SIGACTION(2) Linux Programmers Manual SIGACTION(2) NAME sigaction - examine and change a signal action SYNOPSIS #include 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 The sa_restorer element is obsolete and should not be used.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 sig nal 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 execu tion 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 sig nal. 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)). This flag is only meaningful when establishing a handler for SIGCHLD. SA_NOCLDWAIT (Since Linux 2.6) If signum is SIGCHLD, do not transform children into zombies when they terminate. See also waitpid(2). This flag is only meaningful when establishing a handler for SIGCHLD, or when setting that signals disposition to SIG_DFL. If the SA_NOCLDWAIT flag is set when establishing a handler for SIGCHLD, POSIX.1 leaves it unspecified whether a SIGCHLD signal is generated when a child process terminates. On Linux, a SIGCHLD signal is generated in this case; on some other implementations, it is not. SA_NODEFER Do not prevent the signal from being received from within its own signal handler. This flag is only meaningful when establishing a signal handler. SA_NOMASK is an obsolete, non-standard synonym for this flag. SA_ONSTACK Call the signal handler on an alternate signal stack pro vided by sigaltstack(2). If an alternate stack is not available, the default stack will be used. This flag is only meaningful when establishing a signal handler. SA_RESETHAND Restore the signal action to the default state once the sig nal handler has been called. This flag is only meaningful when establishing a signal handler. SA_ONESHOT is an obso lete, non-standard synonym for this flag. SA_RESTART Provide behavior compatible with BSD signal semantics by making certain system calls restartable across signals. This flag is only meaningful when establishing a signal han dler. See signal(7) for a discussion of system call restarting. SA_SIGINFO (since Linux 2.2) The signal handler takes 3 arguments, not one. In this case, sa_sigaction should be set instead of sa_handler. This flag is only meaningful when establishing a signal han dler. The siginfo_t argument 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 */ int si_trapno; /* Trap number that caused hardware-generated signal (unused on most architectures) */ 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 */ int si_overrun; /* Timer overrun count; POSIX.1b timers */ int si_timerid; /* Timer ID; POSIX.1b timers */ 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 generally 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. * POSIX.1b timers (since Linux 2.6) fill in si_overrun and si_timerid. The si_timerid field is an internal ID used by the kernel to identify the timer; it is not the same as the timer ID returned by timer_cre ate(3). * SIGCHLD 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 which can be placed in si_code for any signal, along with reason that the signal was gener ated. 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 placed 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 placed 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 placed 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 placed in si_code for a SIGBUS signal: BUS_ADRALN invalid address alignment BUS_ADRERR nonexistent physical address BUS_OBJERR object-specific hardware error The following values can be placed in si_code for a SIGTRAP signal: TRAP_BRKPT process breakpoint TRAP_TRACE process trace trap The following values can be placed 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 placed 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 per form a wait(2) or similar. POSIX.1-1990 only specified SA_NOCLDSTOP. POSIX.1-2001 added SA_NOCLD WAIT, 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. 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 implementa tion 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 cur rent signal handler. It can also be used to check whether a given sig nal 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 argu ment 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 dur ing 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), sig nalfd(2), sigpending(2), sigprocmask(2), sigqueue(2), sigsuspend(2), wait(2), raise(3), siginterrupt(3), sigsetops(3), sigvec(3), core(5), signal(7) COLOPHON This page is part of release 3.05 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/. Linux 2008-07-08 SIGACTION(2)