typedef void (*sighandler_t)(int);
sighandler_t signal(int signum, sighandler_t handler);
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:
The signals SIGKILL and SIGSTOP cannot be caught or ignored.
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);
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:
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).)