|SA_NOCLDSTOP||If this bit is set when installing a catching function 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 processs child processes terminate, and then returns a value of -1 with errno set to ECHILD. The same effect of avoiding zombie creation can also be achieved by setting sa_handler for SIGCHLD to SIG_IGN.|
|SA_ONSTACK||If this bit is set, the system will deliver the signal to the process on a signal stack, specified by each thread with sigaltstack(2).|
|SA_NODEFER||If this bit is set, further occurrences of the delivered signal are not masked during the execution 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_RESTART||See paragraph below.|
If this bit is set, the handler function is assumed to be pointed to by the
.Vt struct sigaction and should match the prototype shown above or as below in EXAMPLES. This bit should not be set when assigning SIG_DFL or SIG_IGN.
If a signal is caught during the system calls listed 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. Restart of pending calls is requested by setting the SA_RESTART bit in sa_flags. The affected system calls include open(2), 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 already committed are not restarted, but instead return a partial success (for example, a short read count).
After a pthread_create(3) the signal mask is inherited by the new thread and the set of pending signals and the signal stack for the new thread are empty.
The execve(2) system call 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
.In 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
SIGBUS create core image bus error
SIGSEGV create core image segmentation violation
SIGSYS create core image non-existent system call invoked
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 signals and are async-signal safe. Therefore applications may invoke them, without restriction, from signal-catching functions or from a child process after calling fork(2) in a multi-threaded process:
_Exit, _exit, accept, access, alarm, bind, cfgetispeed, cfgetospeed, cfsetispeed, cfsetospeed, chdir, chmod, chown, close, connect, creat, dup, dup2, execl, execle, execv, execve, faccessat, fchdir, fchmod, fchmodat, fchown, fchownat, fcntl, fork, fstat, fstatat, fsync, ftruncate, getegid, geteuid, getgid, getgroups, getpeername, getpgrp, getpid, getppid, getsockname, getsockopt, getuid, kill, link, linkat, listen, lseek, lstat, mkdir, mkdirat, mkfifo, mkfifoat, mknod, mknodat, open, openat, pause, pipe, poll, pselect, pthread_sigmask, raise, read, readlink, readlinkat, recv, recvfrom, recvmsg, rename, renameat, rmdir, select, send, sendmsg, sendto, setgid, setpgid, setsid, setsockopt, setuid, shutdown, sigaction, sigaddset, sigdelset, sigemptyset, sigfillset, sigismember, signal, sigpending, sigprocmask, sigsuspend, sleep, sockatmark, socket, socketpair, stat, symlink, symlinkat, tcdrain, tcflow, tcflush, tcgetattr, tcgetpgrp, tcsendbreak, tcsetattr, tcsetpgrp, time, times, umask, uname, unlink, unlinkat, utime, wait, waitpid, write.
X/Open Systems Interfaces:
sigpause, sigset, utimes.
aio_error, clock_gettime, timer_getoverrun, aio_return, fdatasync, sigqueue, timer_gettime, aio_suspend, sem_post, timer_settime.
Base Interfaces not specified as async-signal safe by POSIX:
fpathconf, pathconf, sysconf.
Base Interfaces not specified as async-signal safe by POSIX, but planned to be:
ffs, htonl, htons, memccpy, memchr, memcmp, memcpy, memmove, memset, ntohl, ntohs, stpcpy, stpncpy, strcat, strchr, strcmp, strcpy, strcspn, strlen, strncat, strncmp, strncpy, strnlen, strpbrk, strrchr, strspn, strstr, strtok_r, wcpcpy, wcpncpy, wcscat, wcschr, wcscmp, wcscpy, wcscspn, wcslen, wcsncat, wcsncmp, wcsncpy, wcsnlen, wcspbrk, wcsrchr, wcsspn, wcsstr, wcstok, wmemchr, wmemcmp, wmemcpy, wmemmove, wmemset.
accept4, bindat, closefrom, connectat, eaccess, ffsl, ffsll, flock, fls, flsl, flsll, futimesat, pipe2, strlcat. strlcpy, strsep.
In addition, reading or writing errno is async-signal safe.
All functions not in the above lists are considered to be unsafe with respect to signals. That is to say, the behaviour of such functions is undefined when they are called from a signal handler that interrupted an unsafe function. In general though, signal handlers should do little more than set a flag; most other actions are not safe.
Also, it is good practice to make a copy of the global variable errno and restore it before returning from the signal handler. This protects against the side effect of errno being set by functions called from inside the signal handler.
.Rv -std sigaction
There are three possible prototypes the handler may match:
ANSI C: void handler int; Traditional BSD style: void handler int int code struct sigcontext *scp; POSIX SA_SIGINFO: void handler int siginfo_t *info ucontext_t *uap;
The handler function should match the SA_SIGINFO prototype if the SA_SIGINFO bit is set in sa_flags. It then should be pointed to by the sa_sigaction member of
.Vt struct sigaction . Note that you should not assign SIG_DFL or SIG_IGN this way.
If the SA_SIGINFO flag is not set, the handler function should match either the ANSI C or traditional BSD prototype and be pointed to by the sa_handler member of
.Vt struct sigaction . In practice,
.Fx always sends the three arguments of the latter and since the ANSI C prototype is a subset, both will work. The sa_handler member declaration in
.Fx include files is that of ANSI C (as required by POSIX), so a function pointer of a BSD Ns -style function needs to be casted to compile without warning. The traditional BSD style is not portable and since its capabilities are a full subset of a SA_SIGINFO handler, its use is deprecated.
The sig argument is the signal number, one of the SIG... values from
.In signal.h .
The code argument of the BSD Ns -style handler and the si_code member of the info argument to a SA_SIGINFO handler contain a numeric code explaining the cause of the signal, usually one of the SI_... values from
.In sys/signal.h or codes specific to a signal, i.e., one of the FPE_... values for SIGFPE.
The scp argument to a BSD Ns -style handler points to an instance of
.Vt struct sigcontext .
The uap argument to a POSIX SA_SIGINFO handler points to an instance of ucontext_t.
The sigaction system call will fail and no new signal handler will be installed if one of the following occurs:
[EINVAL] The sig argument 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), sigpending(2), sigprocmask(2), sigsuspend(2), wait(2), fpsetmask(3), setjmp(3), siginfo(3), siginterrupt(3), sigsetops(3), ucontext(3), tty(4)
The sigaction system call is expected to conform to -p1003.1-90. 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. Those signals are available on most BSD Ns -derived systems. The SA_NODEFER and SA_RESETHAND flags are intended for backwards compatibility with other operating systems. The SA_NOCLDSTOP, and SA_NOCLDWAIT flags are featuring options commonly found in other operating systems. The flags are approved by -susv2, along with the option to avoid zombie creation by ignoring SIGCHLD.