2. System calls (functions provided by the kernel)
SIGNALSection: Linux Programmer's Manual (2)
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NAMEsignal - ANSI C signal handling
typedef void (*sighandler_t)(int);
sighandler_t signal(int signum, sighandler_t handler);
the behavior of signal() varies across UNIX versions, and has also varied historically across different versions of Linux. Avoid its use: use sigaction(2) instead. See Portability below.
signal() sets the disposition of the signal signum to handler, which is either SIG_IGN, SIG_DFL, or the address of a programmer-defined function (a "signal handler").
If the signal signum is delivered to the process, then one of the following happens:
- If the disposition is set to SIG_IGN, then the signal is ignored.
- If the disposition is set to SIG_DFL, then the default action associated with the signal (see signal(7)) occurs.
- If the disposition is set to a function, then first either the disposition is reset to SIG_DFL, or the signal is blocked (see Portability below), and then handler is called with argument signum. If invocation of the handler caused the signal to be blocked, then the signal is unblocked upon return from the handler.
The signals SIGKILL and SIGSTOP cannot be caught or ignored.
RETURN VALUEsignal() returns the previous value of the signal handler, or SIG_ERR on error. In the event of an error, errno is set to indicate the cause.
- signum is invalid.
CONFORMING TOPOSIX.1-2001, POSIX.1-2008, C89, C99.
NOTESThe effects of signal() in a multithreaded process are unspecified.
According to POSIX, the behavior of a process is undefined after it ignores a SIGFPE, SIGILL, or SIGSEGV signal that was not generated by kill(2) or raise(3). Integer division by zero has undefined result. On some architectures it will generate a SIGFPE signal. (Also dividing the most negative integer by -1 may generate SIGFPE.) Ignoring this signal might lead to an endless loop.
See sigaction(2) for details on what happens when the disposition SIGCHLD is set to SIG_IGN.
See signal-safety(7) for a list of the async-signal-safe functions that can be safely called from inside a signal handler.
The use of sighandler_t is a GNU extension, exposed if _GNU_SOURCE is defined; glibc also defines (the BSD-derived) sig_t if _BSD_SOURCE (glibc 2.19 and earlier) or _DEFAULT_SOURCE (glibc 2.19 and later) is defined. Without use of such a type, the declaration of signal() is the somewhat harder to read:
void ( *signal(int signum, void (*handler)(int)) ) (int);
PortabilityThe only portable use of signal() is to set a signal's disposition to SIG_DFL or SIG_IGN. The semantics when using signal() to establish a signal handler vary across systems (and POSIX.1 explicitly permits this variation); do not use it for this purpose.
POSIX.1 solved the portability mess by specifying sigaction(2), which provides explicit control of the semantics when a signal handler is invoked; use that interface instead of signal().
In the original UNIX systems, when a handler that was established using signal() was invoked by the delivery of a signal, the disposition of the signal would be reset to SIG_DFL, and the system did not block delivery of further instances of the signal. This is equivalent to calling sigaction(2) with the following flags:
sa.sa_flags = SA_RESETHAND | SA_NODEFER;
System V also provides these semantics for signal(). This was bad because the signal might be delivered again before the handler had a chance to reestablish itself. Furthermore, rapid deliveries of the same signal could result in recursive invocations of the handler.
BSD improved on this situation, but unfortunately also changed the semantics of the existing signal() interface while doing so. On BSD, when a signal handler is invoked, the signal disposition is not reset, and further instances of the signal are blocked from being delivered while the handler is executing. Furthermore, certain blocking system calls are automatically restarted if interrupted by a signal handler (see signal(7)). The BSD semantics are equivalent to calling sigaction(2) with the following flags:
sa.sa_flags = SA_RESTART;
The situation on Linux is as follows:
- The kernel's signal() system call provides System V semantics.
- By default, in glibc 2 and later, the signal() wrapper function does not invoke the kernel system call. Instead, it calls sigaction(2) using flags that supply BSD semantics. This default behavior is provided as long as a suitable feature test macro is defined: _BSD_SOURCE on glibc 2.19 and earlier or _DEFAULT_SOURCE in glibc 2.19 and later. (By default, these macros are defined; see feature_test_macros(7) for details.) If such a feature test macro is not defined, then signal() provides System V semantics.
SEE ALSOkill(1), alarm(2), kill(2), pause(2), sigaction(2), signalfd(2), sigpending(2), sigprocmask(2), sigsuspend(2), bsd_signal(3), killpg(3), raise(3), siginterrupt(3), sigqueue(3), sigsetops(3), sigvec(3), sysv_signal(3), signal(7)
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