sigaction(2)sigaction(2)NAMEsigaction - Specify 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 stan‐
dards as follows:
sigaction(): XSH4.0, XSH4.2, XSH5.0
Refer to the standards(5) reference page for more information about
industry standards and associated tags.
PARAMETERS
Defines the signal. Points to a sigaction structure that describes the
action to be taken upon receipt of the signal specified by the signal
parameter. 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 iden‐
tifies the signal that is to be affected. Specifying the action param‐
eter, 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. Specify‐
ing the o_action parameter, if not null, points to a sigaction struc‐
ture that contains the action previously attached to the specified sig‐
nal.
The XSH specification 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 POSIX realtime and UNIX programs that conform to
XSH:
void(*) (int, siginfo_t *, void *) sa_sigaction;
These sigaction members are described as follows: [XSH4.2] A pointer
to a signal-handling function. 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 deliv‐
ered. 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 <signal.h>. 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 execut‐
ing. This field can set the following flags to enable further control
over the actions taken when a signal is delivered: [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. [XSH4.2] Setting this bit causes
the signal to be reset to SIG_DFL. Note that SIGILL, SIGTRAP, and SIG‐
PWR cannot be automatically reset. [XSH4.2] Setting this bit causes
the signal not to be blocked automatically by the kernel as it is being
caught. [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 termi‐
nal, 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].
[XSH4.2] If this bit is clear, the signal-catching function adheres to
the following format:
void func ( int signo );
In this format, signo is the only argument to the signal-catch‐
ing function. This argument contains the signal number. In this
case, the sa_handler member of the sigaction structure must be
used to describe the signal-catching function, and the applica‐
tion must not modify the sa_sigaction member.
If the SA_SIGINFO bit is set and the signal is caught, the sig‐
nal-catching function adheres to the following format:
void func (
int signo,
siginfo_t *info,
void *context );
In this format, two additional arguments are passed to the sig‐
nal-catching function: If the info argument is not a null
pointer, it points to a structure of type siginfo_t that
explains the reason why the signal was generated. The si_signo
member of this structure contains the system-generated signal
number. If non-zero, the si_errno member contains an error num‐
ber identifying the condition that caused the signal to be gen‐
erated. 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 the
si_pid and si_uid members indicate the process ID and the real
user ID of the sender. The values of si_pid and si_uid are oth‐
erwise meaningless. The context argument can be cast to a
pointer to an object of type ucontext_t to refer to the receiv‐
ing process' context that was interrupted when the signal was
delivered. In this case, the sa_sigaction member of the sigac‐
tion structure must be used to describe the signal catching
function, and the application must not modify the sa_handler
member.
[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. If this bit is set and the signal param‐
eter is equal to SIGCHLD, a SIGCHLD signal is not sent to the
calling process when child processes terminate. [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 a wait(), waitid(),
waitpid(), or wait3() call is subsequently issued by the calling
process, it blocks until all of its child processes terminate.
The call 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.
[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 par‐
ent process that the child process is now a zombie and can be
cleaned up with a call to wait().
When SA_CLDNOTIFY is used without SA_SIGINFO, the early notifi‐
cation signal is not sent to the parent.
Once an action is installed for a specific signal, it remains installed
until one of the following occurs: Another sigaction() call explicitly
requests a different action. [XSH4.2] A signal causes the SA_RESET‐
HAND flag to reset the signal handler. A call is made 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 asyn‐
chronous signals.
NOTES
Only otherwise noted, sections in this reference page that are marked
by [XSH4.2] also apply to compilation environments conforming to
higher revisions of the XSH specification.
[Tru64 UNIX] When compiled in XSH4.2 and higher compilation environ‐
ments, 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 or a more current XSH compilation environment
has been defined, use _Esigaction to refer to the sigaction() call.
See standards(5) for information about compilation macro definitions
for industry standards.
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: [Tru64 UNIX] The
action or o_action parameter points to a location outside of the allo‐
cated address space of the process. 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.
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 shown 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);
}
SEE ALSO
Functions: acct(2), exit(2), kill(2), ptrace(2), sigblock(2), sigproc‐
mask(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)sigaction(2)
