NAME

signal − simplified software signal facilities

SYNOPSIS

#include <signal.h>

(∗signal(sig, func))()
int (∗func)();

DESCRIPTION

Signal is a simplified interface to the more general sigvec(2) facility.

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>:

SIGHUP1hangup
SIGINT2interrupt
SIGQUIT3∗quit
SIGILL4∗illegal instruction
SIGTRAP5∗trace trap
SIGIOT6∗IOT instruction
SIGEMT7∗EMT instruction
SIGFPE8∗floating point exception
SIGKILL9kill (cannot be caught or ignored)
SIGBUS10∗bus error
SIGSEGV11∗segmentation violation
SIGSYS12∗bad argument to system call
SIGPIPE13write on a pipe with no one to read it
SIGALRM14alarm clock
SIGTERM15software termination signal
SIGURG16•urgent condition present on socket
SIGSTOP17†stop (cannot be caught or ignored)
SIGTSTP18†stop signal generated from keyboard
SIGCONT19•continue after stop
SIGCHLD20•child status has changed
SIGTTIN21†background read attempted from control terminal
SIGTTOU22†background write attempted to control terminal
SIGIO23•i/o is possible on a descriptor (see fcntl(2))
SIGXCPU24cpu time limit exceeded (see setrlimit(2))
SIGXFSZ25file size limit exceeded (see setrlimit(2))
SIGVTALRM26virtual time alarm (see setitimer(2))
SIGPROF27profiling timer alarm (see setitimer(2))
SIGWINCH28•Window size change
SIGUSR130User defined signal 1
SIGUSR231User 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 • or †.  Signals marked with • are discarded if the action is SIG_DFL; signals marked with † 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.  In particular this can occur during a read or write(2) on a slow device (such as a terminal; but not a file) and during a wait(2).

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.

RETURN VALUE

The previous action is returned on a successful call.  Otherwise, −1 is returned and errno is set to indicate the error. 

ERRORS

Signal will fail and no action will take place if one of the following occur:

[EINVAL] Sig is not a valid signal number. 

[EINVAL] An attempt is made to ignore or supply a handler for SIGKILL or SIGSTOP. 

[EINVAL] An attempt is made to ignore SIGCONT (by default SIGCONT is ignored). 

SEE ALSO

kill(1), ptrace(2), kill(2), sigvec(2), sigblock(2), sigsetmask(2), sigpause(2), sigstack(2), setjmp(3), tty(4)

NOTES  (VAX-11)

The handler routine can be declared:

    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 conditionSignalCode
 Arithmetic traps:
   Integer overflowSIGFPEFPE_INTOVF_TRAP
   Integer division by zeroSIGFPEFPE_INTDIV_TRAP
   Floating overflow trapSIGFPEFPE_FLTOVF_TRAP
   Floating/decimal division by zeroSIGFPEFPE_FLTDIV_TRAP
   Floating underflow trapSIGFPEFPE_FLTUND_TRAP
   Decimal overflow trapSIGFPEFPE_DECOVF_TRAP
   Subscript-rangeSIGFPEFPE_SUBRNG_TRAP
   Floating overflow faultSIGFPEFPE_FLTOVF_FAULT
   Floating divide by zero faultSIGFPEFPE_FLTDIV_FAULT
   Floating underflow faultSIGFPEFPE_FLTUND_FAULT
Length access controlSIGSEGV
Protection violationSIGBUS
Reserved instructionSIGILLILL_RESAD_FAULT
Customer-reserved instr.SIGEMT
Reserved operandSIGILLILL_PRIVIN_FAULT
Reserved addressingSIGILLILL_RESOP_FAULT
Trace pendingSIGTRAP
Bpt instructionSIGTRAP
Compatibility-modeSIGILLhardware supplied code
ChmeSIGSEGV
ChmsSIGSEGV
ChmuSIGSEGV

NOTES  (PDP-11)

The handler routine can be declared:

    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 a constant as given below.  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 conditionSignalCode
 Arithmetic traps:
   Floating overflow trapSIGFPEFPE_FLTOVF_TRAP
   Floating/decimal division by zeroSIGFPEFPE_FLTDIV_TRAP
   Floating underflow trapSIGFPEFPE_FLTUND_TRAP
   Decimal overflow trapSIGFPEFPE_DECOVF_TRAP
   Illegal return codeSIGFPEFPE_CRAZY
   Bad op codeSIGFPEFPE_OPCODE_TRAP
   Bad operandSIGFPEFPE_OPERAND_TRAP
   Maintenance trapSIGFPEFPE_MAINT_TRAP
Length access controlSIGSEGV
Protection violation (odd address)SIGBUS
Reserved instructionSIGILLILL_RESAD_FAULT
Customer-reserved instr.SIGEMT
Trace pendingSIGTRAP
Bpt instructionSIGTRAP

The handler routine must save any registers it uses and restore them before returning.  On the PDP-11, the kernel saves r0 and r1 before calling the handler routine, but expect the handler to save any other registers it uses.  The standard entry code generated by the C compiler for handler routines written in C automatically saves the remaining general registers, but floating point registers are not saved.  As a result there is currently no [standard] method for a handler routine written in C to perform floating point operations without blowing the interrupted program out of the water. 

4th Berkeley Distribution  —  May 20, 1986