signal2
Ansi c signal handling
Usage
#include <signal.h>
typedef typeof(void (int)) *sighandler_t;
sighandler_t signal(int signum, sighandler_t handler);
Library
Standard C library (libc, -lc)
Overview
WARNING: 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 value
signal() returns the previous value of the signal handler. On failure, it returns SIG_ERR, and errno is set to indicate the error.
Errors
- EINVAL
-
signum is invalid.
Versions
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. The standard definition of signal() is:
typeof(void (int)) *signal(int signum, typeof(void (int)) *handler);
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.
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().
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 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.
Releases
19 releases since Debian 1.1 (Buzz); the text changed 18 times.