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%sccs.include.redist.man%
@(#)sigaction.2 6.1 (Berkeley) 07/01/90
#include <signal.h>struct sigaction { void (*sa_handler)(); sigset_t sa_mask; int sa_flags; };
sigaction(sig, act, oact) int sig; struct sigaction *act, *oact;
Signal routines execute with the signal that caused their invocation 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., during a system call, page fault or trap, or clock interrupt). If multiple signals are ready to be delivered at the same time, any signals that could be caused by traps are delivered first. Additional signals may be processed 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 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 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.
Sigaction assigns an action for a specific signal. 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 remains installed until another sigaction call is made, or an execve (2) is performed. The default action for a signal may be reinstated by setting sa_handler to SIG_DFL. The default actions are termination, possibly with a core image; 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_IGN the signal is subsequently ignored, and pending instances of the signal are discarded.
Options may be specified by setting sa_flags . If the SA_NOCLDSTOP 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. Further, if the SA_ONSTACK bit is set in sa_flags, the system will deliver the signal to the process on a "signal stack" , specified with sigstack (2).
If a caught signal occurs during certain system calls, the call may be forced to terminate prematurely with an EINTR error return, 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 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) the child inherits all signals, the signal mask, the signal stack, and the restart/interrupt flags.
Execve (2) resets all caught signals to default action 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 >:
SIGHUP 1 hangup SIGINT 2 interrupt SIGQUIT 3* quit SIGILL 4* illegal instruction SIGTRAP 5*\*p trace trap SIGABRT 6* abort() call (formerly SIGIOT) SIGEMT 7*\*p EMT instruction SIGFPE 8* floating point exception SIGKILL 9 kill (cannot be caught, blocked, or ignored) SIGBUS 10*\*p bus error SIGSEGV 11* segmentation violation SIGSYS 12*\*p bad argument to system call SIGPIPE 13 write on a pipe with no one to read it SIGALRM 14 alarm clock SIGTERM 15 software termination signal SIGURG 16\*b\*p urgent condition present on socket SIGSTOP 17\*d stop (cannot be caught, blocked, or ignored) SIGTSTP 18\*d stop signal generated from keyboard SIGCONT 19\*b continue after stop SIGCHLD 20\*b child status has changed SIGTTIN 21\*d background read attempted from control terminal SIGTTOU 22\*d background write attempted to control terminal SIGIO 23\*b\*p i/o is possible on a descriptor (see fcntl(2)) SIGXCPU 24\*p cpu time limit exceeded (see setrlimit(2)) SIGXFSZ 25\*p file size limit exceeded (see setrlimit(2)) SIGVTALRM 26\*p virtual time alarm (see setitimer(2)) SIGPROF 27\*p profiling timer alarm (see setitimer(2)) SIGWINCH 28\*b\*p window size change SIGINFO 29\*b\*p status request from keyboard SIGUSR1 30 user-defined signal 1 SIGUSR2 31 user-defined signal 2
The default signal action is termination if the signal is not caught or ignored, except for signals marked with \*b or \*d. The starred signals in the list above cause termination with a core image. Signals marked with \*b are discarded if the action is SIG_DFL; signals marked with \*d cause the process to stop. The signals marked with \*p are not defined by POSIX.
15 [EFAULT] Either act or oact points to memory that is not a valid part of the process address space.
15 [EINVAL] Sig is not a valid signal number.
15 [EINVAL] An attempt is made to ignore or supply a handler for SIGKILL or SIGSTOP.
void handler(sig, code, scp) int sig, code; struct sigcontext *scp;
Here sig is the signal number, into which the hardware faults and traps are mapped as defined below. Code is a parameter that is either a constant as given below or, for compatibility mode faults, the code provided by the hardware (Compatibility mode faults are distinguished from the other SIGILL traps by having PSL_CM set in the psl). Scp is a pointer to the sigcontext structure (defined in < signal.h >), used to restore the context from before the signal.
The following defines the mapping of hardware traps to signals and codes. All of these symbols are defined in < signal.h >:
Hardware condition Signal Code Arithmetic traps: Integer overflow SIGFPE FPE_INTOVF_TRAP Integer division by zero SIGFPE FPE_INTDIV_TRAP Floating overflow trap SIGFPE FPE_FLTOVF_TRAP Floating/decimal division by zero SIGFPE FPE_FLTDIV_TRAP Floating underflow trap SIGFPE FPE_FLTUND_TRAP Decimal overflow trap SIGFPE FPE_DECOVF_TRAP Subscript-range SIGFPE FPE_SUBRNG_TRAP Floating overflow fault SIGFPE FPE_FLTOVF_FAULT Floating divide by zero fault SIGFPE FPE_FLTDIV_FAULT Floating underflow fault SIGFPE FPE_FLTUND_FAULT Length access control SIGSEGV Protection violation SIGBUS Reserved instruction SIGILL ILL_RESAD_FAULT Customer-reserved instr. SIGEMT Reserved operand SIGILL ILL_PRIVIN_FAULT Reserved addressing SIGILL ILL_RESOP_FAULT Trace pending SIGTRAP Bpt instruction SIGTRAP Compatibility-mode SIGILL hardware supplied code Chme SIGSEGV Chms SIGSEGV Chmu SIGSEGV