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SIGACTION(3P) POSIX Programmer's Manual SIGACTION(3P)
PROLOG
This manual page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (con-
sult the corresponding Linux manual page for details of Linux behavior), or the interface may not be implemented on
Linux.
NAME
sigaction - examine and change a signal action
SYNOPSIS
#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 behav-
ior of signal.
void(*) (int,
siginfo_t *, sa_sigaction Pointer to a signal-catching
void *) function.
The storage occupied by sa_handler and sa_sigaction may overlap, and a conforming application shall not use both simulta-
neously.
If the argument act is not a null pointer, it points to a structure specifying the action to be associated with the spec-
ified 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,
and the implementation supports the Realtime Signals Extension option or the XSI Extension option, the sa_sigaction field
specifies a signal-catching function. If the SA_SIGINFO bit is cleared and the sa_handler field specifies a signal-
catching function, or if the SA_SIGINFO bit is set, the sa_mask field identifies a set of signals that shall be added to
the signal mask of the thread before the signal-catching function is invoked. If the sa_handler field specifies a signal-
catching function, the sa_mask field identifies a set of signals that shall be added to the process' signal mask before
the signal-catching function is invoked.
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 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 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
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() behaves 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 the flag is not set, interruptible func-
tions 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 process' 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 applica-
tion shall not modify the sa_handler member.
The si_signo member contains the system-generated signal number.
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.
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,
respectively, indicate the process ID and the real user ID of the sender. The <signal.h> header description contains
information about the signal-specific contents of the elements of the siginfo_t type.
SA_NOCLDWAIT
If set, and sig equals SIGCHLD, child processes of the calling processes shall not be transformed into zombie pro-
cesses when they terminate. If the calling process subsequently waits for its children, and the process has no
unwaited-for children that were transformed into zombie processes, it shall block until all of its children termi-
nate, and wait(), waitid(), and waitpid() shall fail and set errno to [ECHILD]. Otherwise, terminating child pro-
cesses shall be transformed into zombie processes, unless SIGCHLD is set to SIG_IGN.
SA_NODEFER
If set and sig is caught, sig shall not be added to the process' signal mask on entry to the signal handler unless
it is included in sa_mask. Otherwise, sig shall always be added to the process' 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 unless SA_NODEFER or SA_RESETHAND is set, and 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 sig-
nal(). 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 the implementation sup-
ports the Realtime Signals Extension option, and 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 applica-
tion-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 unspec-
ified.
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.
ENOTSUP
The SA_SIGINFO bit flag is set in the sa_flags field of the sigaction structure, and the implementation does not
support either the Realtime Signals Extension option, or the XSI Extension option.
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).
The following sections are informative.
EXAMPLES
None.
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 reentrant functions, as
defined in the DESCRIPTION, is as specified by this volume of IEEE Std 1003.1-2001, regardless of invocation from a sig-
nal-catching function. This is the only intended meaning of the statement that reentrant 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 writers 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 reentrant does not necessarily mean that invocation of that function from a signal-catch-
ing function is recommended.
In order to prevent errors arising from interrupting non-reentrant 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, may want to save and restore its value. Naturally,
the same principles apply to the reentrancy of application routines and asynchronous data access. Note that longjmp() and
siglongjmp() are not in the list of reentrant functions. This is because the code executing after longjmp() and sig-
longjmp() 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-reentrant manner. Since any combination of different functions using a common data structure can cause reentrancy
problems, this volume of IEEE Std 1003.1-2001 does not define the behavior when any unsafe function is called in a signal
handler that interrupts an unsafe function.
If the signal occurs other than as the result of calling abort(), kill(), or raise(), the behavior is undefined if the
signal handler calls any function in the standard library other than one of the functions listed in the table above or
refers to any object with static storage duration other than by assigning a value to a static storage duration variable
of type volatile sig_atomic_t. Furthermore, if such a call fails, the value of errno is unspecified.
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 process resumes execution at the point it was interrupted unless the sig-
nal handler makes other arrangements. If longjmp() or _longjmp() is used to leave the signal handler, then the signal
mask must be explicitly restored by the process.
This volume of IEEE Std 1003.1-2001 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 IEEE Std 1003.1-2001. 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.
RATIONALE
Although this volume of IEEE Std 1003.1-2001 requires that signals that cannot be ignored shall not be added to the sig-
nal mask when a signal-catching function is entered, there is no explicit requirement that subsequent calls to sigac-
tion() 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 seman-
tics 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, act-
ing 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 IEEE Std 1003.1-2001.
This volume of IEEE Std 1003.1-2001 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 IEEE Std 1003.1-2001 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
Signal Concepts, bsd_signal(), kill(), _longjmp(), longjmp(), raise(), semget(), sem_init() , sem_open(), sigaddset(),
sigaltstack(), sigdelset(), sigemptyset(), sigfillset(), sigismember(), signal(), sigprocmask(), sigsuspend(), wait(),
waitid(), waitpid(), the Base Definitions volume of IEEE Std 1003.1-2001, <signal.h>, <ucontext.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for
Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copy-
right (C) 2001-2003 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any
discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group
Standard is the referee document. The original Standard can be obtained online at http://www.open-
group.org/unix/online.html .
IEEE/The Open Group 2003 SIGACTION(3P)
