SIGNAL(S) UNIX System V SIGNAL(S)
Name
signal - specify what to do upon receipt of a signal
Syntax
#include <signal.h>
void (*signal (sig, func))()
int sig;
void (*func)();
Description
The signal system call allows the calling process to choose
one of three ways in which it is possible to handle the
receipt of a specific signal. sig specifies the signal and
func specifies the choice.
sig can be assigned any one of the following except SIGKILL:
SIGHUP 01 hangup
SIGINT 02 interrupt
SIGQUIT 03[1] quit
SIGILL 04[1] illegal instruction (not reset when caught)
SIGTRAP 05[1] trace trap (not reset when caught)
SIGIOT 06[1] IOT instruction
SIGABRT 06 used by abort, replaces SIG10T
SIGEMT 07[1] EMT instruction
SIGFPE 08[1] floating point exception
SIGKILL 09 kill (cannot be caught or ignored)
SIGBUS 10[1] bus error
SIGSEGV 11[1] segmentation violation
SIGSYS 12[1] 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
SIGUSR1 16 user-defined signal 1
SIGUSR2 17 user-defined signal 2
SIGCLD 18[2] death of a child
SIGPWR 19[2] power fail
SIGPOLL 22[3] selectable event pending
func is assigned one of three values: SIG_DFL, SIG_IGN, or a
function address. SIG_DFL, and SIG_IGN, are defined in the
include file <signal.h>. Each is a macro that expands to a
constant expression of type pointer to function returning
void, and has a unique value that matches no declarable
function.
The actions prescribed by the values of func are as follows:
SIG_DFL -terminate process upon receipt of a signal
Upon receipt of the signal sig, the receiving
process is to be terminated with all of the
consequences outlined in exit(S). See Note [1]
below.
SIG_IGN -ignore signal
The signal sig is to be ignored.
Note: the signal SIGKILL cannot be ignored.
function address -catch signal
Upon receipt of the signal sig, the receiving
process is to execute the signal-catching function
pointed to by func. The signal number sig will be
passed as the only argument to the signal-catching
function. Additional arguments are passed to the
signal-catching function for hardware-generated
signals. Before entering the signal-catching
function, the value of func for the caught signal
will be set to SIG_DFL unless the signal is
SIGILL, SIGTRAP, or SIGPWR.
Upon return from the signal-catching function, the
receiving process will resume execution at the
point it was interrupted.
When a signal that is to be caught occurs during a
read(S), a write(S), an open(S), or an ioctl(S)
system call on a slow device (like a terminal; but
not a file), during a pause(S) system call, or
during a wait(S) system call that does not return
immediately due to the existence of a previously
stopped or zombie process, the signal catching
function will be executed. Then the interrupted
system call may return a -1 to the calling process
with errno set to EINTR.
The signal system call will not catch an invalid
function argument, func, and results are undefined
when an attempt is made to execute the function at
the bad address.
Note: The signal SIGKILL cannot be caught.
A call to signal cancels a pending signal sig except for a
pending SIGKILL signal.
The signal system call will fail if sig is an illegal signal
number, including SIGKILL. [EINVAL]
See Also
intro(S), kill(S), pause(S), ptrace(S), wait(S), setjmp(S),
sigset(S), kill(C)
Diagnostics
Upon successful completion, signal returns the previous
value of func for the specified signal sig. Otherwise, a
value of SIG_ERR is returned and errno is set to indicate
the error. SIG_ERR is defined in the include file
<signal.h>.
Notes
[1] If SIG_DFL is assigned for these signals, in addition to
the process being terminated, a ``core image'' will be
constructed in the current working directory of the
process, if the following conditions are met:
The effective user ID and the real user ID of
the receiving process are equal.
An ordinary file named core exists and is
writable or can be created. If the file must be
created, it will have the following properties:
+ a mode of 0666 modified by the file
creation mask (see umask(S))
+ a file owner ID that is the same as
the effective user ID of the receiving
process
+ a file group ID that is the same as
the effective group ID of the receiving
process.
[2] For the signals SIGCLD and SIGPWR, func is assigned one
of three values: SIG_DFL, SIG_IGN, or a function
address. The actions prescribed by these values are:
SIG_DFL -ignore signal
The signal is to be ignored.
SIG_IGN -ignore signal
The signal is to be ignored. Also, if sig is
SIGCLD, the calling process's child processes
will not create zombie processes when they
terminate (see exit(S)).
function address -catch signal
If the signal is SIGPWR, the action to be
taken is the same as that described above for
func equal to function address. The same is
true if the signal is SIGCLD with one
exception: while the process is executing the
signal-catching function, any received SIGCLD
signals will be ignored. (This is the default
action.) A wait call must proceed any call to
signal that specifies SIGCLD.
In addition, SIGCLD affects the wait and exit system
calls as follows:
wait If the func value of SIGCLD is set to SIG_IGN
and a wait is executed, the wait will block
until all of the calling process's child
processes terminate; it will then return a
value of -1 with errno set to ECHILD.
exit If in the exiting process's parent process the
func value of SIGCLD is set to SIG_IGN, the
exiting process will not create a zombie
process.
When processing a pipeline, the shell makes the last
process in the pipeline the parent of the preceding
processes. A process that may be piped into in this
manner (and thus become the parent of other processes)
should take care not to set SIGCLD to be caught.
[3] SIGPOLL is issued when a file descriptor corresponding
to a STREAMS (see intro(S)) file has a "selectable"
event pending. A process must specifically request that
this signal be sent using the I_SETSIG ioctl call.
Otherwise, the process will never receive SIGPOLL.
Standards Conformance
signal is conformant with:
AT&T SVID Issue 2, Select Code 307-127;
The X/Open Portability Guide II of January 1987;
and ANSI X3.159-198X C Language Draft Standard, May 13,
1988.
(printed 6/20/89)