SIGACTION(2)

NAME
sigaction - examine and change a signal action

SYNOPSIS
#include <signal.h>

int sigaction(int signum, const struct sigaction *act, struct sigaction *oldact);

DESCRIPTION
The 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

    struct sigaction {
        void (*sa_handler)(int);
        void (*sa_sigaction)(int, siginfo_t *, void *);
        sigset_t sa_mask;
        int sa_flags;
        void (*sa_restorer)(void);
    }

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The sa_restorer element is obsolete and should not be used.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.

If SA_SIGINFO is specified in sa_flags, then sa_sigaction (instead of sa_handler) specifies the signal-handling function for 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 flag is 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:

    SA_NOCLDSTOP
        If signum is SIGCHLD, do not receive notification when child processes stop (i.e., when they receive one of SIGSTOP, SIGTSTP, SIGTTIN or SIGTTOU) or resume (i.e., they receive SIGCONT) (see wait(2)).

    SA_NOCLDWAIT
        (Linux 2.6 and later) If signum is SIGCHLD, do not transform children into zombies when they terminate. See also waitpid(2).

    SA_RESETHAND
        Restore the signal action to the default state once the signal handler has been called. SA_ONESHOT is an obsolete, non-standard synonym for this flag.

    SA_ONSTACK
        Call the signal handler on an alternate signal stack provided by sigaltstack(2). If an alternate stack is not available, the default stack will be used.

    SA_RESTART
        Provide behaviour compatible with BSD signal semantics by making certain system calls restartable across signals.

    SA_NODEFER
        Do not prevent the signal from being received from within its own signal handler. SA_NOMASK is an obsolete, non-standard synonym for this flag.

    SA_SIGINFO
        The signal handler takes 3 arguments, not one. In this case, sa_sigaction should be set instead of sa_handler. (The sa_sigaction field was added in Linux 2.1.86.)

The siginfo_t parameter to sa_sigaction is a struct with the following elements

    siginfo_t {
    	int	si_signo;	/* Signal number */
    	int	si_errno;	/* An errno value */
    	int	si_code;	/* Signal code */
    	pid_t	si_pid;	/* Sending process ID */
    	uid_t	si_uid;	/* Real user ID of sending process */
    	int	si_status;	/* Exit value or signal */
    	clock_t	si_utime;	/* User time consumed */
    	clock_t	si_stime;	/* System time consumed */
    	sigval_t	si_value;	/* Signal value */
    	int	si_int;	/* POSIX.1b signal */
    	void *	si_ptr;	/* POSIX.1b signal */
    	void *	si_addr;	/* Memory location which caused fault */
    	int	si_band;	/* Band event */
    	int	si_fd;	/* File descriptor */
    }
si_signo, si_errno and si_code are defined for all signals. (si_signo is unused on Linux.) The rest of the struct may be a union, so that one should only read the fields that are meaningful for the given signal. POSIX.1b signals and SIGCHLD fill in si_pid and si_uid. SIGCHLD also fills in 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:
si_code
Value | Signal origin
SI_USER | kill(), sigsend(), or raise()
SI_KERNEL | The kernel
SI_QUEUE | sigqueue()
SI_TIMER | POSIX timer expired
SI_MESGQ | POSIX message queue state changed (since Linux 2.6.6)
SI_ASYNCIO | AIO completed
SI_SIGIO | queued SIGIO
SI_TKILL | tkill() or tgkill() (since Linux 2.4.19)
SIGILL
ILL_ILLOPC | illegal opcode
ILL_ILLOPN | illegal operand
ILL_ILLADR | illegal addressing mode
ILL_ILLTRP | illegal trap
ILL_PRVOPC | privileged opcode
ILL_PRVREG | privileged register
ILL_COPROC | coprocessor error
ILL_BADSTK | internal stack error
SIGFPE
FPE_INTDIV | integer divide by zero
FPE_INTOVF | integer overflow
FPE_FLTDIV | floating point divide by zero
FPE_FLTOVF | floating point overflow
FPE_FLTUND | floating point underflow
FPE_FLTRES | floating point inexact result
FPE_FLTINV | floating point invalid operation
FPE_FLTSUB | subscript out of range
SIGSEGV
SEGV_MAPERR | address not mapped to object
SEGV_ACCERR | invalid permissions for mapped object
SIGBUS
BUS_ADRALN | invalid address alignment
BUS_ADRERR | non-existent physical address
BUS_OBJERR | object specific hardware error
SIGTRAP
TRAP_BRKPT | process breakpoint
TRAP_TRACE | process trace trap
SIGCHLD
CLD_EXITED | child has exited
CLD_KILLED | child was killed
CLD_DUMPED | child terminated abnormally
CLD_TRAPPED | traced child has trapped
CLD_STOPPED | child has stopped
CLD_CONTINUED | stopped child has continued (since Linux 2.6.9)
SIGPOLL
POLL_IN | data input available
POLL_OUT | output buffers available
POLL_MSG | input message available
POLL_ERR | i/o error
POLL_PRI | high priority input available
POLL_HUP | device disconnected

RETURN VALUE
sigaction() returns 0 on success and -1 on error.

ERRORS

EFAULT
    act or oldact points to memory which is not a valid part of the process address space.

EINVAL
    An invalid signal was specified. This will also be generated if an attempt is made to change the action for SIGKILL or SIGSTOP, which cannot be caught or ignored.

NOTES

According to POSIX, the behaviour of a process is undefined after it ignores a SIGFPE, SIGILL, or SIGSEGV signal that was not generated by kill() or raise(). 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.1-1990 disallowed setting the action for SIGCHLD to SIG_IGN. POSIX.1-2001 allows this possibility, so that ignoring SIGCHLD can be used to prevent the creation of zombies (see wait(2)). Nevertheless, the historical BSD and System V behaviours for ignoring SIGCHLD differ, so that the only completely portable method of ensuring that terminated children do not become zombies is to catch the SIGCHLD signal and perform a wait(2) or similar.

POSIX.1-1990 only specified SA_NOCLDSTOP. POSIX.1-2001 added SA_NOCLDWAIT, SA_RESETHAND, SA_NODEFER, and SA_SIGINFO. Use of these latter values in sa_flags may be less portable in applications intended for older Unix implementations.

Support for SA_SIGINFO was added in Linux 2.2.

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).

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.

It is not possible to block SIGKILL or SIGSTOP (by specifying them in sa_mask). Attempts to do so are silently ignored.

See sigsetops(3) for details on manipulating signal sets.

BUGS
In kernels up to and including 2.6.13, specifying SA_NODEFER in sa_flags preventing not only the delivered signal from being masked during execution of the handler, but also the signals specified in sa_mask. This bug is was fixed in kernel 2.6.14.

CONFORMING TO
POSIX.1-2001, SVr4.

UNDOCUMENTED
Before 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 ALSO
kill(1), kill(2), pause(2), sigaltstack(2), signal(2), sigpending(2), sigprocmask(2), sigqueue(2), sigsuspend(2), wait(2), killpg(3), raise(3), siginterrupt(3), sigsetops(3), sigvec(3), core(5), signal(7)