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sigaction(2)
NAME
sigaction - Specifies the action to take upon delivery of a signal
SYNOPSIS
#include <signal.h>
int sigaction(
int signal,
const struct sigaction *action,
struct sigaction *o_action);
STANDARDS
Interfaces documented on this reference page conform to industry standards
as follows:
sigaction(): XPG4, XPG4-UNIX
Refer to the standards(5) reference page for more information about
industry standards and associated tags.
PARAMETERS
signal
Defines the signal.
action
Points to a sigaction structure that describes the action to be taken
upon receipt of the signal specified by the signal parameter.
o_action
Points to a sigaction structure. When the sigaction() function returns
from a call, the action previously attached to the specified signal is
stored in this structure.
DESCRIPTION
When a process requests the sigaction() function, the process can both
examine or specify what action is to be performed when the specified signal
is delivered. The parameters determine the behavior of the sigaction()
function as follows:
· Specifying the signal parameter identifies the signal that is to be
affected.
· Specifying the action parameter, if not null, points to a sigaction
structure that defines what action is to be performed when the signal
is received. If the action parameter is null, signal handling remains
unchanged; thus the call can be used to inquire about the current
handling of the signal.
· Specifying the o_action parameter, if not null, points to a sigaction
structure that contains the action previously attached to the
specified signal.
The X/Open Base Standard defines the following members for the sigaction
structure:
void (*sa_handler)(int);
sigset_t sa_mask;
int sa_flags;
[XSH4.2] The following additional member is defined in the sigaction
structure for use by realtime and X/Open UNIX programs:
void(*) (int, siginfo_t *, void *)
sa_sigaction;
The members are defined as follows:
sa_sigaction
[XSH4.2] A pointer to a signal-handling function.
sa_handler
This field can contain a value of SIG_DFL or SIG_IGN, or it can point
to a function. A SIG_DFL value specifies the default action is to be
taken when the signal is delivered. A value of SIG_IGN specifies that
the signal has no effect on the receiving process. A pointer to a
function requests that the signal be caught; that is, the signal should
cause the function to be called. These actions are more fully described
in the <signal.h> file.
sa_mask
This field can request that individual signals, in addition to those in
the current signal mask, be blocked from being delivered while the
signal handler function specified by the sa_handler or sa_sigaction
field is executing.
sa_flags
This field can set the following flags to enable further control over
the actions taken when a signal is delivered:
SA_ONSTACK
[XSH4.2] Setting this bit causes the system to run the signal-
catching function on the signal stack specified by the
sigaltstack() and sigstack() functions. If this bit is not set,
the function runs on the stack of the process to which the signal
is delivered.
SA_RESETHAND
[XSH4.2] Setting this bit causes the signal to be reset to
SIG_DFL. Note that SIGILL, SIGTRAP, and SIGPWR cannot be
automatically reset.
SA_NODEFER
[XSH4.2] Setting this bit causes the signal not to be blocked
automatically by the kernel as it is being caught.
SA_RESTART
[XSH4.2] Setting this bit enables a function that has been
interrupted by the execution of this signal's handler to be
restarted transparently by the system. The affected functions
include wait(), and the read() and write() functions on a slow
device (such as a terminal, but not a regular file). If this bit is
not set and one of the previously mentioned functions is
interrupted by a signal which is caught, the function returns the
value -1 and sets errno to [EINTR].
SA_SIGINFO
[XSH4.2] If this bit is clear, the signal-catching function is
entered as:
void func(int signo);
where signo is the only argument to the signal catching function.
In this case, the sa_handler member must be used to describe the
signal-catching function, and the application must not modify the
sa_sigaction member.
If this bit is set and the signal is caught, the signal-catching
function is entered as:
void func(int signo, siginfo_t *info, void *context);
where two additional arguments are passed to the signal-catching
function. If the second argument is not a null pointer, it points
to an object of type siginfo_t explaining the reason why the signal
was generated. The third argument can be cast to a pointer to an
object of type ucontext_t to refer to the receiving process'
context that was interrupted when the signal was delivered. In
this case, the sa_sigaction member must be used to describe the
signal catching function and the application must not modify the
sa_handler member.
The si_signo member contains the system-generated signal number.
If non-zero, the si_errno member contains an error number
identifying the condition that caused the signal to be generated.
The si_code member contains a code identifying the cause of the
signal. If the value of si_code is less than or equal to 0, then
the signal was generated by a process, and si_pid and si_uid
indicate the process ID and the real user ID of the sender. The
values of si_pid and si_uid are otherwise meaningless.
[Tru64 UNIX] In the backward-compatible version of sigaction(),
the second and third arguments available to the signal are defined
as follows:
·
If SA_SIGINFO is clear, the second argument is an integer value
providing additional error information for exception signals (see
<machine/signal.h>).
·
The third argument points to a sigcontext structure containing
context information in a different format.
SA_NOCLDSTOP
If this bit is set and the signal parameter is equal to SIGCHLD, a
SIGCHLD signal is not sent to the calling process when child
processes terminate.
SA_NOCLDWAIT
[XSH4.2] If this bit is set and the signal parameter is equal to
SIGCHLD, zombie processes are not created by the system when a
child process of the calling process exits. If the wait function
is subsequently issued by the calling process, it blocks until all
of its child processes terminate. It then returns a value of -1
and errno is set to ECHILD to indicate the error. Note: when this
flag is set, exiting child processes do not send SIGCHLD signals to
the parent.
SA_CLDNOTIFY
[Tru64 UNIX] If this bit is set in combination with SA_SIGINFO,
two SIGCHLD signals are queued to the parent for each abnormal
termination of a child process that includes a core dump: an early
notification signal and a final notification signal.
The early notification signal delivers a siginfo structure with the
si_signo field set to SIGCHLD, the si_code field set to
CLD_SIGEXITING, and the si_pid field set to the process ID of the
child process that is about to write a core file and then terminate
abnormally. This signal tells the parent process that it can start
a failover operation or take other appropriate action.
The final notification signal delivers a siginfo structure with the
si_signo field set to SIGCHLD, the si_code field set to CLD_DUMPED,
and the si_pid field set to the process ID of the child that has
terminated abnormally. This signal tells the parent process that
the child process is now a zombie and can be cleaned up with a call
to wait(2).
When SA_CLDNOTIFY is used without SA_SIGINFO, the early
notification signal is not sent to the parent.
Once an action is installed for a specific signal, it remains installed
until one of the following events occurs:
· A sigaction() call that explicitly requests a different action.
· [XSH4.2] A signal that causes the SA_RESETHAND flag to reset the
signal handler.
· A call to one of the exec* functions.
The signal parameter can be any one of the signal values defined in the
signal.h header file, except SIGKILL and SIGSTOP.
In a multithreaded environment, the sigaction() function should only be
used for the synchronous signals. Use the sigwait() function for
asynchronous signals.
NOTES
[Tru64 UNIX] When compiled in the X/Open UNIX environment, calls to the
sigaction() function are internally renamed by prepending _E to the
function name. When you are debugging a module that includes the
sigaction() function and for which _XOPEN_SOURCE_EXTENDED has been defined,
use _Esigaction to refer to the sigaction() call. See standards(5) for
information on when the _XOPEN_SOURCE_EXTENDED macro is defined.
EXAMPLE
The following program illustrates the use of the SA_CLDNOTIFY flag:
#include <stdio.h>
#include <sys/types.h>
#include <sys/signal.h>
#include <sys/siginfo.h>
volatile pid_t kid1, kid2;
/*
* Handler for SIGHCLD signal. Note: printf() calls
* are technically unsupported from signal handlers
* and are show for illustrative purposes only.
*/
void
sigchld_handler(int sig, siginfo_t *sip, void *extra)
{
pid_t kid;
kid = sip->si_pid;
if (sip->si_code == CLD_SIGEXITING) {
printf("SIGEXITING: Got signal %d, si_code %d"
" for kid %d\n",
sip->si_signo, sip->si_code, kid);
} else if (sip->si_code == CLD_DUMPED) {
printf("EXITED: Got signal %d, si_code %d"
" for kid %d\n",
sip->si_signo, sip->si_code, kid);
kid = wait(0);
printf("Parent got PID %d exiting\n", kid);
if (kid == kid1)
kid1 = 0;
else if (kid == kid2)
kid2 = 0;
}
}
main()
{
struct sigaction sa;
int ret;
/*
* Set up SIGHCLD handler for early exit
* notification.
*/
sa.sa_sigaction = sigchild_handler;
sigemptyset(&sa.sa_mask);
sa.sa_flags = SA_CLDNOTIFY|SA_SIGINFO;
ret = sigaction(SIGCHLD, &sa, 0);
if (ret) {
perror("sigaction");
exit(1);
}
/*
* Create 2 children to die with SIGABRT
* and create core files.
*/
kid1 = fork();
if (kid1)
printf("Parent forked %d\n", kid1);
else {
/*
* First child...
*/
sleep(1);
printf("Kid %d will now die with core file...\n",
getpid());
abort();
/*NOTREACHED*/
}
kid2 = fork();
if (kid2)
printf("Parent forked %d\n", kid2);
else {
/*
* Second child...
*/
sleep(1);
printf("Kid %d will now die with core file...\n",
getpid());
abort();
/*NOTREACHED*/
}
/*
* Parent: keep busy until children exit.
*/
while (kid1 || kid2)
;
/*
* Children have exit: verify by printing zeros.
*/
printf("Parent: done -- kid1 %d, kid2 %d\n", kid1, kid2);
;
/*
* Children have exit: verify by printing zeros.
*/
printf("Parent: done -- kid1 %d, kid2 %d\n", kid1, kid2);
}
RETURN VALUES
Upon successful completion of the sigaction() function, a value of zero (0)
is returned. If the sigaction() function fails, a value of -1 is returned
and errno is set to indicate the error.
ERRORS
If the sigaction() function fails, no new signal handler is installed and
errno may be set to one of the following values:
[EFAULT] [Tru64 UNIX] The action or o_action parameter points to a
location outside of the allocated address space of the process.
[EINVAL] The signal parameter is not a valid signal number.
An attempt was made to ignore or supply a handler for the SIGKILL
and SIGSTOP signals.
RELATED INFORMATION
Functions: acct(2), exit(2), kill(2), ptrace(2), sigblock(2),
sigprocmask(2), sigstack(2), sigsuspend(2), sigvec(2), umask(2), wait(2),
pause(3), setjmp(3), sigpause(3)
Commands: kill(1)
Files: signal(4)
Standards: standards(5)
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