'\" et
.TH SIGACTION "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
.\"
.SH PROLOG
This manual page is part of the POSIX Programmer's Manual.
The Linux implementation of this interface may differ (consult
the corresponding Linux manual page for details of Linux behavior),
or the interface may not be implemented on Linux.
.\"
.SH NAME
sigaction
\(em examine and change a signal action
.SH SYNOPSIS
.LP
.nf
#include <signal.h>
.P
int sigaction(int \fIsig\fP, const struct sigaction *restrict \fIact\fP,
    struct sigaction *restrict \fIoact\fP);
.fi
.SH DESCRIPTION
The
\fIsigaction\fR()
function allows the calling process to examine and/or specify the
action to be associated with a specific signal. The argument
.IR sig
specifies the signal; acceptable values are defined in
.IR <signal.h> .
.P
The structure
.BR sigaction ,
used to describe an action to be taken, is defined in the
.IR <signal.h> 
header to include at least the following members:
.ad l
.TS
center box tab(!);
cB | cB | cB
lw(1.5i)B | lw(1.25i)I | lw(2.5i).
Member Type!Member Name!Description
_
void(*) (int)!sa_handler!T{
Pointer to a signal-catching function or one of the macros
SIG_IGN or SIG_DFL.
T}
sigset_t!sa_mask!T{
Additional set of signals to be blocked during execution of
signal-catching function.
T}
int!sa_flags!T{
Special flags to affect behavior of signal.
T}
T{
void(*) (int,
.br
\0\0siginfo_t *, void *)
T}!sa_sigaction!Pointer to a signal-catching function.
.TE
.ad b
.P
The storage occupied by
.IR sa_handler
and
.IR sa_sigaction
may overlap, and a conforming application shall not use both
simultaneously.
.P
If the argument
.IR act
is not a null pointer, it points to a structure specifying the action
to be associated with the specified signal. If the argument
.IR oact
is not a null pointer, the action previously associated with the signal
is stored in the location pointed to by the argument
.IR oact .
If the argument
.IR 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.
.P
If the SA_SIGINFO flag (see below) is cleared in the
.IR sa_flags
field of the
.BR sigaction
structure, the
.IR sa_handler
field identifies the action to be associated with the specified signal.
If the SA_SIGINFO flag is set in the
.IR sa_flags
field, the
.IR sa_sigaction
field specifies a signal-catching function.
.P
The
.IR sa_flags
field can be used to modify the behavior of the specified signal.
.P
The following flags, defined in the
.IR <signal.h> 
header, can be set in
.IR sa_flags :
.IP SA_NOCLDSTOP 14
Do not generate SIGCHLD when children stop
or stopped children continue.
.RS 14 
.P
If
.IR sig
is SIGCHLD and the SA_NOCLDSTOP flag is not set in
.IR 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
.IR sig
is SIGCHLD and the SA_NOCLDSTOP flag is set in
.IR sa_flags ,
then the implementation shall not generate a SIGCHLD signal in this
way.
.RE
.IP SA_ONSTACK 14
If set and an alternate signal stack has been declared with
\fIsigaltstack\fR(),
the signal shall be delivered to the calling process on that stack.
Otherwise, the signal shall be delivered on the current stack.
.IP SA_RESETHAND 14
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.
.RS 14 
.TP 10
.BR Note:
SIGILL and SIGTRAP cannot be automatically reset when delivered; the
system silently enforces this restriction.
.P
Otherwise, the disposition of the signal shall not be modified on entry
to the signal handler.
.P
In addition, if this flag is set,
\fIsigaction\fR()
may behave as if the SA_NODEFER flag were also set.
.RE
.IP SA_RESTART 14
This flag affects the behavior of interruptible functions; that is,
those specified to fail with
.IR errno
set to
.BR [EINTR] .
If set, and a function specified as interruptible is interrupted by
this signal, the function shall restart and shall not fail with
.BR [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
.IR errno
set to
.BR [EINTR] .
.IP SA_SIGINFO 14
If cleared and the signal is caught, the signal-catching function
shall be entered as:
.RS 14 
.sp
.RS 4
.nf

void func(int \fIsigno\fP);
.fi
.P
.RE
.P
where
.IR signo
is the only argument to the signal-catching function. In this case, the
application shall use the
.IR sa_handler
member to describe the signal-catching function and the application
shall not modify the
.IR sa_sigaction
member.
.P
If SA_SIGINFO is set and the signal is caught, the signal-catching
function shall be entered as:
.sp
.RS 4
.nf

void func(int \fIsigno\fP, siginfo_t *\fIinfo\fP, void *\fIcontext\fP);
.fi
.P
.RE
.P
where two additional arguments are passed to the signal-catching
function. The second argument shall point to an object of type
.BR siginfo_t
explaining the reason why the signal was generated; the third argument
can be cast to a pointer to an object of type
.BR 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
.IR sa_sigaction
member to describe the signal-catching function and the application
shall not modify the
.IR sa_handler
member.
.P
The
.IR si_signo
member contains the system-generated signal number.
.P
The
.IR 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.
.P
The
.IR si_code
member contains a code identifying the cause of the signal, as
described in
.IR "Section 2.4.3" ", " "Signal Actions".
.RE
.IP SA_NOCLDWAIT 14
If
.IR sig
does not equal SIGCHLD, the behavior is unspecified. Otherwise, the
behavior of the SA_NOCLDWAIT flag is as specified in
.IR "Consequences of Process Termination".
.IP SA_NODEFER 14
If set and
.IR sig
is caught,
.IR sig
shall not be added to the thread's signal mask on entry to the signal
handler unless it is included in
.IR sa_mask .
Otherwise,
.IR sig
shall always be added to the thread's signal mask on entry to the
signal handler.
.P
When a signal is caught by a signal-catching function installed by
\fIsigaction\fR(),
a new signal mask is calculated and installed for the duration of the
signal-catching function (or until a call to either
\fIsigprocmask\fR()
or
\fIsigsuspend\fR()
is made). This mask is formed by taking the union of the current
signal mask and the value of the
.IR 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.
.P
Once an action is installed for a specific signal, it shall remain
installed until another action is explicitly requested (by another
call to
\fIsigaction\fR()),
until the SA_RESETHAND flag causes resetting of the handler,
or until one of the
.IR exec
functions is called.
.P
If the previous action for
.IR sig
had been established by
\fIsignal\fR(),
the values of the fields returned in the structure pointed to by
.IR oact
are unspecified, and in particular
.IR oact ->\c
.IR sa_handler
is not necessarily the same value passed to
\fIsignal\fR().
However, if a pointer to the same structure or a copy thereof is passed
to a subsequent call to
\fIsigaction\fR()
via the
.IR act
argument, handling of the signal shall be as if the original call to
\fIsignal\fR()
were repeated.
.P
If
\fIsigaction\fR()
fails, no new signal handler is installed.
.P
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
.IR errno
set to
.BR [EINVAL] .
.P
If SA_SIGINFO is not set in
.IR sa_flags ,
then the disposition of subsequent occurrences of
.IR 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
.IR sa_flags ,
then subsequent occurrences of
.IR sig
generated by
\fIsigqueue\fR()
or as a result of any signal-generating function that supports the
specification of an application-defined value (when
.IR 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
.IR si_value
member of the
.BR siginfo_t
structure.
.P
The result of the use of
\fIsigaction\fR()
and a
\fIsigwait\fR()
function concurrently within a process on the same signal is
unspecified.
.SH "RETURN VALUE"
Upon successful completion,
\fIsigaction\fR()
shall return 0; otherwise, \-1 shall be returned,
.IR errno
shall be set to indicate the error, and no new signal-catching function
shall be installed.
.br
.SH ERRORS
The
\fIsigaction\fR()
function shall fail if:
.TP
.BR EINVAL
The
.IR 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.
.P
The
\fIsigaction\fR()
function may fail if:
.TP
.BR EINVAL
An attempt was made to set the action to SIG_DFL for a signal that
cannot be caught or ignored (or both).
.P
In addition, on systems that do not support the XSI option, the
\fIsigaction\fR()
function may fail if the SA_SIGINFO flag is set in the
.IR sa_flags
field of the
.BR sigaction
structure for a signal not in the range SIGRTMIN to SIGRTMAX.
.LP
.IR "The following sections are informative."
.SH EXAMPLES
.SS "Establishing a Signal Handler"
.P
The following example demonstrates the use of
\fIsigaction\fR()
to establish a handler for the SIGINT signal.
.sp
.RS 4
.nf

#include <signal.h>
.P
static void handler(int signum)
{
    /* Take appropriate actions for signal delivery */
}
.P
int main()
{
    struct sigaction sa;
.P
    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 */;
.P
    /* Further code */
}
.fi
.P
.RE
.SH "APPLICATION USAGE"
The
\fIsigaction\fR()
function supersedes the
\fIsignal\fR()
function, and should be used in preference. In particular,
\fIsigaction\fR()
and
\fIsignal\fR()
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\(hy2017, 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
\fIsleep\fR()
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.
.P
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
.IR "\fIsemget\fR\^(\|)",
.IR "\fIsem_init\fR\^(\|)",
.IR "\fIsem_open\fR\^(\|)",
and so on). Note in particular that even the ``safe'' functions may
modify
.IR 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
.IR errno
and the subsequent examination of
.IR errno
could result in the signal-catching function changing the value of
.IR errno .
Naturally, the same principles apply to the async-signal-safety of
application routines and asynchronous data access. Note that
\fIlongjmp\fR()
and
\fIsiglongjmp\fR()
are not in the list of async-signal-safe functions. This is because
the code executing after
\fIlongjmp\fR()
and
\fIsiglongjmp\fR()
can call any unsafe functions with the same danger as calling those
unsafe functions directly from the signal handler. Applications that
use
\fIlongjmp\fR()
and
\fIsiglongjmp\fR()
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
\fImalloc\fR()
or
\fIfree\fR()
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\(hy2017 does not define the behavior
when any unsafe function is called in a signal handler that interrupts
an unsafe function.
.P
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.
.P
When the signal handler returns, the receiving thread resumes
execution at the point it was interrupted unless the signal handler
makes other arrangements. If
\fIlongjmp\fR()
or
\fI_longjmp\fR()
is used to leave the signal handler, then the signal mask must be
explicitly restored.
.P
This volume of POSIX.1\(hy2017 defines the third argument of a signal handling function when
SA_SIGINFO is set as a
.BR "void *"
instead of a
.BR "ucontext_t *" ,
but without requiring type checking. New applications should
explicitly cast the third argument of the signal handling function to
.BR "ucontext_t *" .
.P
The BSD optional four argument signal handling function is not
supported by this volume of POSIX.1\(hy2017. The BSD declaration would be:
.sp
.RS 4
.nf

void handler(int \fIsig\fP, int \fIcode\fP, struct sigcontext *\fIscp\fP,
    char *\fIaddr\fP);
.fi
.P
.RE
.P
where
.IR sig
is the signal number,
.IR code
is additional information on certain signals,
.IR scp
is a pointer to the
.BR sigcontext
structure, and
.IR 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.
.P
Since the
\fIsigaction\fR()
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.
.P
See also the rationale for Realtime Signal Generation and Delivery in
the Rationale (Informative) volume of POSIX.1\(hy2017,
.IR "Section B.2.4.2" ", " "Signal Generation and Delivery".
.SH RATIONALE
Although this volume of POSIX.1\(hy2017 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
\fIsigaction\fR()
reflect this in the information returned in the
.IR oact
argument. In other words, if SIGKILL
is included in the
.IR sa_mask
field of
.IR act ,
it is unspecified whether or not a subsequent call to
\fIsigaction\fR()
returns with SIGKILL included in the
.IR sa_mask
field of
.IR oact .
.P
The SA_NOCLDSTOP
flag, when supplied in the
.IR act ->\c
.IR 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
\fIwaitpid\fR()
function with the WNOHANG
flag set, acting on each child for which status is returned, until
\fIwaitpid\fR()
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.
.P
Some historical implementations also define other mechanisms for
stopping processes, such as the
\fIptrace\fR()
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\(hy2017.
.P
This volume of POSIX.1\(hy2017 requires that calls to
\fIsigaction\fR()
that supply a NULL
.IR act
argument succeed, even in the case of signals that cannot be caught or
ignored (that is, SIGKILL
or SIGSTOP).
The System V
\fIsignal\fR()
and BSD
\fIsigvec\fR()
functions return
.BR [EINVAL] 
in these cases and, in this respect, their behavior varies from
\fIsigaction\fR().
.P
This volume of POSIX.1\(hy2017 requires that
\fIsigaction\fR()
properly save and restore a signal action set up by the ISO\ C standard
\fIsignal\fR()
function. However, there is no guarantee that the reverse is true, nor
could there be given the greater amount of information conveyed by the
.BR 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
\fIsigaction\fR().
.P
It was intended that the
\fIsignal\fR()
function should be implementable as a library routine using
\fIsigaction\fR().
.P
The POSIX Realtime Extension extends the
\fIsigaction\fR()
function as specified by the POSIX.1\(hy1990 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.
.SH "FUTURE DIRECTIONS"
None.
.SH "SEE ALSO"
.IR "Section 2.4" ", " "Signal Concepts",
.IR "\fIexec\fR\^",
.IR "\fI_Exit\fR\^(\|)",
.IR "\fIkill\fR\^(\|)",
.IR "\fI_longjmp\fR\^(\|)",
.IR "\fIlongjmp\fR\^(\|)",
.IR "\fIpthread_sigmask\fR\^(\|)",
.IR "\fIraise\fR\^(\|)",
.IR "\fIsemget\fR\^(\|)",
.IR "\fIsem_init\fR\^(\|)",
.IR "\fIsem_open\fR\^(\|)",
.IR "\fIsigaddset\fR\^(\|)",
.IR "\fIsigaltstack\fR\^(\|)",
.IR "\fIsigdelset\fR\^(\|)",
.IR "\fIsigemptyset\fR\^(\|)",
.IR "\fIsigfillset\fR\^(\|)",
.IR "\fIsigismember\fR\^(\|)",
.IR "\fIsignal\fR\^(\|)",
.IR "\fIsigsuspend\fR\^(\|)",
.IR "\fIwait\fR\^(\|)",
.IR "\fIwaitid\fR\^(\|)"
.P
The Base Definitions volume of POSIX.1\(hy2017,
.IR "\fB<signal.h>\fP"
.\"
.SH COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1-2017, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 7, 2018 Edition,
Copyright (C) 2018 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.opengroup.org/unix/online.html .
.PP
Any typographical or formatting errors that appear
in this page are most likely
to have been introduced during the conversion of the source files to
man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .