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@(#)signal.3 6.5 (Berkeley) 07/01/90
#include <signal.h>void (*signal(sig, func))() void (*func)();
A signal is generated by some abnormal event, initiated by a user at a terminal (quit, interrupt, stop), by a program error (bus error, etc.), by request of another program (kill), or when a process is stopped because it wishes to access its control terminal while in the background (see tty (4)). Signals are optionally generated when a process resumes after being stopped, when the status of child processes changes, or when input is ready at the control terminal. Most signals cause termination of the receiving process if no action is taken; some signals instead cause the process receiving them to be stopped, or are simply discarded if the process has not requested otherwise. Except for the SIGKILL and SIGSTOP signals, the signal call allows signals either to be ignored or to cause an interrupt to a specified location. 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* trace trap SIGABRT 6* abort() call (formerly SIGIOT) SIGEMT 7* EMT instruction SIGFPE 8* floating point exception SIGKILL 9 kill (cannot be caught or ignored) SIGBUS 10* bus error SIGSEGV 11* segmentation violation SIGSYS 12* 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 urgent condition present on socket SIGSTOP 17\*d stop (cannot be caught 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 i/o is possible on a descriptor (see fcntl(2)) SIGXCPU 24 cpu time limit exceeded (see setrlimit(2)) SIGXFSZ 25 file size limit exceeded (see setrlimit(2)) SIGVTALRM 26 virtual time alarm (see setitimer(2)) SIGPROF 27 profiling timer alarm (see setitimer(2)) SIGWINCH 28\*b Window size change SIGINFO 29\*b status request from keyboard SIGUSR1 30 User defined signal 1 SIGUSR2 31 User defined signal 2
The starred signals in the list above cause a core image if not caught or ignored.
If func is SIG_DFL, the default action for signal sig is reinstated; this default is termination (with a core image for starred signals) except for signals marked with \*b or \*d. Signals marked with \*b are discarded if the action is SIG_DFL; signals marked with \*d cause the process to stop. If func is SIG_IGN the signal is subsequently ignored and pending instances of the signal are discarded. Otherwise, when the signal occurs further occurrences of the signal are automatically blocked and func is called.
A return from the function unblocks the handled signal and continues the process at the point it was interrupted. Unlike previous signal facilities, the handler func remains installed after a signal has been delivered.
If a caught signal occurs during certain system calls, causing the call to terminate prematurely, the call is automatically restarted (the handler is installed using the SA_RESTART flag with sigaction (2)). 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).
The value of signal is the previous (or initial) value of func for the particular signal.
After a fork (2) or vfork (2) the child inherits all signals. Execve (2) resets all caught signals to the default action; ignored signals remain ignored.
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)
Here sig is the signal number, into which the hardware faults and traps are mapped as defined below. Code is a parameter which is either a constant as given below or, for compatibility mode faults, the code provided by the hardware. Scp is a pointer to the "struct sigcontext" used by the system to restore the process context from before the signal. Compatibility mode faults are distinguished from the other SIGILL traps by having PSL_CM set in the psl.
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