NAME
ssiiggaaccttiioonn - software signal facilities
LLIIBBRRAARRYYStandard C Library (libc, -lc)
SYNOPSIS
##iinncclluuddee <
> struct sigaction {
union { void (*sahandler)(int);void (*sasigaction)(int, struct siginfo *, void *);
} sigactionu; /* signal handler */
int saflags; /* see signal options below */ sigsett samask; /* signal mask to apply */ };#define sahandler sigactionu.sahandler
#define sasigaction sigactionu.sasigaction
intssiiggaaccttiioonn(int sig, const struct sigaction *restrict act,
struct sigaction *restrict oact);
DESCRIPTION
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 bechanged, 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 defines 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., duringa 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 couldbe 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) system call. When a caught signal is delivered, the current state of the process is saved, anew signal mask is calculated (as described below), and the signal han-
dler is invoked. The call to the handler is arranged so that if the sig-
nal handling 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 previous 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) system 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 associated with the handler to be invoked. The ssiiggaaccttiioonn() system call assigns an action for a signal specified bysig. If act is non-zero, it specifies an action (SIGDFL, SIGIGN, or a
handler routine) and mask to be used when delivering the specified sig-
nal. If oact is non-zero, the previous handling information for the sig-
nal is returned to the user. Once a signal handler is installed, it normally remains installed until another ssiiggaaccttiioonn() system call is made, or an execve(2) is performed. Asignal-specific default action may be reset by setting sahandler to
SIGDFL. The defaults are process termination, possibly with core dump;no action; stopping the process; or continuing the process. See the sig-
nal list below for each signal's default action. If sahandler is SIGDFL, the default action for the signal is to discard the signal, andif a signal is pending, the pending signal is discarded even if the sig-
nal is masked. If sahandler is set to SIGIGN current and pending instances of the signal are ignored and discarded. Options may be specified by setting saflags. The meaning of the various bits is as follows:SANOCLDSTOP 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. SANOCLDWAIT If this bit is set when calling ssiiggaaccttiioonn() 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, andthen returns a value of -1 with errno set to
ECHILD. SAONSTACK If this bit is set, the system will deliver the signal to the process on a signal stack, specified with sigaltstack(2). SANODEFER If this bit is set, further occurrences of thedelivered signal are not masked during the execu-
tion of the handler. SARESETHAND If this bit is set, the handler is reset back to SIGDFL at the moment the signal is delivered. SARESTART See paragraph below. SASIGINFO If this bit is set, the handler function is assumedto be pointed to by the sasigaction member of
struct sigaction and should match the prototype
shown above or as below in EXAMPLES. This bit
should not be set when assigning SIGDFL or SIGIGN. 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 SARESTART bit in saflags. 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 fork(2) or vfork(2) all signals, the signal mask, the signal stack, and the restart/interrupt flags are inherited by the child. 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: 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 executedSIGFPE create core image floating-point exception
SIGKILL terminate process kill program SIGBUS create core image bus error SIGSEGV create core image segmentation violationSIGSYS create core image non-existent system call invoked
SIGPIPE terminate process write on a pipe with no readerSIGALRM 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 NNOOTTEE The samask 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:
Base Interfaces: eexxiitt(), aacccceessss(), aallaarrmm(), ccffggeettiissppeeeedd(), ccffggeettoossppeeeedd(), ccffsseettiissppeeeedd(), ccffsseettoossppeeeedd(), cchhddiirr(), cchhmmoodd(), cchhoowwnn(), cclloossee(), ccrreeaatt(), dduupp(), dduupp22(), eexxeeccllee(), eexxeeccvvee(), ffccnnttll(), ffoorrkk(), ffppaatthhccoonnff(), ffssttaatt(), ffssyynncc(), ggeetteeggiidd(), ggeetteeuuiidd(), ggeettggiidd(), ggeettggrroouuppss(), ggeettppggrrpp(), ggeettppiidd(), ggeettppppiidd(), ggeettuuiidd(), kkiillll(), lliinnkk(), llsseeeekk(), mmkkddiirr(), mmkkffiiffoo(), ooppeenn(), ppaatthhccoonnff(), ppaauussee(), ppiippee(), rraaiissee(), rreeaadd(), rreennaammee(), rrmmddiirr(), sseettggiidd(), sseettppggiidd(), sseettssiidd(), sseettuuiidd(), ssiiggaaccttiioonn(), ssiiggaaddddsseett(), ssiiggddeellsseett(), ssiiggeemmppttyysseett(), ssiiggffiillllsseett(), ssiiggiissmmeemmbbeerr(), ssiiggnnaall(), ssiiggppeennddiinngg(), ssiiggpprrooccmmaasskk(), ssiiggssuussppeenndd(), sslleeeepp(), ssttaatt(), ssyyssccoonnff(), ttccddrraaiinn(), ttccffllooww(), ttccfflluusshh(), ttccggeettaattttrr(), ttccggeettppggrrpp(), ttccsseennddbbrreeaakk(), ttccsseettaattttrr(), ttccsseettppggrrpp(), ttiimmee(), ttiimmeess(), uummaasskk(), uunnaammee(), uunnlliinnkk(), uuttiimmee(), wwaaiitt(), wwaaiittppiidd(), wwrriittee(). Realtime Interfaces: aaiiooeerrrroorr(), cclloocckkggeettttiimmee(), ssiiggppaauussee(), ttiimmeerrggeettoovveerrrruunn(), aaiioorreettuurrnn(), ffddaattaassyynncc(), ssiiggqquueeuuee(), ttiimmeerrggeettttiimmee(), aaiioossuussppeenndd(), sseemmppoosstt(), ssiiggsseett(), ttiimmeerrsseettttiimmee(). ANSI C Interfaces: ssttrrccppyy(), ssttrrccaatt(), ssttrrnnccppyy(), ssttrrnnccaatt(), and perhaps some others. Extension Interfaces: ssttrrllccppyy(), ssttrrllccaatt(). 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 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. 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.RETURN VALUES
The ssiiggaaccttiioonn() function returns the value 0 if successful; otherwise thevalue -1 is returned and the global variable errno is set to indicate the
error. EEXXAAMMPPLLEESS There are three possible prototypes the handler may match: ANSI C: void hhaannddlleerr(int); POSIX SASIGINFO: void hhaannddlleerr(int, siginfot *info, ucontextt *uap);The handler function should match the SASIGINFO prototype if the SASIG-
INFO bit is set in flags. It then should be pointed to by thesasigaction member of struct sigaction. Note that you should not assign
SIGDFL or SIGIGN this way. If the SASIGINFO flag is not set, the handler function should match either the ANSI C or traditional BSD prototype and be pointed to by thesahandler member of struct sigaction. In practice, FreeBSD always sends
the three arguments of the latter and since the ANSI C prototype is a subset, both will work. The sahandler member declaration in FreeBSD include files is that of ANSI C (as required by POSIX), so a functionpointer of a BSD-style function needs to be casted to compile without
warning. The traditional BSD style is not portable and since its capa-
bilities are a full subset of a SASIGINFO handler, its use is depre-
cated. The sig argument is the signal number, one of the SIG... values from. The code argument of the BSD-style handler and the sicode member of the
info argument to a SASIGINFO handler contain a numeric code explainingthe cause of the signal, usually one of the SI... values from
nal.h> or codes specific to a signal, i.e. one of the FPE... values for SIGFPE. The uap argument to a POSIX SASIGINFO handler points to an instance of ucontextt. EERRRROORRSS The ssiiggaaccttiioonn() system call 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] The sig argument is not a valid signal number. [EINVAL] An attempt is made to ignore or supply a handler for SIGKILL or SIGSTOP. [EINVAL] An attempt was made to set the action to SIGDFL for a signal that cannot be caught or ignored (or both). STANDARDS The ssiiggaaccttiioonn() system call is expected to conform to ISO/IEC 9945-1:1990
(``POSIX.1''). The SAONSTACK and SARESTART flags are Berkeley exten-
sions, as are the signals, SIGTRAP, SIGEMT, SIGBUS, SIGSYS, SIGURG, SIGIO, SIGXCPU, SIGXFSZ, SIGVTALRM, SIGPROF, SIGWINCH, and SIGINFO.Those signals are available on most BSD-derived systems. The SANODEFER
and SARESETHAND flags are intended for backwards compatibility with other operating systems. The SANOCLDSTOP, and SANOCLDWAIT flags are featuring options commonly found in other operating systems.SEE ALSO
kill(1), kill(2), ptrace(2), sigaltstack(2), sigblock(2), sigpause(2), sigpending(2), sigprocmask(2), sigsetmask(2), sigsuspend(2), sigvec(2), wait(2), fpsetmask(3), setjmp(3), siginterrupt(3), sigsetops(3), ucontext(3), tty(4) BSD April 3, 1994 BSD