SIGACTION(2) BSD Programmer's Manual SIGACTION(2)
sigaction - software signal facilities
#include <signal.h> struct sigaction { union { /* signal handler */ void (*__sa_handler)(int); void (*__sa_sigaction)(int, siginfo_t *, void *); } __sigaction_u; sigset_t sa_mask; /* signal mask to apply */ int sa_flags; /* see signal options below */ }; #define sa_handler __sigaction_u.__sa_handler #define sa_sigaction __sigaction_u.__sa_sigaction int sigaction(int sig, const struct sigaction *act, struct sigaction *oact);
The system defines a set of signals that may be delivered to a process. Signal delivery resembles the occurrence of a hardware interrupt: the signal is normally blocked from further occurrence, the current process context is saved, and a new one is built. A process may specify a handler to which a signal is delivered, or specify that a signal is to be ignored. A process may also specify that a default action is to be taken by the system when a signal occurs. A signal may also be blocked, in which case its delivery is postponed until it is unblocked. The action to be taken on delivery is determined at the time of delivery. Normally, signal handlers execute on the current stack of the process. This may be changed, on a per-handler basis, so that signals are taken on a special signal stack. Signal routines normally execute with the signal that caused their invo- cation blocked, but other signals may yet occur. A global signal mask de- fines the set of signals currently blocked from delivery to a process. The signal mask for a process is initialized from that of its parent (normally empty). It may be changed with a sigprocmask(2) call, or when a signal is delivered to the process. When a signal condition arises for a process, the signal is added to a set of signals pending for the process. If the signal is not currently blocked by the process then it is delivered to the process. Signals may be delivered any time a process enters the operating system (e.g., during a system call, page fault or trap, or clock interrupt). If multiple sig- nals are ready to be delivered at the same time, any signals that could be caused by traps are delivered first. Additional signals may be pro- cessed at the same time, with each appearing to interrupt the handlers for the previous signals before their first instructions. The set of pending signals is returned by the sigpending(2) function. When a caught signal is delivered, the current state of the process is saved, a new signal mask is calculated (as described below), and the signal handler is invoked. The call to the handler is arranged so that if the signal han- dling routine returns normally the process will resume execution in the context from before the signal's delivery. If the process wishes to resume in a different context, then it must arrange to restore the previ- ous context itself. When a signal is delivered to a process a new signal mask is installed for the duration of the process' signal handler (or until a sigprocmask(2) call is made). This mask is formed by taking the union of the current signal mask set, the signal to be delivered, and the signal mask sa_mask associated with the handler to be invoked. sigaction() assigns an action for a signal specified by sig. If act is non-zero, it specifies an action (SIG_DFL, SIG_IGN, or a handler routine) and mask to be used when delivering the specified signal. If oact is non- zero, the previous handling information for the signal is returned to the user. Once a signal handler is installed, it normally remains installed until another sigaction() call is made, or an execve(2) is performed. The value of sa_handler (or, if the SA_SIGINFO flag is set, the value of sa_sigaction instead) indicates what action should be performed when a signal arrives. A signal-specific default action may be reset by setting sa_handler to SIG_DFL. Alternately, if the SA_RESETHAND flag is set the default action will be reinstated when the signal is first posted. The defaults are process termination, possibly with core dump; no action; stopping the process; or continuing the process. See the signal list below for each signal's default action. If sa_handler is SIG_DFL, the de- fault action for the signal is to discard the signal, and if a signal is pending, the pending signal is discarded even if the signal is masked. If sa_handler is set to SIG_IGN, current and pending instances of the signal are ignored and discarded. If sig is SIGCHLD and sa_handler is set to SIG_IGN, the SA_NOCLDWAIT flag (described below) is implied. Options may be specified by setting sa_flags. The meaning of the various bits is as follows: SA_NOCLDSTOP If this bit is set when installing a catching func- tion for the SIGCHLD signal, the SIGCHLD signal will be generated only when a child process exits, not when a child process stops. SA_NOCLDWAIT If this bit is set when calling sigaction() for the SIGCHLD signal, the system will not create zombie processes when children of the calling process exit. If the calling process subsequently issues a wait(2) (or equivalent), it blocks until all of the calling process's child processes terminate, and then returns a value of -1 with errno set to ECHILD. SA_ONSTACK If this bit is set, the system will deliver the signal to the process on a signal stack, specified with sigaltstack(2). SA_NODEFER If this bit is set, further occurrences of the delivered signal are not masked during the execu- tion of the handler. SA_RESETHAND If this bit is set, the handler is reset back to SIG_DFL at the moment the signal is delivered. SA_SIGINFO If this bit is set, the 2nd argument of the handler is set to be a pointer to a siginfo_t structure as described in <sys/siginfo.h>. The siginfo_t struc- ture is a part of IEEE Std 1003.1b ("POSIX.1b"). It provides much more information about the causes and attributes of the signal that is being delivered. SA_RESTART If a signal is caught during the system calls list- ed below, the call may be forced to terminate with the error EINTR, the call may return with a data transfer shorter than requested, or the call may be restarted. Restarting of pending calls is requested by setting the SA_RESTART bit in sa_flags. The af- fected system calls include read(2), write(2), sendto(2), recvfrom(2), sendmsg(2) and recvmsg(2) on a communications channel or a slow device (such as a terminal, but not a regular file) and during a wait(2) or ioctl(2). However, calls that have al- ready committed are not restarted, but instead re- turn a partial success (for example, a short read count). After a fork(2) or vfork(2), all signals, the signal mask, the signal stack, and the restart/interrupt flags are inherited by the child. execve(2) reinstates the default action for all signals which were caught and resets all signals to be caught on the user stack. Ignored signals remain ignored; the signal mask remains the same; signals that restart pending system calls continue to do so. The following is a list of all signals with names as in the include file <signal.h>: NAME Default Action Description SIGHUP terminate process terminal line hangup SIGINT terminate process interrupt program SIGQUIT create core image quit program SIGILL create core image illegal instruction SIGTRAP create core image trace trap SIGABRT create core image abort(3) call (formerly SIGIOT) SIGEMT create core image emulate instruction executed SIGFPE create core image floating-point exception SIGKILL terminate process kill program (cannot be caught or ignored) SIGBUS create core image bus error SIGSEGV create core image segmentation violation SIGSYS create core image system call given invalid argument SIGPIPE terminate process write on a pipe with no reader SIGALRM terminate process real-time timer expired SIGTERM terminate process software termination signal SIGURG discard signal urgent condition present on socket SIGSTOP stop process stop (cannot be caught or ignored) SIGTSTP stop process stop signal generated from keyboard SIGCONT discard signal continue after stop SIGCHLD discard signal child status has changed SIGTTIN stop process background read attempted from control terminal SIGTTOU stop process background write attempted to control terminal SIGIO discard signal I/O is possible on a descriptor (see fcntl(2)) SIGXCPU terminate process CPU time limit exceeded (see setrlimit(2)) SIGXFSZ terminate process file size limit exceeded (see setrlimit(2)) SIGVTALRM terminate process virtual time alarm (see setitimer(2)) SIGPROF terminate process profiling timer alarm (see setitimer(2)) SIGWINCH discard signal window size change SIGINFO discard signal status request from keyboard SIGUSR1 terminate process user defined signal 1 SIGUSR2 terminate process user defined signal 2
The sa_mask field specified in act is not allowed to block SIGKILL or SIGSTOP. Any attempt to do so will be silently ignored. The following functions are either reentrant or not interruptible by sig- nals and are async-signal safe. Therefore applications may invoke them, without restriction, from signal-catching functions: _exit(2), access(2), alarm(3), cfgetispeed(3), cfgetospeed(3), cfsetispeed(3), cfsetospeed(3), chdir(2), chmod(2), chown(2), close(2), creat(3), dup(2), dup2(2), execle(3), execve(2), fcntl(2), fork(2), fpathconf(2), fstat(2), fsync(2), getegid(2), geteuid(2), getgid(2), getgroups(2), getpgrp(2), getpid(2), getppid(2), getuid(2), kill(2), link(2), lseek(2), mkdir(2), mkfifo(2), open(2), pathconf(2), pause(3), pipe(2), raise(3), read(2), readv(2), rename(2), rmdir(2), setgid(2), setpgid(2), setsid(2), setuid(2), sigaction(2), sigaddset(3), sigdelset(3), sigemptyset(3), sigfillset(3), sigismember(3), signal(3), sigpause(3), sigpending(2), sigprocmask(2), sigsuspend(2), sleep(3), stat(2), sysconf(3), tcdrain(3), tcflow(3), tcflush(3), tcgetattr(3), tcgetpgrp(3), tcsendbreak(3), tcsetattr(3), tcsetpgrp(3), time(3), times(3), umask(2), uname(3), unlink(2), utime(3), wait(2), waitpid(2), write(2), writev(2). Please see signal(3) for a more detailed list. All functions not in the above list are considered to be unsafe with respect to signals. That is to say, the behaviour of such functions when called from a signal handler is undefined. In general though, signal handlers should do little more than set a flag; most other actions are not safe. Additionally, it is advised that signal handlers guard against modifica- tion of the external symbol errno by the above functions, saving it at entry and restoring it on return, thus: void handler(sig) { int save_errno = errno; ... errno = save_errno; }
A 0 value indicates that the call succeeded. A -1 return value indicates an error occurred and errno is set to indicate the reason.
The handler routine can be declared: void handler(sig) int sig; If the SA_SIGINFO option is enabled, the canonical way to declare it is: void handler(sig, sip, scp) int sig; siginfo_t *sip; struct sigcontext *scp; Here sig is the signal number, into which the hardware faults and traps are mapped. If the SA_SIGINFO option is set, sip is a pointer to a siginfo_t as described in <sys/siginfo.h>. If SA_SIGINFO is not set, this pointer will be NULL instead. The function specified in sa_sigaction will be called instead of the function specified by sa_handler (Note that in some implementations these are in fact the same). scp is a pointer to the sigcontext structure (defined in <signal.h>), used to restore the context from before the signal.
sigaction() will fail and no new signal handler will be installed if one of the following occurs: [EFAULT] Either act or oact points to memory that is not a valid part of the process address space. [EINVAL] sig is not a valid signal number. [EINVAL] An attempt is made to ignore or supply a handler for SIGKILL or SIGSTOP.
kill(1), kill(2), ptrace(2), sigaltstack(2), sigprocmask(2), sigsuspend(2), wait(2), setjmp(3), sigblock(3), sigpause(3), sigsetops(3), sigvec(3), tty(4)
The sigaction() function conforms to IEEE Std 1003.1-1990 ("POSIX.1"). The SA_ONSTACK and SA_RESTART flags are Berkeley extensions, as are the signals SIGTRAP, SIGEMT, SIGBUS, SIGSYS, SIGURG, SIGIO, SIGXCPU, SIGXFSZ, SIGVTALRM, SIGPROF, SIGWINCH, and SIGINFO. These signals are available on most BSD-derived systems. The SA_NODEFER and SA_RESETHAND flags are in- tended for backwards compatibility with other operating systems. The SA_NOCLDSTOP, SA_NOCLDWAIT, and SA_SIGINFO flags are options commonly found in other operating systems. The following functions are either reentrant or not interruptible by signals and are async-signal safe. Therefore applications may invoke them, without restriction, from signal- catching functions: Base Interfaces: _exit(), access(), alarm(), cfgetispeed(), cfgetospeed(), cfsetispeed(), cfsetospeed(), chdir(), chmod(), chown(), close(), creat(), dup(), dup2(), execle(), execve(), fcntl(), fork(), fpathconf(), fstat(), fsync(), getegid(), geteuid(), getgid(), getgroups(), getpgrp(), get- pid(), getppid(), getuid(), kill(), link(), lseek(), mkdir(), mkfifo(), open(), pathconf(), pause(), pipe(), raise(), read(), rename(), rmdir(), setgid(), setpgid(), setsid(), setuid(), sigaction(), sigaddset(), sig- delset(), sigemptyset(), sigfillset(), sigismember(), signal(), sigpend- ing(), sigprocmask(), sigsuspend(), sleep(), stat(), sysconf(), tcdrain(), tcflow(), tcflush(), tcgetattr(), tcgetpgrp(), tcsendbreak(), tcsetattr(), tcsetpgrp(), time(), times(), umask(), uname(), unlink(), utime(), wait(), waitpid(), write(). ANSI C Interfaces: strcat(), strcpy(), strncat(), strncpy(), and perhaps some others. Extension Interfaces: strlcat(), strlcpy(). Most functions not in the above lists are considered to be unsafe with respect to signals. That is to say, the behaviour of such functions when called from a signal handler is undefined. Additionally, inside the signal handler it is also considered safer to make a copy of the global variable errno and restore it before returning from the signal handler. A few other functions are signal race safe in OpenBSD but probably not on other systems: snprintf() Safe. vsnprintf() Safe. syslog_r() Safe if the syslog_data struct is initialized as a local variable. MirBSD #10-current December 5, 2021 5