If this part of the program was in the middle of updating an important data structure, the data structure will remain inconsistent. Since the program does not terminate, the inconsistency is likely to be noticed later on.
There are two ways to avoid this problem. One is to block the signal for the parts of the program that update important data structures. Blocking the signal delays its delivery until it is unblocked, once the critical updating is finished. See section Blocking Signals.
The other way to re-initialize the crucial data structures in the signal handler, or make their values consistent.
Here is a rather schematic example showing the reinitialization of one global variable.
#include <signal.h>
#include <setjmp.h>
jmp_buf return_to_top_level;
volatile sig_atomic_t waiting_for_input;
void
handle_sigint (int signum)
{
/* We may have been waiting for input when the signal arrived,
but we are no longer waiting once we transfer control. */
waiting_for_input = 0;
longjmp (return_to_top_level, 1);
}
int
main (void)
{
...
signal (SIGINT, sigint_handler);
...
while (1) {
prepare_for_command ();
if (setjmp (return_to_top_level) == 0)
read_and_execute_command ();
}
}
/* Imagine this is a subroutine used by various commands. */
char *
read_data ()
{
if (input_from_terminal) {
waiting_for_input = 1;
...
waiting_for_input = 0;
} else {
...
}
}
What happens if another signal arrives when your signal handler function is running?
When the handler for a particular signal is invoked, that signal is
normally blocked until the handler returns. That means that if two
signals of the same kind arrive close together, the second one will be
held until the first has been handled. (The handler can explicitly
unblock the signal using sigprocmask, if you want to allow more
signals of this type to arrive; see section Process Signal Mask.)
However, your handler can still be interrupted by delivery of another
kind of signal. To avoid this, you can use the sa_mask member of
the action structure passed to sigaction to explicitly specify
which signals should be blocked while the signal handler runs. These
signals are in addition to the signal for which the handler was invoked,
and any other signals that are normally blocked by the process.
See section Blocking Signals for a Handler.
Portability Note: Always use sigaction to establish a
handler for a signal that you expect to receive asynchronously, if you
want your program to work properly on System V Unix. On this system,
the handling of a signal whose handler was established with
signal automatically sets the signal's action back to
SIG_DFL, and the handler must re-establish itself each time it
runs. This practice, while inconvenient, does work when signals cannot
arrive in succession. However, if another signal can arrive right away,
it may arrive before the handler can re-establish itself. Then the
second signal would receive the default handling, which could terminate
the process.
If multiple signals of the same type are delivered to your process before your signal handler has a chance to be invoked at all, the handler may only be invoked once, as if only a single signal had arrived. In effect, the signals merge into one. This situation can arise when the signal is blocked, or in a multiprocessing environment where the system is busy running some other processes while the signals are delivered. This means, for example, that you cannot reliably use a signal handler to count signals. The only distinction you can reliably make is whether at least one signal has arrived since a given time in the past.
Here is an example of a handler for SIGCHLD that compensates for
the fact that the number of signals recieved may not equal the number of
child processes generate them. It assumes that the program keeps track
of all the child processes with a chain of structures as follows:
struct process
{
struct process *next;
/* The process ID of this child. */
int pid;
/* The descriptor of the pipe or pseudo terminal
on which output comes from this child. */
int input_descriptor;
/* Nonzero if this process has stopped or terminated. */
sig_atomic_t have_status;
/* The status of this child; 0 if running,
otherwise a status value from waitpid. */
int status;
};
struct process *process_list;
This example also uses a flag to indicate whether signals have arrived since some time in the past--whenever the program last cleared it to zero.
/* Nonzero means some child's status has changed
so look at process_list for the details. */
int process_status_change;
Here is the handler itself:
void
sigchld_handler (int signo)
{
int old_errno = errno;
while (1) {
register int pid;
int w;
struct process *p;
/* Keep asking for a status until we get a definitive result. */
do
{
errno = 0;
pid = waitpid (WAIT_ANY, &w, WNOHANG | WUNTRACED);
}
while (pid <= 0 && errno == EINTR);
if (pid <= 0) {
/* A real failure means there are no more
stopped or terminated child processes, so return. */
errno = old_errno;
return;
}
/* Find the process that signaled us, and record its status. */
for (p = process_list; p; p = p->next)
if (p->pid == pid) {
p->status = w;
/* Indicate that the status field
has data to look at. We do this only after storing it. */
p->have_status = 1;
/* If process has terminated, stop waiting for its output. */
if (WIFSIGNALED (w) || WIFEXITED (w))
if (p->input_descriptor)
FD_CLR (p->input_descriptor, &input_wait_mask);
/* The program should check this flag from time to time
to see if there is any news in process_list. */
++process_status_change;
}
/* Loop around to handle all the processes
that have something to tell us. */
}
}
Here is the proper way to check the flag process_status_change:
if (process_status_change) {
struct process *p;
process_status_change = 0;
for (p = process_list; p; p = p->next)
if (p->have_status) {
... Examine p->status ...
}
}
It is vital to clear the flag before examining the list; otherwise, if a signal were delivered just before the clearing of the flag, and after the appropriate element of the process list had been checked, the status change would go unnoticed until the next signal arrived to set the flag again. You could, of course, avoid this problem by blocking the signal while scanning the list, but it is much more elegant to guarantee correctness by doing things in the right order.
The loop which checks process status avoids examining p->status
until it sees that status has been validly stored. This is to make sure
that the status cannot change in the middle of accessing it. Once
p->have_status is set, it means that the child process is stopped
or terminated, and in either case, it cannot stop or terminate again
until the program has taken notice. See section Atomic Usage Patterns, for more
information about coping with interruptions during accessings of a
variable.
Here is another way you can test whether the handler has run since the last time you checked. This technique uses a counter which is never changed outside the handler. Instead of clearing the count, the program remembers the previous value and sees whether it has changed since the previous check. The advantage of this method is that different parts of the program can check independently, each part checking whether there has been a signal since that part last checked.
sig_atomic_t process_status_change;
sig_atomic_t last_process_status_change;
...
{
sig_atomic_t prev = last_process_status_change;
last_process_status_change = process_status_change;
if (last_process_status_change != prev) {
struct process *p;
for (p = process_list; p; p = p->next)
if (p->have_status) {
... Examine p->status ...
}
}
}
Handler functions usually don't do very much. The best practice is to
write a handler that does nothing but set an external variable that the
program checks regularly, and leave all serious work to the program.
This is best because the handler can be called at asynchronously, at
unpredictable times--perhaps in the middle of a system call, or even
between the beginning and the end of a C operator that requires multiple
instructions. The data structures being manipulated might therefore be
in an inconsistent state when the handler function is invoked. Even
copying one int variable into another can take two instructions
on most machines.
This means you have to be very careful about what you do in a signal handler.
volatile. This tells the compiler that
the value of the variable might change asynchronously, and inhibits
certain optimizations that would be invalidated by such modifications.
A function can be non-reentrant if it uses memory that is not on the stack.
For example, suppose that the signal handler uses gethostbyname.
This function returns its value in a static object, reusing the same
object each time. If the signal happens to arrive during a call to
gethostbyname, or even after one (while the program is still
using the value), it will clobber the value that the program asked for.
However, if the program does not use gethostbyname or any other
function that returns information in the same object, or if it always
blocks signals around each use, then you are safe.
There are a large number of library functions that return values in a fixed object, always reusing the same object in this fashion, and all of them cause the same problem. The description of a function in this manual always mentions this behavior.
This case arises when you do I/O using streams. Suppose that the
signal handler prints a message with fprintf. Suppose that the
program was in the middle of an fprintf call using the same
stream when the signal was delivered. Both the signal handler's message
and the program's data could be corrupted, because both calls operate on
the same data structure--the stream itself.
However, if you know that the stream that the handler uses cannot possibly be used by the program at a time when signals can arrive, then you are safe. It is no problem if the program uses some other stream.
malloc and free are not reentrant,
because they use a static data structure which records what memory
blocks are free. As a result, no library functions that allocate or
free memory are reentrant. This includes functions that allocate space
to store a result.
The best way to avoid the need to allocate memory in a handler is to allocate in advance space for signal handlers to use.
The best way to avoid freeing memory in a handler is to flag or record the objects to be freed, and have the program check from time to time whether anything is waiting to be freed. But this must be done with care, because placing an object on a chain is not atomic, and if it is interrupted by another signal handler that does the same thing, you could "lose" one of the objects.
On the GNU system, malloc and free are safe to use in
signal handlers because it blocks signals. As a result, the library
functions that allocate space for a result are also safe in signal
handlers. The obstack allocation functions are safe as long as you
don't use the same obstack both inside and outside of a signal handler.
The relocating allocation functions (see section Relocating Allocator) are certainly not safe to use in a signal handler.
errno is non-reentrant, but you can
correct for this: in the handler, save the original value of
errno and restore it before returning normally. This prevents
errors that occur within the signal handler from being confused with
errors from system calls at the point the program is interrupted to run
the handler.
This technique is generally applicable; if you want to call in a handler a function that modifies a particular object in memory, you can make this safe by saving and restoring that object.
Whether the data in your application concerns atoms, or mere text, you have to be careful about the fact that access to a single datum is not necessarily atomic. This means that it can take more than one instruction to read or write a single object. In such cases, a signal handler can run in the middle of reading or writing the object.
There are three ways you can cope with this problem. You can use data types that are always accessed atomically; you can carefully arrange that nothing untoward happens if an access is interrupted, or you can block all signals around any access that had better not be interrupted (see section Blocking Signals).
Here is an example which shows what can happen if a signal handler runs in the middle of modifying a variable. (Interrupting the reading of a variable can also lead to paradoxical results, but here we only show writing.)
#include <signal.h>
#include <stdio.h>
struct two_words { int a, b; } memory;
void
handler(int signum)
{
printf ("%d,%d\n", memory.a, memory.b);
alarm (1);
}
int
main (void)
{
static struct two_words zeros = { 0, 0 }, ones = { 1, 1 };
signal (SIGALRM, handler);
memory = zeros;
alarm (1);
while (1)
{
memory = zeros;
memory = ones;
}
}
This program fills memory with zeros, ones, zeros, ones,
alternating forever; meanwhile, once per second, the alarm signal handler
prints the current contents. (Calling printf in the handler is
safe in this program because it is certainly not being called outside
the handler when the signal happens.)
Clearly, this program can print a pair of zeros or a pair of ones. But
that's not all it can do! On most machines, it takes several
instructions to store a new value in memory, and the value is
stored one word at a time. If the signal is delivered in between these
instructions, the handler might find that memory.a is zero and
memory.b is one (or vice versa).
On some machines it may be possible to store a new value in
memory with just one instruction that cannot be interrupted. On
these machines, the handler will always print two zeros or two ones.
To avoid uncertainty about interrupting access to a variable, you can
use a particular data type for which access is always atomic:
sig_atomic_t. Reading and writing this data type is guaranteed
to happen in a single instruction, so there's no way for a handler to
run "in the middle" of an access.
The type sig_atomic_t is always an integer data type, but which
one it is, and how many bits it contains, may vary from machine to
machine.
This is an integer data type. Objects of this type are always accessed atomically.
In practice, you can assume that int and other integer types no
longer than int are atomic. You can also assume that pointer
types are atomic; that is very convenient. Both of these are true on
all of the machines that the GNU C library supports, and on all POSIX
systems we know of.
Certain patterns of access avoid any problem even if an access is interrupted. For example, a flag which is set by the handler, and tested and cleared by the main program from time to time, is always safe even if access actually requires two instructions. To show that this is so, we must consider each access that could be interrupted, and show that there is no problem if it is interrupted.
An interrupt in the middle of testing the flag is safe because either it's recognized to be nonzero, in which case the precise value doesn't matter, or it will be seen to be nonzero the next time it's tested.
An interrupt in the middle of clearing the flag is no problem because either the value ends up zero, which is what happens if a signal comes in just before the flag is cleared, or the value ends up nonzero, and subsequent events occur as if the signal had come in just after the flag was cleared. As long as the code handles both of these cases properly, it can also handle a signal in the middle of clearing the flag. (This is an example of the sort of reasoning you need to do to figure out whether non-atomic usage is safe.)
Sometimes you can insure uninterrupted access to one object by protecting its use with another object, perhaps one whose type guarantees atomicity. See section Signals Close Together Merge into One, for an example.
A signal can arrive and be handled while an I/O primitive such as
open or read is waiting for an I/O device. If the signal
handler returns, the system faces the question: what should happen next?
POSIX specifies one approach: make the primitive fail right away. The
error code for this kind of failure is EINTR. This is flexible,
but usually inconvenient. Typically, POSIX applications that use signal
handlers must check for EINTR after each library function that
can return it, in order to try the call again. Often programmers forget
to check, which is a common source of error.
The GNU library provides a convenient way to retry a call after a
temporary failure, with the macro TEMP_FAILURE_RETRY:
Macro: TEMP_FAILURE_RETRY (expression)
This macro evaluates expression once. If it fails and reports
error code EINTR, TEMP_FAILURE_RETRY evaluates it again,
and over and over until the result is not a temporary failure.
The value returned by TEMP_FAILURE_RETRY is whatever value
expression produced.
BSD avoids EINTR entirely and provides a more convenient
approach: to restart the interrupted primitive, instead of making it
fail. If you choose this approach, you need not be concerned with
EINTR.
You can choose either approach with the GNU library. If you use
sigaction to establish a signal handler, you can specify how that
handler should behave. If you specify the SA_RESTART flag,
return from that handler will resume a primitive; otherwise, return from
that handler will cause EINTR. See section Flags for sigaction.
Another way to specify the choice is with the siginterrupt
function. See section POSIX and BSD Signal Facilities.
When you don't specify with sigaction or siginterrupt what
a particular handler should do, it uses a default choice. The default
choice in the GNU library depends on the feature test macros you have
defined. If you define _BSD_SOURCE or _GNU_SOURCE before
calling signal, the default is to resume primitives; otherwise,
the default is to make them fail with EINTR. (The library
contains alternate versions of the signal function, and the
feature test macros determine which one you really call.) See section Feature Test Macros.
The primitives affected by this issue are close, fcntl
(operation F_SETLK), open, read, recv,
recvfrom, select, send, sendto,
tcdrain, waitpid, wait, and write.
There is one situation where resumption never happens no matter which
choice you make: when a data-transfer function such as read or
write is interrupted by a signal after transferring part of the
data. In this case, the function returns the number of bytes already
transferred, indicating partial success.
This might at first appear to cause unreliable behavior on
record-oriented devices (including datagram sockets; see section Datagram Socket Operations),
where splitting one read or write into two would read or
write two records. Actually, there is no problem, because interruption
after a partial transfer cannot happen on such devices; they always
transfer an entire record in one burst, with no waiting once data
transfer has started.
Besides signals that are generated as a result of a hardware trap or interrupt, your program can explicitly send signals to itself or to another process.
A process can send itself a signal with the raise function. This
function is declared in `signal.h'.
Function: int raise (int signum)
The raise function sends the signal signum to the calling
process. It returns zero if successful and a nonzero value if it fails.
About the only reason for failure would be if the value of signum
is invalid.
Function: int gsignal (int signum)
The gsignal function does the same thing as raise; it is
provided only for compatibility with SVID.
One convenient use for raise is to reproduce the default behavior
of a signal that you have trapped. For instance, suppose a user of your
program types the SUSP character (usually C-z; see section Special Characters) to send it an interactive stop stop signal
(SIGTSTP), and you want to clean up some internal data buffers
before stopping. You might set this up like this:
#include <signal.h>
/* When a stop signal arrives, set the action back to the default
and then resend the signal after doing cleanup actions. */
void
tstp_handler (int sig)
{
signal (SIGTSTP, SIG_DFL);
/* Do cleanup actions here. */
...
raise (SIGTSTP);
}
/* When the process is continued again, restore the signal handler. */
void
cont_handler (int sig)
{
signal (SIGCONT, cont_handler);
signal (SIGTSTP, tstp_handler);
}
/* Enable both handlers during program initialization. */
int
main (void)
{
signal (SIGCONT, cont_handler);
signal (SIGTSTP, tstp_handler);
...
}
Portability note: raise was invented by the ANSI C
committee. Older systems may not support it, so using kill may
be more portable. See section Signaling Another Process.
The kill function can be used to send a signal to another process.
In spite of its name, it can be used for a lot of things other than
causing a process to terminate. Some examples of situations where you
might want to send signals between processes are:
This section assumes that you know a little bit about how processes work. For more information on this subject, see section Child Processes.
The kill function is declared in `signal.h'.
Function: int kill (pid_t pid, int signum)
The kill function sends the signal signum to the process
or process group specified by pid. Besides the signals listed in
section Standard Signals, signum can also have a value of zero to
check the validity of the pid.
The pid specifies the process or process group to receive the signal:
pid > 0
pid == 0
pid < -1
pid == -1
A process can send a signal to itself with kill (getpid(),
signum);. If kill is used by a process to send a signal to
itself, and the signal is not blocked, then kill delivers at
least one signal (which might be some other pending unblocked signal
instead of the signal signum) to that process before it returns.
The return value from kill is zero if the signal can be sent
successfully. Otherwise, no signal is sent, and a value of -1 is
returned. If pid specifies sending a signal to several processes,
kill succeeds if it can send the signal to at least one of them.
There's no way you can tell which of the processes got the signal
or whether all of them did.
The following errno error conditions are defined for this function:
EINVAL
EPERM
ESCRH
Function: int killpg (int pgid, int signum)
This is similar to kill, but sends signal signum to the
process group pgid. This function is provided for compatibility
with BSD; using kill to do this is more portable.
As a simple example of kill, the call kill (getpid (),
sig) has the same effect as raise (sig).
kill
There are restrictions that prevent you from using kill to send
signals to any random process. These are intended to prevent antisocial
behavior such as arbitrarily killing off processes belonging to another
user. In typical use, kill is used to pass signals between
parent, child, and sibling processes, and in these situations you
normally do have permission to send signals. The only common execption
is when you run a setuid program in a child process; if the program
changes its real UID as well as its effective UID, you may not have
permission to send a signal. The su program does this.
Whether a process has permission to send a signal to another process is determined by the user IDs of the two processes. This concept is discussed in detail in section The Persona of a Process.
Generally, for a process to be able to send a signal to another process, either the sending process must belong to a privileged user (like `root'), or the real or effective user ID of the sending process must match the real or effective user ID of the receiving process. If the receiving process has changed its effective user ID from the set-user-ID mode bit on its process image file, then the owner of the process image file is used in place of its current effective user ID. In some implementations, a parent process might be able to send signals to a child process even if the user ID's don't match, and other implementations might enforce other restrictions.
The SIGCONT signal is a special case. It can be sent if the
sender is part of the same session as the receiver, regardless of
user IDs.
kill for CommunicationSIGUSR1 and
SIGUSR2 signals are provided for. Since these signals are fatal
by default, the process that is supposed to receive them must trap them
through signal or sigaction.
In this example, a parent process forks a child process and then waits
for the child to complete its initialization. The child process tells
the parent when it is ready by sending it a SIGUSR1 signal, using
the kill function.
#include <signal.h>
#include <stdio.h>
#include <sys/types.h>
#include <unistd.h>
/* When a SIGUSR1 signal arrives, set this variable. */
volatile sig_atomic_t usr_interrupt = 0;
void
synch_signal (int sig)
{
usr_interrupt = 1;
}
/* The child process executes this function. */
void
child_function (void)
{
/* Perform initialization. */
printf ("I'm here!!! My pid is %d.\n", (int) getpid ());
/* Let parent know you're done. */
kill (getppid (), SIGUSR1);
/* Continue with execution. */
puts ("Bye, now....");
exit (0);
}
int
main (void)
{
struct sigaction usr_action;
sigset_t block_mask;
pid_t child_id;
/* Establish the signal handler. */
sigfillset (&block_mask);
usr_action.sa_handler = synch_signal;
usr_action.sa_mask = block_mask;
usr_action.sa_flags = 0;
sigaction (SIGUSR1, &usr_action, NULL);
/* Create the child process. */
child_id = fork ();
if (child_id == 0)
child_function (); /* Does not return. */
/* Busy wait for the child to send a signal. */
while (!usr_interrupt)
;
/* Now continue execution. */
puts ("That's all, folks!");
return 0;
}
This example uses a busy wait, which is bad, because it wastes CPU cycles that other programs could otherwise use. It is better to ask the system to wait until the signal arrives. See the example in section Waiting for a Signal.
Blocking a signal means telling the operating system to hold it and
deliver it later. Generally, a program does not block signals
indefinitely--it might as well ignore them by setting their actions to
SIG_IGN. But it is useful to block signals briefly, to prevent
them from interrupting sensitive operations. For instance:
sigprocmask function to block signals while you
modify global variables that are also modified by the handlers for these
signals.
sa_mask in your sigaction call to block
certain signals while a particular signal handler runs. This way, the
signal handler can run without being interrupted itself by signals.
Temporary blocking of signals with sigprocmask gives you a way to
prevent interrupts during critical parts of your code. If signals
arrive in that part of the program, they are delivered later, after you
unblock them.
One example where this is useful is for sharing data between a signal
handler and the rest of the program. If the type of the data is not
sig_atomic_t (see section Atomic Data Access and Signal Handling), then the signal
handler could run when the rest of the program has only half finished
reading or writing the data. This would lead to confusing consequences.
To make the program reliable, you can prevent the signal handler from running while the rest of the program is examining or modifying that data--by blocking the appropriate signal around the parts of the program that touch the data.
Blocking signals is also necessary when you want to perform a certain
action only if a signal has not arrived. Suppose that the handler for
the signal sets a flag of type sig_atomic_t; you would like to
test the flag and perform the action if the flag is not set. This is
unreliable. Suppose the signal is delivered immediately after you test
the flag, but before the consequent action: then the program will
perform the action even though the signal has arrived.
The only way to test reliably for whether a signal has yet arrived is to test while the signal is blocked.
All of the signal blocking functions use a data structure called a signal set to specify what signals are affected. Thus, every activity involves two stages: creating the signal set, and then passing it as an argument to a library function.
These facilities are declared in the header file `signal.h'.
The sigset_t data type is used to represent a signal set.
Internally, it may be implemented as either an integer or structure
type.
For portability, use only the functions described in this section to
initialize, change, and retrieve information from sigset_t
objects--don't try to manipulate them directly.
There are two ways to initialize a signal set. You can initially
specify it to be empty with sigemptyset and then add specified
signals individually. Or you can specify it to be full with
sigfillset and then delete specified signals individually.
You must always initialize the signal set with one of these two
functions before using it in any other way. Don't try to set all the
signals explicitly because the sigset_t object might include some
other information (like a version field) that needs to be initialized as
well. (In addition, it's not wise to put into your program an
assumption that the system has no signals aside from the ones you know
about.)
Function: int sigemptyset (sigset_t *set)
This function initializes the signal set set to exclude all of the
defined signals. It always returns 0.
Function: int sigfillset (sigset_t *set)
This function initializes the signal set set to include
all of the defined signals. Again, the return value is 0.
Function: int sigaddset (sigset_t *set, int signum)
This function adds the signal signum to the signal set set.
All sigaddset does is modify set; it does not block or
unblock any signals.
The return value is 0 on success and -1 on failure.
The following errno error condition is defined for this function:
EINVAL
Function: int sigdelset (sigset_t *set, int signum)
This function removes the signal signum from the signal set
set. All sigdelset does is modify set; it does not
block or unblock any signals. The return value and error conditions are
the same as for sigaddset.
Finally, there is a function to test what signals are in a signal set:
Function: int sigismember (const sigset_t *set, int signum)
The sigismember function tests whether the signal signum is
a member of the signal set set. It returns 1 if the signal
is in the set, 0 if not, and -1 if there is an error.
The following errno error condition is defined for this function:
EINVAL
The collection of signals that are currently blocked is called the signal mask. Each process has its own signal mask. When you create a new process (see section Creating a Process), it inherits its parent's mask. You can block or unblock signals with total flexibility by modifying the signal mask.
The prototype for the sigprocmask function is in `signal.h'.
Function: int sigprocmask (int how, const sigset_t *set, sigset_t *oldset)
The sigprocmask function is used to examine or change the calling
process's signal mask. The how argument determines how the signal
mask is changed, and must be one of the following values:
SIG_BLOCK
set---add them to the existing mask. In
other words, the new mask is the union of the existing mask and
set.
SIG_UNBLOCK
SIG_SETMASK
The last argument, oldset, is used to return information about the
old process signal mask. If you just want to change the mask without
looking at it, pass a null pointer as the oldset argument.
Similarly, if you want to know what's in the mask without changing it,
pass a null pointer for set (in this case the how argument
is not significant). The oldset argument is often used to
remember the previous signal mask in order to restore it later. (Since
the signal mask is inherited over fork and exec calls, you
can't predict what its contents are when your program starts running.)
If invoking sigprocmask causes any pending signals to be
unblocked, at least one of those signals is delivered to the process
before sigprocmask returns. The order in which pending signals
are delivered is not specified, but you can control the order explicitly
by making multiple sigprockmask calls to unblock various signals
one at a time.
The sigprocmask function returns 0 if successful, and -1
to indicate an error. The following errno error conditions are
defined for this function:
EINVAL
You can't block the SIGKILL and SIGSTOP signals, but
if the signal set includes these, sigprocmask just ignores
them instead of returning an error status.
Remember, too, that blocking program error signals such as SIGFPE
leads to undesirable results for signals generated by an actual program
error (as opposed to signals sent with raise or kill).
This is because your program may be too broken to be able to continue
executing to a point where the signal is unblocked again.
See section Program Error Signals.
Now for a simple example. Suppose you establish a handler for
SIGALRM signals that sets a flag whenever a signal arrives, and
your main program checks this flag from time to time and then resets it.
You can prevent additional SIGALRM signals from arriving in the
meantime by wrapping the critical part of the code with calls to
sigprocmask, like this:
/* This variable is set by the SIGALRM signal handler. */
volatile sig_atomic_t flag = 0;
int
main (void)
{
sigset_t block_alarm;
...
/* Initialize the signal mask. */
sigemptyset (&block_alarm);
sigaddset (&block_alarm, SIGALRM);
while (1)
{
/* Check if a signal has arrived; if so, reset the flag. */
sigprocmask (SIG_BLOCK, &block_alarm, NULL);
if (flag)
{
actions-if-not-arrived
flag = 0;
}
sigprocmask (SIG_UNBLOCK, &block_alarm, NULL);
...
}
}
When a signal handler is invoked, you usually want it to be able to finish without being interrupted by another signal. From the moment the handler starts until the moment it finishes, you must block signals that might confuse it or corrupt its data.
When a handler function is invoked on a signal, that signal is
automatically blocked (in addition to any other signals that are already
in the process's signal mask) during the time the handler is running.
If you set up a handler for SIGTSTP, for instance, then the
arrival of that signal forces further SIGTSTP signals to wait
during the execution of the handler.
However, by default, other kinds of signals are not blocked; they can arrive during handler execution.
The reliable way to block other kinds of signals during the execution of
the handler is to use the sa_mask member of the sigaction
structure.
Here is an example:
#include <signal.h>
#include <stddef.h>
void catch_stop ();
void
install_handler (void)
{
struct sigaction setup_action;
sigset_t block_mask;
sigemptyset (&block_mask);
/* Block other terminal-generated signals while handler runs. */
sigaddset (&block_mask, SIGINT);
sigaddset (&block_mask, SIGQUIT);
setup_action.sa_handler = catch_stop;
setup_action.sa_mask = block_mask;
setup_action.sa_flags = 0;
sigaction (SIGTSTP, &setup_action, NULL);
}
This is more reliable than blocking the other signals explicitly in the code for the handler. If you block signals explicity in the handler, you can't avoid at least a short interval at the beginning of the handler where they are not yet blocked.
You cannot remove signals from the process's current mask using this
mechanism. However, you can make calls to sigprocmask within
your handler to block or unblock signals as you wish.
In any case, when the handler returns, the system restores the mask that was in place before the handler was entered.
You can find out which signals are pending at any time by calling
sigpending. This function is declared in `signal.h'.
Function: int sigpending (sigset_t *set)
The sigpending function stores information about pending signals
in set. If there is a pending signal that is blocked from
delivery, then that signal is a member of the returned set. (You can
test whether a particular signal is a member of this set using
sigismember; see section Signal Sets.)
The return value is 0 if successful, and -1 on failure.
Testing whether a signal is pending is not often useful. Testing when that signal is not blocked is almost certainly bad design.
Here is an example.
#include <signal.h>
#include <stddef.h>
sigset_t base_mask, waiting_mask;
sigemptyset (&base_mask);
sigaddset (&base_mask, SIGINT);
sigaddset (&base_mask, SIGTSTP);
/* Block user interrupts while doing other processing. */
sigprocmask (SIG_SETMASK, &base_mask, NULL);
...
/* After a while, check to see whether any signals are pending. */
sigpending (&waiting_mask);
if (sigismember (&waiting_mask, SIGINT)) {
/* User has tried to kill the process. */
}
else if (sigismember (&waiting_mask, SIGTSTP)) {
/* User has tried to stop the process. */
}
Remember that if there is a particular signal pending for your process,
additional signals of that same type that arrive in the meantime might
be discarded. For example, if a SIGINT signal is pending when
another SIGINT signal arrives, your program will probably only
see one of them when you unblock this signal.
Portability Note: The sigpending function is new in
POSIX.1. Older systems have no equivalent facility.
Instead of blocking a signal using the library facilities, you can get almost the same results by making the handler set a flag to be tested later, when you "unblock". Here is an example:
/* If this flag is nonzero, don't handle the signal right away. */
volatile sig_atomic_t signal_pending;
/* This is nonzero if a signal arrived and was not handled. */
volatile sig_atomic_t defer_signal;
void
handler (int signum)
{
if (defer_signal)
signal_pending = signum;
else
... /* "Really" handle the signal. */
}
...
void
update_mumble (int frob)
{
/* Prevent signals from having immediate effect. */
defer_signal++;
/* Now update mumble, without worrying about interruption. */
mumble.a = 1;
mumble.b = hack ();
mumble.c = frob;
/* We have updated mumble. Handle any signal that came in. */
defer_signal--;
if (defer_signal == 0 && signal_pending != 0)
raise (signal_pending);
}
Note how the particular signal that arrives is stored in
signal_pending. That way, we can handle several types of
inconvenient signals with the same mechanism.
We increment and decrement defer_signal so that nested critical
sections will work properly; thus, if update_mumble were called
with signal_pending already nonzero, signals would be deferred
not only within update_mumble, but also within the caller. This
is also why we do not check signal_pending if defer_signal
is still nonzero.
The incrementing and decrementing of defer_signal require more
than one instruction; it is possible for a signal to happen in the
middle. But that does not cause any problem. If the signal happens
early enough to see the value from before the increment or decrement,
that is equivalent to a signal which came before the beginning of the
increment or decrement, which is a case that works properly.
It is absolutely vital to decrement defer_signal before testing
signal_pending, because this avoids a subtle bug. If we did
these things in the other order, like this,
if (defer_signal == 1 && signal_pending != 0)
raise (signal_pending);
defer_signal--;
then a signal arriving in between the if statement and the decrement
would be effetively "lost" for an indefinite amount of time. The
handler would merely set defer_signal, but the program having
already tested this variable, it would not test the variable again.
Bugs like these are called timing errors. They are especially bad because they happen only rarely and are nearly impossible to reproduce. You can't expect to find them with a debugger as you would find a reproducible bug. So it is worth being especially careful to avoid them.
(You would not be tempted to write the code in this order, given the use
of defer_signal as a counter which must be tested along with
signal_pending. After all, testing for zero is cleaner than
testing for one. But if you did not use defer_signal as a
counter, and gave it values of zero and one only, then either order
might seem equally simple. This is a further advantage of using a
counter for defer_signal: it will reduce the chance you will
write the code in the wrong order and create a subtle bug.)
If your program is driven by external events, or uses signals for synchronization, then when it has nothing to do it should probably wait until a signal arrives.
pause
The simple way to wait until a signal arrives is to call pause.
Please read about its disadvantages, in the following section, before
you use it.
The pause function suspends program execution until a signal
arrives whose action is either to execute a handler function, or to
terminate the process.
If the signal causes a handler function to be executed, then
pause returns. This is considered an unsuccessful return (since
"successful" behavior would be to suspend the program forever), so the
return value is -1. Even if you specify that other primitives
should resume when a system handler returns (see section Primitives Interrupted by Signals), this has no effect on pause; it always fails when a
signal is handled.
The following errno error conditions are defined for this function:
EINTR
If the signal causes program termination, pause doesn't return
(obviously).
The pause function is declared in `unistd.h'.
pause
The simplicity of pause can conceal serious timing errors that
can make a program hang mysteriously.
It is safe to use pause if the real work of your program is done
by the signal handlers themselves, and the "main program" does nothing
but call pause. Each time a signal is delivered, the handler
will do the next batch of work that is to be done, and then return, so
that the main loop of the program can call pause again.
You can't safely use pause to wait until one more signal arrives,
and then resume real work. Even if you arrange for the signal handler
to cooperate by setting a flag, you still can't use pause
reliably. Here is an example of this problem:
/* usr_interrupt is set by the signal handler. */
if (!usr_interrupt)
pause ();
/* Do work once the signal arrives. */
...
This has a bug: the signal could arrive after the variable
usr_interrupt is checked, but before the call to pause.
If no further signals arrive, the process would never wake up again.
You can put an upper limit on the excess waiting by using sleep
in a loop, instead of using pause. (See section Sleeping, for more
about sleep.) Here is what this looks like:
/* usr_interrupt is set by the signal handler.
while (!usr_interrupt)
sleep (1);
/* Do work once the signal arrives. */
...
For some purposes, that is good enough. But with a little more
complexity, you can wait reliably until a particular signal handler is
run, using sigsuspend.
sigsuspend
The clean and reliable way to wait for a signal to arrive is to block it
and then use sigsuspend. By using sigsuspend in a loop,
you can wait for certain kinds of signals, while letting other kinds of
signals be handled by their handlers.
Function: int sigsuspend (const sigset_t *set)
This function replaces the process's signal mask with set and then suspends the process until a signal is delivered whose action is either to terminate the process or invoke a signal handling function. In other words, the program is effectively suspended until one of the signals that is not a member of set arrives.
If the process is woken up by deliver of a signal that invokes a handler
function, and the handler function returns, then sigsuspend also
returns.
The mask remains set only as long as sigsuspend is waiting.
The function sigsuspend always restores the previous signal mask
when it returns.
The return value and error conditions are the same as for pause.
With sigsuspend, you can replace the pause or sleep
loop in the previous section with something completely reliable:
sigset_t mask, oldmask; ... /* Set up the mask of signals to temporarily block. */ sigemptyset (&mask); sigaddset (&mask, SIGUSR1); ... /* Wait for a signal to arrive. */ sigprocmask (SIG_BLOCK, &mask, &oldmask); while (!usr_interrupt) sigsuspend (&oldmask); sigprocmask (SIG_UNBLOCK, &mask, NULL);
This last piece of code is a little tricky. The key point to remember
here is that when sigsuspend returns, it resets the process's
signal mask to the original value, the value from before the call to
sigsuspend---in this case, the SIGUSR1 signal is once
again blocked. The second call to sigprocmask is
necessary to explicitly unblock this signal.
One other point: you may be wondering why the while loop is
necessary at all, since the program is apparently only waiting for one
SIGUSR1 signal. The answer is that the mask passed to
sigsuspend permits the process to be woken up by the delivery of
other kinds of signals, as well--for example, job control signals. If
the process is woken up by a signal that doesn't set
usr_interrupt, it just suspends itself again until the "right"
kind of signal eventually arrives.
This technique takes a few more lines of preparation, but that is needed just once for each kind of wait criterion you want to use. The code that actually waits is just four lines.
This section describes alternative signal handling functions derived from BSD Unix. These facilities were an advance, in their time; today, they are mostly obsolete, and supported mainly for compatibility with BSD Unix.
They do provide one feature that is not available through the POSIX functions: You can specify a separate stack for use in certain signal handlers. Using a signal stack is the only way you can handle a signal caused by stack overflow.
There are many similarities between the BSD and POSIX signal handling facilities, because the POSIX facilities were inspired by the BSD facilities. Besides having different names for all the functions to avoid conflicts, the main differences between the two are:
int bit mask, rather than
as a sigset_t object.
The BSD facilities are declared in `signal.h'.
This data type is the BSD equivalent of struct sigaction
(see section Advanced Signal Handling); it is used to specify signal actions
to the sigvec function. It contains the following members:
sighandler_t sv_handler
int sv_mask
int sv_flags
sv_onstack.
These symbolic constants can be used to provide values for the
sv_flags field of a sigvec structure. This field is a bit
mask value, so you bitwise-OR the flags of interest to you together.
If this bit is set in the sv_flags field of a sigvec
structure, it means to use the signal stack when delivering the signal.
If this bit is set in the sv_flags field of a sigvec
structure, it means that system calls interrupted by this kind of signal
should not be restarted if the handler returns; instead, the system
calls should return with a EINTR error status. See section Primitives Interrupted by Signals.
If this bit is set in the sv_flags field of a sigvec
structure, it means to reset the action for the signal back to
SIG_DFL when the signal is received.
Function: int sigvec (int signum, const struct sigvec *action,struct sigvec *old_action)
This function is the equivalent of sigaction (see section Advanced Signal Handling); it installs the action action for the signal signum,
returning information about the previous action in effect for that signal
in old_action.
Function: int siginterrupt (int signum, int failflag)
This function specifies which approach to use when certain primitives
are interrupted by handling signal signum. If failflag is
false, signal signum restarts primitives. If failflag is
true, handling signum causes these primitives to fail with error
code EINTR. See section Primitives Interrupted by Signals.
Macro: int sigmask (int signum)
This macro returns a signal mask that has the bit for signal signum
set. You can bitwise-OR the results of several calls to sigmask
together to specify more than one signal. For example,
(sigmask (SIGTSTP) | sigmask (SIGSTOP) | sigmask (SIGTTIN) | sigmask (SIGTTOU))
specifies a mask that includes all the job-control stop signals.
Function: int sigblock (int mask)
This function is the equivalent of sigprocmask (see section Process Signal Mask) with a how argument of SIG_BLOCK: it adds the
signals specified by mask to the calling process's signal mask.
The return value is the previous set of blocked signals.
Function: int sigsetmask (int mask)
This function is the equivalent of sigprocmask (see section Process Signal Mask) with a how argument of SIG_SETMASK: it sets
the calling process's signal mask to mask. The return value is
the previous set of blocked signals.
Function: int sigpause (int mask)
This function is the equivalent of sigsuspend (see section Waiting for a Signal): it sets the calling process's signal mask to mask,
and waits for a signal to arrive. On return the previous set of blocked
signals is restored.
A signal stack is a special area of memory to be used as the execution
stack during signal handlers. It should be fairly large, to avoid any
danger that it will overflow in turn--we recommend at least 16,000
bytes. You can use malloc to allocate the space for the stack.
Then call sigstack to tell the system to use that space for the
signal stack.
You don't need to write signal handlers differently in order to use a signal stack. Switching from one stack to the other happens automatically. However, some debuggers on some machines may get confused if you examine a stack trace while a handler that uses the signal stack is running.
This structure describes a signal stack. It contains the following members:
void *ss_sp
int ss_onstack
Function: int sigstack (const struct sigstack *stack, struct sigstack *oldstack)
The sigstack function specifies an alternate stack for use during
signal handling. When a signal is received by the process and its
action indicates that the signal stack is used, the system arranges a
switch to the currently installed signal stack while the handler for
that signal is executed.
If oldstack is not a null pointer, information about the currently installed signal stack is returned in the location it points to. If stack is not a null pointer, then this is installed as the new stack for use by signal handlers.
The return value is 0 on success and 1 on failure.
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