|SIGNAL(2)||Linux Programmer's Manual||SIGNAL(2)|
NAME¶signal - ANSI C signal handling
DESCRIPTION¶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.
- 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.
RETURN VALUE¶signal() 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 TO¶C89, C99, POSIX.1-2001.
NOTES¶The 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 SIGCHLD is set to SIG_IGN. See signal(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 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);
Portability¶The 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.
sa.sa_flags = SA_RESETHAND | SA_NODEFER;
sa.sa_flags = SA_RESTART;
- 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 the _BSD_SOURCE feature test macro is defined. By default, _BSD_SOURCE is defined; it is also implicitly defined if one defines _GNU_SOURCE, and can of course be explicitly defined.
- On glibc 2 and later, if the _BSD_SOURCE feature test macro is not defined, then signal() provides System V semantics. (The default implicit definition of _BSD_SOURCE is not provided if one invokes gcc(1) in one of its standard modes (-std=xxx or -ansi) or defines various other feature test macros such as _POSIX_SOURCE, _XOPEN_SOURCE, or _SVID_SOURCE; see feature_test_macros(7).)
SEE ALSO¶kill(1), alarm(2), kill(2), killpg(2), pause(2), sigaction(2), signalfd(2), sigpending(2), sigprocmask(2), sigsuspend(2), bsd_signal(3), raise(3), siginterrupt(3), sigqueue(3), sigsetops(3), sigvec(3), sysv_signal(3), signal(7)
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