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NAMEsigaction, sigprocmask, sigpending, sigsuspend - POSIX signal handling functions
int sigaction(int signum, const struct sigaction *act, struct sigaction *oldact);
int sigprocmask(int how, const sigset_t *set, sigset_t *oldset);
int sigpending(sigset_t *set);
int sigsuspend(const sigset_t *mask);
DESCRIPTIONThe sigaction system call is used to change the action taken by a process on receipt of a specific signal.
signum specifies the signal and can be any valid signal except SIGKILL and SIGSTOP.
If act is non-null, the new action for signal signum is installed from act. If oldact is non-null, the previous action is saved in oldact.
The sigaction structure is defined as something like
On some architectures a union is involved - do not assign to both sa_handler and sa_sigaction.
The sa_restorer element is obsolete and should not be used. POSIX does not specify a sa_restorer element.
sa_handler specifies the action to be associated with signum and may be SIG_DFL for the default action, SIG_IGN to ignore this signal, or a pointer to a signal handling function. This function receives the signal number as its only argument.
sa_sigaction also specifies the action to be associated with signum. This function receives the signal number as its first argument, a pointer to a siginfo_t as its second argument and a pointer to a ucontext_t (cast to void *) as its third argument.
sa_mask gives a mask of signals which should be blocked during execution of the signal handler. In addition, the signal which triggered the handler will be blocked, unless the SA_NODEFER or SA_NOMASK flags are used.
sa_flags specifies a set of flags which modify the behaviour of the signal handling process. It is formed by the bitwise OR of zero or more of the following:
The siginfo_t parameter to sa_sigaction is a struct with the following elements
si_signo, si_errno and si_code are defined for all signals. The rest of the struct may be a union, so that one should only read the fields that are meaningful for the given signal. kill(2), POSIX.1b signals and SIGCHLD fill in si_pid and si_uid. SIGCHLDalsofillsin si_status, si_utime and si_stime. si_int and si_ptr are specified by the sender of the POSIX.1b signal. SIGILL, SIGFPE, SIGSEGV and SIGBUS fill in si_addr with the address of the fault. SIGPOLL fills in si_band and si_fd.
si_code indicates why this signal was sent. It is a value, not a bitmask. The values which are possible for any signal are listed in this table:
The sigprocmask call is used to change the list of currently blocked signals. The behaviour of the call is dependent on the value of how, as follows.
If oldset is non-null, the previous value of the signal mask is stored in oldset.
The sigpending call allows the examination of pending signals (ones which have been raised while blocked). The signal mask of pending signals is stored in set.
The sigsuspend call temporarily replaces the signal mask for the process with that given by mask and then suspends the process until a signal is received.
RETURN VALUEThe functions sigaction, sigprocmask, and sigpending return 0 on success and -1 on error. The function sigsuspend always returns -1, normally with the error EINTR.
NOTESIt is not possible to block SIGKILL or SIGSTOP with the sigprocmask call. Attempts to do so will be silently ignored.
According to POSIX, the behaviour of a process is undefined after it ignores a SIGFPE, SIGILL, or SIGSEGV signal that was not generated by the fIkill()fP or the fIraise()fP functions. 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.
POSIX (B.188.8.131.52) disallows setting the action for SIGCHLD to SIG_IGN. The BSD and SYSV behaviours differ, causing BSD software that sets the action for SIGCHLD to SIG_IGN to fail on Linux.
The POSIX spec only defines SA_NOCLDSTOP. Use of other sa_flags is non-portable.
The SA_RESETHAND flag is compatible with the SVr4 flag of the same name.
The SA_NODEFER flag is compatible with the SVr4 flag of the same name under kernels 1.3.9 and newer. On older kernels the Linux implementation allowed the receipt of any signal, not just the one we are installing (effectively overriding any sa_mask settings).
The SA_RESETHAND and SA_NODEFER names for SVr4 compatibility are present only in library versions 3.0.9 and greater.
The SA_SIGINFO flag is specified by POSIX.1b. Support for it was added in Linux 2.2.
sigaction can be called with a null second argument to query the current signal handler. It can also be used to check whether a given signal is valid for the current machine by calling it with null second and third arguments.
See sigsetops(3) for details on manipulating signal sets.
CONFORMING TOPOSIX, SVr4. SVr4 does not document the EINTR condition.
UNDOCUMENTEDBefore the introduction of SA_SIGINFO it was also possible to get some additional information, namely by using a sa_handler with second argument of type struct sigcontext. See the relevant kernel sources for details. This use is obsolete now.
SEE ALSOkill(1), kill(2), killpg(2), pause(2), sigaltstack(2), raise(3), siginterrupt(3), signal(2), signal(7), sigsetops(3), sigvec(2)