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The sa_restorer element is obsolete and should not be used.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 behavior 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 behavior 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_errno 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. See sigqueue(2) for more details.

*   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 is a value (not a bit mask) indicating why this signal was sent. The following list shows the values can be placed in si_code for any signal, along with reason that the signal was generated.

    SI_USER
        kill(2) or raise(3)

    SI_KERNEL
        Sent by the kernel.

    SI_QUEUE
        sigqueue(2)

    SI_TIMER
        POSIX timer expired

    SI_MESGQ
        POSIX message queue state changed (since Linux 2.6.6); see mq_notify(3)

    SI_ASYNCIO
        AIO completed

    SI_SIGIO
        queued SIGIO

    SI_TKILL
        tkill(2) or tgkill(2) (since Linux 2.4.19)

The following values can be place in si_code for a SIGILL signal:

    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

The following values can be place in si_code for a SIGFPE signal:

    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

The following values can be place in si_code for a SIGSEGV signal:

    SEGV_MAPERR
        address not mapped to object

    SEGV_ACCERR
        invalid permissions for mapped object

The following values can be place in si_code for a SIGBUS signal:

    BUS_ADRALN
        invalid address alignment

    BUS_ADRERR
        non-existent physical address

    BUS_OBJERR
        object specific hardware error

The following values can be place in si_code for a SIGTRAP signal:

    TRAP_BRKPT
        process breakpoint

    TRAP_TRACE
        process trace trap

The following values can be place in si_code for a SIGCHLD signal:

    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)

The following values can be place in si_code for a SIGPOLL signal:

    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.

CONFORMING TO
POSIX.1-2001, SVr4.

NOTES

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.

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 behaviors 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.

See signal(7) for a list of the async-signal-safe functions that can be safely called inside from inside a signal handler.

  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.

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

EXAMPLE
See mprotect(2).

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