sigaction(2) System Calls Manual sigaction(2)
sigaction - examine and change signal action
The 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
The structure sigaction, used to describe an action to be taken, is defined in the header to include at least the following members:
Member Type Member Name Description
void(*)(int) sa_handler SIG_DFL, SIG_IGN or
pointer to a func-
sigset_t sa_mask Additional set of
signals to be
blocked during exe-
cution of signal-
int sa_flags Special flags to
affect behavior of
void(*)(int, siginfo_t*,void *) sa_sigaction signal-catching
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 argu-
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 sa_handler field of the structure identifies the action to be associated with the specified signal. If the sa_handler
field specifies a signal-catching function, the sa_mask field identifies a set of signals that will be added to the thread's signal mask
before the signal-catching function is invoked. The and signals will not be added to the signal mask using this mechanism; this restric-
tion will be enforced by the system without causing an error to be indicated.
The sa_flags field can be used to modify the behavior of the specified signal. The following flags, defined in the header can be set in
Do not generate when children stop.
If set and an alternate signal stack has been declared with
or the signal will be delivered to the calling process on that stack. Otherwise, the signal will be deliv-
ered on the current stack.
If set, the disposition of the signal will be reset to
and the flag will be cleared on entry to the signal handler (Note: and cannot be automatically reset when
delivered; the system silently enforces this restriction). Otherwise, the disposition of the signal will not
be modified on entry to the signal handler. In addition, if this flag is set, behaves as if the flag were
This flag affects the behavior of interruptible functions; that
is, those specified to fail with set to If set, and a function specified as interruptible is interrupted by
this signal, the function will restart and will not fail with unless otherwise specified. If the flag is not
set, interruptible functions interrupted by this signal will fail with set to
If cleared and the signal is caught, the signal-catching
function will be entered as:
where 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 is set and the signal is caught, the signal-catching function will be entered as:
where two additional arguments are passed to the signal catching function. If the second argument is not a
null pointer, it will point to an object of type siginfo_t explaining the reason why the signal was gener-
ated; the third argument can be cast to a pointer to an object of type ucontext_t to refer to the context of
the receiving process or thread 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 of info contains the system-generated signal number.
The si_errno member may contain implementation-dependent additional error information; if non-zero, it con-
tains 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 values of si_pid and si_uid are otherwise meaningless.
If is set in sa_flags, subsequent occurrences of sig generated by or as a result of any signal-generating
function that supports the specification of an application-defined value - when sig is already pending - will
be queued in FIFO order until delivered, and the application specified value will be passed to the signal-
catching function as the si_value member of info (See If is not set in sa_flags, then the disposition of sub-
sequent occurrences of sig when it is already pending is implementation-defined.
If set, and sig equals child processes of the calling process will not be transformed into zombie processes 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 will block until all of its children terminate, and
and will fail and set errno to Otherwise, terminating child processes will be transformed into zombie pro-
cesses, unless is set to
If set and sig is caught, sig will not be added to the process' signal mask on entry to the signal handler unless it is
included in sa_mask. Otherwise, sig will always be added to the process' signal mask on entry to the signal
If sig is and the flag is not set in sa_flags, and the implementation supports the signal, then a signal will be generated for the calling
process whenever any of its child processes stop. If sig is and the flag is set in sa_flags, then the implementation will not generate a
signal in this way.
When a signal is caught by a signal-catching function installed by a new signal mask is calculated and installed for the duration of the
signal-catching function (or until a call to either or 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 or 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 remains installed until another action is explicitly requested (by another call to
until the 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 the values of the fields returned in the structure pointed to by oact are unspeci-
fied, and in particular oact->sa_handler is not necessarily the same value passed to However, if a pointer to the same structure or a copy
thereof is passed to a subsequent call to via the act argument, handling of the signal will be as if the original call to were repeated.
If 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 is ignored or causes an error to be
returned with set to
A signal is said to be for (or sent to) a process when the event that causes the signal first occurs. Examples of such events include
detection of hardware faults, timer expiration and terminal activity, as well as the invocation of and In some circumstances, the same
event generates signals for multiple processes.
Each process has an action to be taken in response to each signal defined by the system (see Signal Actions). A signal is said to be
delivered to a process when the appropriate action for the process and signal is taken.
During the time between the generation of a signal and its delivery, the signal is said to be Ordinarily, this interval cannot be detected
by an application.
However, a signal can be from delivery to a thread. If the action associated with a blocked signal is anything other than to ignore the
signal, and if that signal is generated for the thread, the signal will remain pending until either it is unblocked or the action associ-
ated with it is set to ignore the signal.
If the action associated with a blocked signal is to ignore the signal and if that signal is generated for the process, it is unspecified
whether the signal is discarded immediately upon generation or remains pending.
Each thread has a signal mask that defines the set of signals currently blocked from delivery to it. The signal mask for a thread is ini-
tialized from that of its parent. The and functions control the manipulation of the signal mask.
The determination of which action is taken in response to a signal is made at the time the signal is delivered, allowing for any changes
since the time of generation. This determination is independent of the means by which the signal was originally generated. If a subse-
quent occurrence of a pending signal is generated, it is implementation-dependent as to whether the signal is delivered more than once.
The order in which multiple, simultaneously pending signals are delivered to a process is unspecified.
When any stop signal is generated for a process, any pending signals for that process will be discarded. Conversely, when is generated for
a process, all pending stop signals for that process will be discarded. When is generated for a process that is stopped, the process will
be continued, even if the signal is blocked or ignored. If is blocked and not ignored, it will remain pending until it is either unblocked
or a stop signal is generated for the process.
Some signal-generating functions, such as high-resolution timer expiration, asynchronous I/O completion, interprocess message arrival, and
the function, support the specification of an application-defined value, either explicitly as a parameter to the function or in a structure
parameter (see signal(5)).
Realtime Signals Extension
When a signal is generated by or any signal-generating function that supports the specification of an application-defined value, and if the
flag is set for that signal, the signal will be queued to the process along with the application-specified signal value.
Multiple occurrences of signals so generated are queued in FIFO order. When multiple unblocked signals, all in the range to are pending,
the implementation delivers the pending unblocked signal with the lowest signal number within that range.
The selection order between realtime and nonrealtime signals, or between multiple pending nonrealtime signals, is unspecified.
Signals generated by or other events that cause signals to occur, such as detection of hardware faults, timer expiration, or terminal
activity, and for which the implementation does not support queueing, will have no effect on signals already queued for the same signal
If, when a pending signal is delivered, there are additional signals to be queued to that signal number, the signal will remain pending.
Otherwise, the pending indication will be reset.
An implementation will document any condition not specified by this document under which the implementation generates signals.
There are three types of action that can be associated with a signal: or a pointer to a function. Initially, all signals will be set to or
prior to entry of the routine (see the functions). The actions prescribed by these values are as follows:
- signal-specific default action
o The default actions for the signals defined in this document are specified under signal(5).
o If the default action is to stop the process, the execution of that process is temporarily suspended. When a process stops, a
signal will be generated for its parent process, unless the parent process has set the flag. While a process is stopped, any
additional signals that are sent to the process will not be delivered until the process is continued, except which always termi-
nates the receiving process. A process that is a member of an orphaned process group will not be allowed to stop in response to
the or signals. In cases where delivery of one of these signals would stop such a process, the signal will be discarded.
o Setting a signal action to for a signal that is pending, and whose default action is to ignore the signal (for example, will
cause the pending signal to be discarded, whether or not it is blocked. Any queued values pending will be discarded, and the
system resources used to queue them will be released and made available to queue other signals.
- ignore signal
o Delivery of the signal will have no effect on the process. The behavior of a process is undefined after it ignores a or signal
that was not generated by
o The system will not allow the action for the signals or to be set to
o Setting a signal action to for a signal that is pending will cause the pending signal to be discarded, whether or not it is
blocked. Any queued values pending will be discarded, and the system resources used to queue them will be released and made
available to queue other signals.
o If a process sets the action for the signal to the behavior is unspecified, except as specified below.
If the action for the signal is set to child processes of the calling processes will not be transformed into zombie processes
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 will block until all of its children terminate, and and will fail and set to
Pointer to a function - catch signal
o On delivery of the signal, the receiving process is to execute the signal-catching function at the specified address. After
returning from the signal-catching function, the receiving process will resume execution at the point at which it was inter-
o If is cleared, the signal-catching function will be entered as:
where is the specified signal-catching function and signo is the signal number of the signal being delivered.
o If is set, the signal-catching function will be entered as:
where is the specified signal-catching function, is the signal number of the signal being delivered, siginfo points to an object
of type siginfo_t associated with the signal being delivered, and points to a ucontext_t.
o The behavior of a process is undefined after it returns normally from a signal- catching function for a or signal that was not
generated by or
o The system will not allow a process to catch the signals and
o If a process establishes a signal-catching function for the signal while it has a terminated child process for which it has not
waited, a signal is delivered to the parent process immediately.
o When signal-catching functions are invoked asynchronously with process execution, the behavior of some of the functions defined
by this document is unspecified if they are called from a signal-catching function.
Refer to the "Async Signal Safe" section of thread_safety(5) which defines a set of functions that are either reentrant or not
interruptible by signals. Therefore applications may invoke them, without restriction, from signal-catching functions.
All functions not mentioned under the "Async Signal Safe" section of thread_safety(5) are considered to be unsafe with respect to
signals. In the presence of signals, all functions defined by this document will behave as defined when called from or inter-
rupted by a signal-catching function, with a single exception: when a signal interrupts an unsafe function and the signal-catch-
ing function calls an unsafe function, the behavior is undefined.
Signal Effects on Other Functions
Signals affect the behavior of certain functions (defined under the section of thread_safety(5)) if delivered to a process while it is exe-
cuting such a function. If the action of the signal is to terminate the process, the process will be terminated and the function will not
If the action of the signal is to stop the process, the process will stop until continued or terminated. Generation of a signal for the
process causes the process to be continued, and the original function will continue at the point the process was stopped.
If the action of the signal is to invoke a signal-catching function, the signal-catching function will be invoked; in this case the origi-
nal function is said to be by the signal. If the signal-catching function executes a return statement, the behavior of the interrupted
function will be as described individually for that function. Signals that are ignored will not affect the behavior of any function; sig-
nals that are blocked will not affect the behavior of any function until they are unblocked and then delivered.
The function supersedes the interface, and should be used in preference. In particular, and should not be used in the same process to con-
trol the same signal.
The behavior of reentrant functions, as defined in the description, is as specified by this document, 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 func-
tions 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 and interactions
among multiple handles for a file descriptor.
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.
This document does not address the more general problem of synchronizing access to shared data structures. Note in particular that even
the "safe" functions may modify the global variable the signal-catching function 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 and are not in the list of reentrant functions. This is because the code executing after and can call any unsafe functions with
the same danger as calling those unsafe functions directly from the signal handler. Applications that use and 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 or functions or the standard I/O
library, both of which traditionally use data structures in a non-reentrant manner.
Because any combination of different functions using a common data structure can cause reentrancy problems, this document 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 or the behavior is undefined if the signal handler calls any function in the stan-
dard 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 is
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 will resume execution at the point it was interrupted unless the signal handler
makes other arrangements. If or is used to leave the signal handler, then the signal mask must be explicitly restored by the process.
POSIX.4-1993 defines the third argument of a signal handling function when 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 uncontext_t *.
The BSD optional four argument signal handling function is not supported by this specification. The BSD declaration would be
where sig is the signal number, code is additional information on certain signals, scp is a pointer to the structure, and addr is addi-
tional address information. Much the same information is available in the objects pointed to by the second argument of the signal handler
specified when is set.
The signal disposition, catch/ignore/default, established by is shared by all threads in the process.
If the signal disposition for sig is set to or is set to and the default action for sig is to ignore the signal, any instances of sig pend-
ing on the process or any of the threads will be discarded. The signals are discarded regardless of whether the signal is blocked by any
of the threads.
For more information regarding signals and threads, see signal(5).
Upon successful completion, returns 0. Otherwise -1 is returned, is set to indicate the error and no new signal-catching function will be
The function will fail if:
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.
act or oact points to an invalid address. The reliable detection of this error is implementation dependent.
The function may fail if:
An attempt was made to set the action to
for a signal that cannot be caught or ignored (or both).
The function is marked as an extension in the list of safe functions because it is not included in the corresponding list in the ISO
POSIX-1 standard, but it is expected to be added in a future revision of that standard.
was derived from the IEEE POSIX 1003.1-1988 Standard.
kill(2), sigaltstack(2), signal(2), sigpending(2), sigprocmask(2), sigqueue(2), sigspace(2), sigsuspend(2), ttrace(2), wait(2), waitid(2),
setjmp(3C), sigsetops(3C), thread_safety(5).