NAME

sigaction — examine and change a signal action

SYNOPSIS

[CX] #include <signal.h>

int sigaction(int sig, const struct sigaction *restrict act,
       struct sigaction *restrict oact);

DESCRIPTION

The sigaction() function allows the calling process to examine and/or specify the action to be associated with a specific signal. The argument sig specifies the signal; acceptable values are defined in <signal.h>.

The structure sigaction, used to describe an action to be taken, is defined in the <signal.h> header to include at least the following members:

Member Type	Member Name	Description
void(*) (int)	sa_handler	Pointer to a signal-catching function or one of the macros SIG_IGN or SIG_DFL.
sigset_t	sa_mask	Additional set of signals to be blocked during execution of signal-catching function.
int	sa_flags	Special flags to affect behavior of signal.
void(*) (int,   siginfo_t *, void *)	sa_sigaction	Pointer to a signal-catching function.

The storage occupied by sa_handler and sa_sigaction may overlap, and a conforming application shall not use both simultaneously.

If the argument act is not a null pointer, it points to a structure specifying the action to be associated with the specified signal. If the argument oact is not a null pointer, the action previously associated with the signal is stored in the location pointed to by the argument oact. If the argument act is a null pointer, signal handling is unchanged; thus, the call can be used to enquire about the current handling of a given signal. The SIGKILL and SIGSTOP signals shall not be added to the signal mask using this mechanism; this restriction shall be enforced by the system without causing an error to be indicated.

If the SA_SIGINFO flag (see below) is cleared in the sa_flags field of the sigaction structure, the sa_handler field identifies the action to be associated with the specified signal. If the SA_SIGINFO flag is set in the sa_flags field, the sa_sigaction field specifies a signal-catching function.

The sa_flags field can be used to modify the behavior of the specified signal.

The following flags, defined in the <signal.h> header, can be set in sa_flags:

SA_NOCLDSTOP

Do not generate SIGCHLD when children stop ^[XSI]  or stopped children continue.

If sig is SIGCHLD and the SA_NOCLDSTOP flag is not set in sa_flags, and the implementation supports the SIGCHLD signal, then a SIGCHLD signal shall be generated for the calling process whenever any of its child processes stop ^[XSI]  and a SIGCHLD signal may be generated for the calling process whenever any of its stopped child processes are continued.  If sig is SIGCHLD and the SA_NOCLDSTOP flag is set in sa_flags, then the implementation shall not generate a SIGCHLD signal in this way.

SA_ONSTACK

^[XSI] If set and an alternate signal stack has been declared with sigaltstack(), the signal shall be delivered to the calling process on that stack. Otherwise, the signal shall be delivered on the current stack.

SA_RESETHAND

If set, the disposition of the signal shall be reset to SIG_DFL and the SA_SIGINFO flag shall be cleared on entry to the signal handler.

Note:

SIGILL and SIGTRAP cannot be automatically reset when delivered; the system silently enforces this restriction.

Otherwise, the disposition of the signal shall not be modified on entry to the signal handler.

In addition, if this flag is set, sigaction() may behave as if the SA_NODEFER flag were also set.

SA_RESTART

This flag affects the behavior of interruptible functions; that is, those specified to fail with errno set to [EINTR]. If set, and a function specified as interruptible is interrupted by this signal, the function shall restart and shall not fail with [EINTR] unless otherwise specified. If an interruptible function which uses a timeout is restarted, the duration of the timeout following the restart is set to an unspecified value that does not exceed the original timeout value. If the flag is not set, interruptible functions interrupted by this signal shall fail with errno set to [EINTR].

SA_SIGINFO

If cleared and the signal is caught, the signal-catching function shall be entered as:

void func(int signo);

where signo is the only argument to the signal-catching function. In this case, the application shall use the sa_handler member to describe the signal-catching function and the application shall not modify the sa_sigaction member.

If SA_SIGINFO is set and the signal is caught, the signal-catching function shall be entered as:

void func(int signo, siginfo_t *info, void *context);

where two additional arguments are passed to the signal-catching function. The second argument shall point 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 thread's context that was interrupted when the signal was delivered. In this case, the application shall use the sa_sigaction member to describe the signal-catching function and the application shall not modify the sa_handler member.

The si_signo member contains the system-generated signal number.

^[XSI] The si_errno member may contain implementation-defined additional error information; if non-zero, it 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, as described in 2.4.3 Signal Actions.

SA_NOCLDWAIT

^[XSI] If sig does not equal SIGCHLD, the behavior is unspecified. Otherwise, the behavior of the SA_NOCLDWAIT flag is as specified in Consequences of Process Termination.

SA_NODEFER

If set and sig is caught, sig shall not be added to the thread's signal mask on entry to the signal handler unless it is included in sa_mask. Otherwise, sig shall always be added to the thread's signal mask on entry to the signal handler.

When a signal is caught by a signal-catching function installed by sigaction(), a new signal mask is calculated and installed for the duration of the signal-catching function (or until a call to either sigprocmask() or sigsuspend() is made). This mask is formed by taking the union of the current signal mask and the value of the sa_mask for the signal being delivered, and unless SA_NODEFER or SA_RESETHAND is set, then including the signal being delivered. If and when the user's signal handler returns normally, the original signal mask is restored.

Once an action is installed for a specific signal, it shall remain installed until another action is explicitly requested (by another call to sigaction()), until the SA_RESETHAND flag causes resetting of the handler, or until one of the exec functions is called.

If the previous action for sig had been established by signal(), the values of the fields returned in the structure pointed to by oact are unspecified, and in particular oact->sa_handler is not necessarily the same value passed to signal(). However, if a pointer to the same structure or a copy thereof is passed to a subsequent call to sigaction() via the act argument, handling of the signal shall be as if the original call to signal() were repeated.

If sigaction() fails, no new signal handler is installed.

It is unspecified whether an attempt to set the action for a signal that cannot be caught or ignored to SIG_DFL is ignored or causes an error to be returned with errno set to [EINVAL].

If SA_SIGINFO is not set in sa_flags, then the disposition of subsequent occurrences of sig when it is already pending is implementation-defined; the signal-catching function shall be invoked with a single argument. If SA_SIGINFO is set in sa_flags, then subsequent occurrences of sig generated by sigqueue() or as a result of any signal-generating function that supports the specification of an application-defined value (when sig is already pending) shall be queued in FIFO order until delivered or accepted; the signal-catching function shall be invoked with three arguments. The application specified value is passed to the signal-catching function as the si_value member of the siginfo_t structure.

The result of the use of sigaction() and a sigwait() function concurrently within a process on the same signal is unspecified.

RETURN VALUE

Upon successful completion, sigaction() shall return 0; otherwise, -1 shall be returned, errno shall be set to indicate the error, and no new signal-catching function shall be installed.

ERRORS

The sigaction() function shall fail if:

[EINVAL]

The sig argument is not a valid signal number or an attempt is made to catch a signal that cannot be caught or ignore a signal that cannot be ignored.

The sigaction() function may fail if:

[EINVAL]

An attempt was made to set the action to SIG_DFL for a signal that cannot be caught or ignored (or both).

In addition, on systems that do not support the XSI option, the sigaction() function may fail if the SA_SIGINFO flag is set in the sa_flags field of the sigaction structure for a signal not in the range SIGRTMIN to SIGRTMAX.

EXAMPLES

Establishing a Signal Handler

The following example demonstrates the use of sigaction() to establish a handler for the SIGINT signal.

#include <signal.h>


static void handler(int signum)
{
    /* Take appropriate actions for signal delivery */
}


int main(void)
{
    struct sigaction sa;


    sa.sa_handler = handler;
    sigemptyset(&sa.sa_mask);
    sa.sa_flags = SA_RESTART; /* Restart functions if
                                 interrupted by handler */
    if (sigaction(SIGINT, &sa, NULL) == -1)
        /* Handle error */;


    /* Further code */
}

APPLICATION USAGE

The sigaction() function supersedes the signal() function, and should be used in preference. In particular, sigaction() and signal() should not be used in the same process to control the same signal. The behavior of async-signal-safe functions, as defined in their respective DESCRIPTION sections, is as specified by this volume of POSIX.1-2024, regardless of invocation from a signal-catching function. This is the only intended meaning of the statement that async-signal-safe functions may be used in signal-catching functions without restrictions. Applications must still consider all effects of such functions on such things as data structures, files, and process state. In particular, application developers need to consider the restrictions on interactions when interrupting sleep() and interactions among multiple handles for a file description. The fact that any specific function is listed as async-signal-safe does not necessarily mean that invocation of that function from a signal-catching function is recommended.

In order to prevent errors arising from interrupting non-async-signal-safe function calls, applications should protect calls to these functions either by blocking the appropriate signals or through the use of some programmatic semaphore (see semget, sem_init, sem_open, and so on). Note in particular that even the "safe" functions may modify errno; the signal-catching function, if not executing as an independent thread, should save and restore its value in order to avoid the possibility that delivery of a signal in between an error return from a function that sets errno and the subsequent examination of errno could result in the signal-catching function changing the value of errno. Naturally, the same principles apply to the async-signal-safety of application routines and asynchronous data access. Note that longjmp() and siglongjmp() are not in the list of async-signal-safe functions. This is because the code executing after longjmp() and siglongjmp() can call any unsafe functions with the same danger as calling those unsafe functions directly from the signal handler. Applications that use longjmp() and siglongjmp() from within signal handlers require rigorous protection in order to be portable. Many of the other functions that are excluded from the list are traditionally implemented using either malloc() or free() functions or the standard I/O library, both of which traditionally use data structures in a non-async-signal-safe manner. Since any combination of different functions using a common data structure can cause async-signal-safety problems, this volume of POSIX.1-2024 does not define the behavior when any unsafe function is called in a signal handler that interrupts an unsafe function.

Usually, the signal is executed on the stack that was in effect before the signal was delivered. An alternate stack may be specified to receive a subset of the signals being caught.

When the signal handler returns, the receiving thread resumes execution at the point it was interrupted unless the signal handler makes other arrangements. If longjmp() is used to leave the signal handler, then the signal mask must be explicitly restored.

This volume of POSIX.1-2024 defines the third argument of a signal handling function when SA_SIGINFO is set as a void * instead of a ucontext_t *, but without requiring type checking. New applications should explicitly cast the third argument of the signal handling function to ucontext_t *.

The BSD optional four argument signal handling function is not supported by this volume of POSIX.1-2024. The BSD declaration would be:

void handler(int sig, int code, struct sigcontext *scp,
    char *addr);

where sig is the signal number, code is additional information on certain signals, scp is a pointer to the sigcontext structure, and addr is additional address information. Much the same information is available in the objects pointed to by the second argument of the signal handler specified when SA_SIGINFO is set.

Since the sigaction() function is allowed but not required to set SA_NODEFER when the application sets the SA_RESETHAND flag, applications which depend on the SA_RESETHAND functionality for the newly installed signal handler must always explicitly set SA_NODEFER when they set SA_RESETHAND in order to be portable.

See also the rationale for Realtime Signal Generation and Delivery in XRAT B.2.4.2 Realtime Signal Generation and Delivery.

RATIONALE

Although this volume of POSIX.1-2024 requires that signals that cannot be ignored shall not be added to the signal mask when a signal-catching function is entered, there is no explicit requirement that subsequent calls to sigaction() reflect this in the information returned in the oact argument. In other words, if SIGKILL is included in the sa_mask field of act, it is unspecified whether or not a subsequent call to sigaction() returns with SIGKILL included in the sa_mask field of oact.

The SA_NOCLDSTOP flag, when supplied in the act->sa_flags parameter, allows overloading SIGCHLD with the System V semantics that each SIGCLD signal indicates a single terminated child. Most conforming applications that catch SIGCHLD are expected to install signal-catching functions that repeatedly call the waitpid() function with the WNOHANG flag set, acting on each child for which status is returned, until waitpid() returns zero. If stopped children are not of interest, the use of the SA_NOCLDSTOP flag can prevent the overhead from invoking the signal-catching routine when they stop.

Some historical implementations also define other mechanisms for stopping processes, such as the ptrace() function. These implementations usually do not generate a SIGCHLD signal when processes stop due to this mechanism; however, that is beyond the scope of this volume of POSIX.1-2024.

This volume of POSIX.1-2024 requires that calls to sigaction() that supply a NULL act argument succeed, even in the case of signals that cannot be caught or ignored (that is, SIGKILL or SIGSTOP). The System V signal() and BSD sigvec() functions return [EINVAL] in these cases and, in this respect, their behavior varies from sigaction().

This volume of POSIX.1-2024 requires that sigaction() properly save and restore a signal action set up by the ISO C standard signal() function. However, there is no guarantee that the reverse is true, nor could there be given the greater amount of information conveyed by the sigaction structure. Because of this, applications should avoid using both functions for the same signal in the same process. Since this cannot always be avoided in case of general-purpose library routines, they should always be implemented with sigaction().

It was intended that the signal() function should be implementable as a library routine using sigaction().

The POSIX Realtime Extension extends the sigaction() function as specified by the POSIX.1-1990 standard to allow the application to request on a per-signal basis via an additional signal action flag that the extra parameters, including the application-defined signal value, if any, be passed to the signal-catching function.

FUTURE DIRECTIONS

None.

SEE ALSO

2.4 Signal Concepts, exec, _Exit, kill, longjmp, pthread_sigmask, raise, semget, sem_init, sem_open, sig2str, sigaddset, sigaltstack, sigdelset, sigemptyset, sigfillset, sigismember, signal, sigsuspend, wait, waitid

XBD <signal.h>

CHANGE HISTORY

First released in Issue 3. Included for alignment with the POSIX.1-1988 standard.

Issue 5

The DESCRIPTION is updated for alignment with the POSIX Realtime Extension and POSIX Threads Extension.

In the DESCRIPTION, the second argument to func when SA_SIGINFO is set is no longer permitted to be NULL, and the description of permitted siginfo_t contents is expanded by reference to <signal.h>.

Since the X/OPEN UNIX Extension functionality is now folded into the BASE, the [ENOTSUP] error is deleted.

Issue 6

The Open Group Corrigendum U028/7 is applied. In the paragraph entitled "Signal Effects on Other Functions", a reference to sigpending() is added.

In the DESCRIPTION, the text "Signal Generation and Delivery", "Signal Actions", and "Signal Effects on Other Functions" are moved to a separate section of this volume of POSIX.1-2024.

Text describing functionality from the Realtime Signals Extension option is marked.

The following changes are made for alignment with the ISO POSIX-1:1996 standard:

• The [ENOTSUP] error condition is added.

The normative text is updated to avoid use of the term "must" for application requirements.

The restrict keyword is added to the sigaction() prototype for alignment with the ISO/IEC 9899:1999 standard.

References to the wait3() function are removed.

The SYNOPSIS is marked CX since the presence of this function in the <signal.h> header is an extension over the ISO C standard.

IEEE Std 1003.1-2001/Cor 1-2002, item XSH/TC1/D6/57 is applied, changing text in the table describing the sigaction structure.

IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/127 is applied, removing text from the DESCRIPTION duplicated later in the same section.

IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/128 is applied, updating the DESCRIPTION and APPLICATION USAGE sections. Changes are made to refer to the thread rather than the process.

IEEE Std 1003.1-2001/Cor 2-2004, item XSH/TC2/D6/129 is applied, adding the example to the EXAMPLES section.

Issue 7

Austin Group Interpretation 1003.1-2001 #004 is applied.

Austin Group Interpretations 1003.1-2001 #065 and #084 are applied, clarifying the role of the SA_NODEFER flag with respect to the signal mask, and clarifying the SA_RESTART flag for interrupted functions which use timeouts.

Austin Group Interpretation 1003.1-2001 #156 is applied.

SD5-XSH-ERN-167 is applied, updating the APPLICATION USAGE section.

SD5-XSH-ERN-172 is applied, updating the DESCRIPTION to make optional the requirement that when the SA_RESETHAND flag is set, sigaction() shall behave as if the SA_NODEFER flag were also set.

Functionality relating to the Realtime Signals Extension option is moved to the Base.

The description of the si_code member is replaced with a reference to 2.4.3 Signal Actions.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0578 [66] and XSH/TC1-2008/0579 [140] are applied.

POSIX.1-2008, Technical Corrigendum 2, XSH/TC2-2008/0329 [690] and XSH/TC2-2008/0330 [491] are applied.

Issue 8

Austin Group Defect 1138 is applied, adding sig2str() to the SEE ALSO section.

Austin Group Defect 1195 is applied, changing "main()" to "main(void)".

Austin Group Defect 1330 is applied, removing obsolescent interfaces.