SIGACTION(2)

NAME
sigaction, sigprocmask, sigpending, sigsuspend - POSIX signal handling functions

SYNOPSIS
#include <signal.h>

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);

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The sa_restorer element is obsolete and should not be used.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

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:

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

    SA_ONESHOT or SA_RESETHAND
        Restore the signal action to the default state once the signal handler has been called.

    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_NOMASK or SA_NODEFER
        Do not prevent the signal from being received from within its own signal handler.

    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. 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. 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 | timer expired
SI_MESGQ | mesq state changed
SI_ASYNCIO | AIO completed
SI_SIGIO | queued SIGIO
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
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

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.

    SIG_BLOCK
        The set of blocked signals is the union of the current set and the set argument.

    SIG_UNBLOCK
        The signals in set are removed from the current set of blocked signals. It is legal to attempt to unblock a signal which is not blocked.

    SIG_SETMASK
        The set of blocked signals is set to the argument set.

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 VALUE
The functions sigaction, sigprocmask, and sigpending return 0 on success and -1 on error. The function sigsuspend always returns -1, normally with the error EINTR.

ERRORS

EFAULT
    act, oldact, set, oldset or mask point to memory which is not a valid part of the process address space.

EINTR
    System call was interrupted.

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.

NOTES
It 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 kill() or the raise() 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.3.3.1.3) 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 TO
POSIX, SVr4. SVr4 does not document the EINTR condition.

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), killpg(2), pause(2), sigaltstack(2), signal(2), sigvec(2), raise(3), siginterrupt(3), sigsetops(3), signal(7)