path: root/man2/sigaction.2

.\" Copyright (c) 1994,1995 Mike Battersby <mib@deakin.edu.au>
.\" and Copyright 2004, 2005 Michael Kerrisk <mtk.manpages@gmail.com>
.\" based on work by faith@cs.unc.edu
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" Modified, aeb, 960424
.\" Modified Fri Jan 31 17:31:20 1997 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified Thu Nov 26 02:12:45 1998 by aeb - add SIGCHLD stuff.
.\" Modified Sat May  8 17:40:19 1999 by Matthew Wilcox
.\"	add POSIX.1b signals
.\" Modified Sat Dec 29 01:44:52 2001 by Evan Jones <ejones@uwaterloo.ca>
.\"	SA_ONSTACK
.\" Modified 2004-11-11 by Michael Kerrisk <mtk.manpages@gmail.com>
.\"	Added mention of SIGCONT under SA_NOCLDSTOP
.\"	Added SA_NOCLDWAIT
.\" Modified 2004-11-17 by Michael Kerrisk <mtk.manpages@gmail.com>
.\"	Updated discussion for POSIX.1-2001 and SIGCHLD and sa_flags.
.\"	Formatting fixes
.\" 2004-12-09, mtk, added SI_TKILL + other minor changes
.\" 2005-09-15, mtk, split sigpending(), sigprocmask(), sigsuspend()
.\"	out of this page into separate pages.
.\" 2010-06-11 Andi Kleen, add hwpoison signal extensions
.\" 2010-06-11 mtk, improvements to discussion of various siginfo_t fields.
.\" 2015-01-17, Kees Cook <keescook@chromium.org>
.\"	Added notes on ptrace SIGTRAP and SYS_SECCOMP.
.\"
.TH sigaction 2 2023-05-03 "Linux man-pages 6.05.01"
.SH NAME
sigaction, rt_sigaction \- examine and change a signal action
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <signal.h>
.PP
.BI "int sigaction(int " signum ,
.BI "              const struct sigaction *_Nullable restrict " act ,
.BI "              struct sigaction *_Nullable restrict " oldact );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR sigaction ():
.nf
    _POSIX_C_SOURCE
.fi
.PP
.IR siginfo_t :
.nf
    _POSIX_C_SOURCE >= 199309L
.fi
.SH DESCRIPTION
The
.BR sigaction ()
system call is used to change the action taken by a process on
receipt of a specific signal.
(See
.BR signal (7)
for an overview of signals.)
.PP
.I signum
specifies the signal and can be any valid signal except
.B SIGKILL
and
.BR SIGSTOP .
.PP
If
.I act
is non-NULL, the new action for signal
.I signum
is installed from
.IR act .
If
.I oldact
is non-NULL, the previous action is saved in
.IR oldact .
.PP
The
.I sigaction
structure is defined as something like:
.PP
.in +4n
.EX
struct sigaction {
    void     (*sa_handler)(int);
    void     (*sa_sigaction)(int, siginfo_t *, void *);
    sigset_t   sa_mask;
    int        sa_flags;
    void     (*sa_restorer)(void);
};
.EE
.in
.PP
On some architectures a union is involved: do not assign to both
.I sa_handler
and
.IR sa_sigaction .
.PP
The
.I sa_restorer
field is not intended for application use.
(POSIX does not specify a
.I sa_restorer
field.)
Some further details of the purpose of this field can be found in
.BR sigreturn (2).
.PP
.I sa_handler
specifies the action to be associated with
.I signum
and can be one of the following:
.IP \[bu] 3
.B SIG_DFL
for the default action.
.IP \[bu]
.B SIG_IGN
to ignore this signal.
.IP \[bu]
A pointer to a signal handling function.
This function receives the signal number as its only argument.
.PP
If
.B SA_SIGINFO
is specified in
.IR sa_flags ,
then
.I sa_sigaction
(instead of
.IR sa_handler )
specifies the signal-handling function for
.IR signum .
This function receives three arguments, as described below.
.PP
.I sa_mask
specifies a mask of signals which should be blocked
(i.e., added to the signal mask of the thread in which
the signal handler is invoked)
during execution of the signal handler.
In addition, the signal which triggered the handler
will be blocked, unless the
.B SA_NODEFER
flag is used.
.PP
.I sa_flags
specifies a set of flags which modify the behavior of the signal.
It is formed by the bitwise OR of zero or more of the following:
.TP
.B SA_NOCLDSTOP
If
.I signum
is
.BR SIGCHLD ,
do not receive notification when child processes stop (i.e., when they
receive one of
.BR SIGSTOP ", " SIGTSTP ", " SIGTTIN ,
or
.BR SIGTTOU )
or resume (i.e., they receive
.BR SIGCONT )
(see
.BR wait (2)).
This flag is meaningful only when establishing a handler for
.BR SIGCHLD .
.TP
.BR SA_NOCLDWAIT " (since Linux 2.6)"
.\" To be precise: Linux 2.5.60 -- MTK
If
.I signum
is
.BR SIGCHLD ,
do not transform children into zombies when they terminate.
See also
.BR waitpid (2).
This flag is meaningful only when establishing a handler for
.BR SIGCHLD ,
or when setting that signal's disposition to
.BR SIG_DFL .
.IP
If the
.B SA_NOCLDWAIT
flag is set when establishing a handler for
.BR SIGCHLD ,
POSIX.1 leaves it unspecified whether a
.B SIGCHLD
signal is generated when a child process terminates.
On Linux, a
.B SIGCHLD
signal is generated in this case;
on some other implementations, it is not.
.TP
.B SA_NODEFER
Do not add the signal to the thread's signal mask while the
handler is executing, unless the signal is specified in
.IR act.sa_mask .
Consequently, a further instance of the signal may be delivered
to the thread while it is executing the handler.
This flag is meaningful only when establishing a signal handler.
.IP
.B SA_NOMASK
is an obsolete, nonstandard synonym for this flag.
.TP
.B SA_ONSTACK
Call the signal handler on an alternate signal stack provided by
.BR sigaltstack (2).
If an alternate stack is not available, the default stack will be used.
This flag is meaningful only when establishing a signal handler.
.TP
.B SA_RESETHAND
Restore the signal action to the default upon entry to the signal handler.
This flag is meaningful only when establishing a signal handler.
.IP
.B SA_ONESHOT
is an obsolete, nonstandard synonym for this flag.
.TP
.B SA_RESTART
Provide behavior compatible with BSD signal semantics by making certain
system calls restartable across signals.
This flag is meaningful only when establishing a signal handler.
See
.BR signal (7)
for a discussion of system call restarting.
.TP
.B SA_RESTORER
.IR "Not intended for application use" .
This flag is used by C libraries to indicate that the
.I sa_restorer
field contains the address of a "signal trampoline".
See
.BR sigreturn (2)
for more details.
.TP
.BR SA_SIGINFO " (since Linux 2.2)"
The signal handler takes three arguments, not one.
In this case,
.I sa_sigaction
should be set instead of
.IR sa_handler .
This flag is meaningful only when establishing a signal handler.
.\" (The
.\" .I sa_sigaction
.\" field was added in Linux 2.1.86.)
.\"
.TP
.BR SA_UNSUPPORTED " (since Linux 5.11)"
Used to dynamically probe for flag bit support.
.IP
If an attempt to register a handler succeeds with this flag set in
.I act\->sa_flags
alongside other flags that are potentially unsupported by the kernel,
and an immediately subsequent
.BR sigaction ()
call specifying the same signal number and with a non-NULL
.I oldact
argument yields
.B SA_UNSUPPORTED
.I clear
in
.IR oldact\->sa_flags ,
then
.I oldact\->sa_flags
may be used as a bitmask
describing which of the potentially unsupported flags are,
in fact, supported.
See the section "Dynamically probing for flag bit support"
below for more details.
.TP
.BR SA_EXPOSE_TAGBITS " (since Linux 5.11)"
Normally, when delivering a signal,
an architecture-specific set of tag bits are cleared from the
.I si_addr
field of
.IR siginfo_t .
If this flag is set,
an architecture-specific subset of the tag bits will be preserved in
.IR si_addr .
.IP
Programs that need to be compatible with Linux versions older than 5.11
must use
.B SA_UNSUPPORTED
to probe for support.
.SS The siginfo_t argument to a SA_SIGINFO handler
When the
.B SA_SIGINFO
flag is specified in
.IR act.sa_flags ,
the signal handler address is passed via the
.I act.sa_sigaction
field.
This handler takes three arguments, as follows:
.PP
.in +4n
.EX
void
handler(int sig, siginfo_t *info, void *ucontext)
{
    ...
}
.EE
.in
.PP
These three arguments are as follows
.TP
.I sig
The number of the signal that caused invocation of the handler.
.TP
.I info
A pointer to a
.IR siginfo_t ,
which is a structure containing further information about the signal,
as described below.
.TP
.I ucontext
This is a pointer to a
.I ucontext_t
structure, cast to \fIvoid\ *\fP.
The structure pointed to by this field contains
signal context information that was saved
on the user-space stack by the kernel; for details, see
.BR sigreturn (2).
Further information about the
.I ucontext_t
structure can be found in
.BR getcontext (3)
and
.BR signal (7).
Commonly, the handler function doesn't make any use of the third argument.
.PP
The
.I siginfo_t
data type is a structure with the following fields:
.PP
.in +4n
.EX
siginfo_t {
    int      si_signo;     /* Signal number */
    int      si_errno;     /* An errno value */
    int      si_code;      /* Signal code */
    int      si_trapno;    /* Trap number that caused
                              hardware\-generated signal
                              (unused on most architectures) */
.\" FIXME
.\" The siginfo_t 'si_trapno' field seems to be used
.\" only on SPARC and Alpha; this page could use
.\" a little more detail on its purpose there.
    pid_t    si_pid;       /* Sending process ID */
    uid_t    si_uid;       /* Real user ID of sending process */
    int      si_status;    /* Exit value or signal */
    clock_t  si_utime;     /* User time consumed */
    clock_t  si_stime;     /* System time consumed */
    union sigval si_value; /* Signal value */
    int      si_int;       /* POSIX.1b signal */
    void    *si_ptr;       /* POSIX.1b signal */
    int      si_overrun;   /* Timer overrun count;
                              POSIX.1b timers */
    int      si_timerid;   /* Timer ID; POSIX.1b timers */
.\" In the kernel: si_tid
    void    *si_addr;      /* Memory location which caused fault */
    long     si_band;      /* Band event (was \fIint\fP in
                              glibc 2.3.2 and earlier) */
    int      si_fd;        /* File descriptor */
    short    si_addr_lsb;  /* Least significant bit of address
                              (since Linux 2.6.32) */
    void    *si_lower;     /* Lower bound when address violation
                              occurred (since Linux 3.19) */
    void    *si_upper;     /* Upper bound when address violation
                              occurred (since Linux 3.19) */
    int      si_pkey;      /* Protection key on PTE that caused
                              fault (since Linux 4.6) */
    void    *si_call_addr; /* Address of system call instruction
                              (since Linux 3.5) */
    int      si_syscall;   /* Number of attempted system call
                              (since Linux 3.5) */
    unsigned int si_arch;  /* Architecture of attempted system call
                              (since Linux 3.5) */
}
.EE
.in
.PP
.IR si_signo ", " si_errno " and " si_code
are defined for all signals.
.RI ( si_errno
is generally unused on Linux.)
The rest of the struct may be a union, so that one should
read only the fields that are meaningful for the given signal:
.IP \[bu] 3
Signals sent with
.BR kill (2)
and
.BR sigqueue (3)
fill in
.IR si_pid " and " si_uid .
In addition, signals sent with
.BR sigqueue (3)
fill in
.IR si_int " and " si_ptr
with the values specified by the sender of the signal;
see
.BR sigqueue (3)
for more details.
.IP \[bu]
Signals sent by POSIX.1b timers (since Linux 2.6) fill in
.I si_overrun
and
.IR si_timerid .
The
.I si_timerid
field is an internal ID used by the kernel to identify
the timer; it is not the same as the timer ID returned by
.BR timer_create (2).
The
.I si_overrun
field is the timer overrun count;
this is the same information as is obtained by a call to
.BR timer_getoverrun (2).
These fields are nonstandard Linux extensions.
.IP \[bu]
Signals sent for message queue notification (see the description of
.B SIGEV_SIGNAL
in
.BR mq_notify (3))
fill in
.IR si_int / si_ptr ,
with the
.I sigev_value
supplied to
.BR mq_notify (3);
.IR si_pid ,
with the process ID of the message sender; and
.IR si_uid ,
with the real user ID of the message sender.
.IP \[bu]
.B SIGCHLD
fills in
.IR si_pid ", " si_uid ", " si_status ", " si_utime ", and " si_stime ,
providing information about the child.
The
.I si_pid
field is the process ID of the child;
.I si_uid
is the child's real user ID.
The
.I si_status
field contains the exit status of the child (if
.I si_code
is
.BR CLD_EXITED ),
or the signal number that caused the process to change state.
The
.I si_utime
and
.I si_stime
contain the user and system CPU time used by the child process;
these fields do not include the times used by waited-for children (unlike
.BR getrusage (2)
and
.BR times (2)).
Up to Linux 2.6, and since Linux 2.6.27, these fields report
CPU time in units of
.IR sysconf(_SC_CLK_TCK) .
In Linux 2.6 kernels before Linux 2.6.27,
a bug meant that these fields reported time in units
of the (configurable) system jiffy (see
.BR time (7)).
.\" FIXME .
.\" When si_utime and si_stime where originally implemented, the
.\" measurement unit was HZ, which was the same as clock ticks
.\" (sysconf(_SC_CLK_TCK)).  In Linux 2.6, HZ became configurable, and
.\" was *still* used as the unit to return the info these fields,
.\" with the result that the field values depended on the
.\" configured HZ.  Of course, the should have been measured in
.\" USER_HZ instead, so that sysconf(_SC_CLK_TCK) could be used to
.\" convert to seconds.  I have a queued patch to fix this:
.\" http://thread.gmane.org/gmane.linux.kernel/698061/ .
.\" This patch made it into Linux 2.6.27.
.\" But note that these fields still don't return the times of
.\" waited-for children (as is done by getrusage() and times()
.\" and wait4()).  Solaris 8 does include child times.
.IP \[bu]
.BR SIGILL ,
.BR SIGFPE ,
.BR SIGSEGV ,
.BR SIGBUS ,
and
.B SIGTRAP
fill in
.I si_addr
with the address of the fault.
On some architectures,
these signals also fill in the
.I si_trapno
field.
.IP
Some suberrors of
.BR SIGBUS ,
in particular
.B BUS_MCEERR_AO
and
.BR BUS_MCEERR_AR ,
also fill in
.IR si_addr_lsb .
This field indicates the least significant bit of the reported address
and therefore the extent of the corruption.
For example, if a full page was corrupted,
.I si_addr_lsb
contains
.IR log2(sysconf(_SC_PAGESIZE)) .
When
.B SIGTRAP
is delivered in response to a
.BR ptrace (2)
event (PTRACE_EVENT_foo),
.I si_addr
is not populated, but
.I si_pid
and
.I si_uid
are populated with the respective process ID and user ID responsible for
delivering the trap.
In the case of
.BR seccomp (2),
the tracee will be shown as delivering the event.
.B BUS_MCEERR_*
and
.I si_addr_lsb
are Linux-specific extensions.
.IP
The
.B SEGV_BNDERR
suberror of
.B SIGSEGV
populates
.I si_lower
and
.IR si_upper .
.IP
The
.B SEGV_PKUERR
suberror of
.B SIGSEGV
populates
.IR si_pkey .
.IP \[bu]
.BR SIGIO / SIGPOLL
(the two names are synonyms on Linux)
fills in
.I si_band
and
.IR si_fd .
The
.I si_band
event is a bit mask containing the same values as are filled in the
.I revents
field by
.BR poll (2).
The
.I si_fd
field indicates the file descriptor for which the I/O event occurred;
for further details, see the description of
.B F_SETSIG
in
.BR fcntl (2).
.IP \[bu]
.BR SIGSYS ,
generated (since Linux 3.5)
.\" commit a0727e8ce513fe6890416da960181ceb10fbfae6
when a seccomp filter returns
.BR SECCOMP_RET_TRAP ,
fills in
.IR si_call_addr ,
.IR si_syscall ,
.IR si_arch ,
.IR si_errno ,
and other fields as described in
.BR seccomp (2).
.\"
.SS
The si_code field
The
.I si_code
field inside the
.I siginfo_t
argument that is passed to a
.B SA_SIGINFO
signal handler is a value (not a bit mask)
indicating why this signal was sent.
For a
.BR ptrace (2)
event,
.I si_code
will contain
.B SIGTRAP
and have the ptrace event in the high byte:
.PP
.in +4n
.EX
(SIGTRAP | PTRACE_EVENT_foo << 8).
.EE
.in
.PP
For a
.RB non- ptrace (2)
event, the values that can appear in
.I si_code
are described in the remainder of this section.
Since glibc 2.20,
the definitions of most of these symbols are obtained from
.I <signal.h>
by defining feature test macros (before including
.I any
header file) as follows:
.IP \[bu] 3
.B _XOPEN_SOURCE
with the value 500 or greater;
.IP \[bu]
.B _XOPEN_SOURCE
and
.BR _XOPEN_SOURCE_EXTENDED ;
or
.IP \[bu]
.B _POSIX_C_SOURCE
with the value 200809L or greater.
.PP
For the
.B TRAP_*
constants, the symbol definitions are provided only in the first two cases.
Before glibc 2.20, no feature test macros were required to obtain these symbols.
.PP
For a regular signal, the following list shows the values which can be
placed in
.I si_code
for any signal, along with the reason that the signal was generated.
.RS 4
.TP
.B SI_USER
.BR kill (2).
.TP
.B SI_KERNEL
Sent by the kernel.
.TP
.B SI_QUEUE
.BR sigqueue (3).
.TP
.B SI_TIMER
POSIX timer expired.
.TP
.BR SI_MESGQ " (since Linux 2.6.6)"
POSIX message queue state changed; see
.BR mq_notify (3).
.TP
.B SI_ASYNCIO
AIO completed.
.TP
.B SI_SIGIO
Queued
.B SIGIO
(only up to Linux 2.2; from Linux 2.4 onward
.BR SIGIO / SIGPOLL
fills in
.I si_code
as described below).
.TP
.BR SI_TKILL " (since Linux 2.4.19)"
.BR tkill (2)
or
.BR tgkill (2).
.\" SI_DETHREAD is defined in Linux 2.6.9 sources, but isn't implemented
.\" It appears to have been an idea that was tried during 2.5.6
.\" through to Linux 2.5.24 and then was backed out.
.RE
.PP
The following values can be placed in
.I si_code
for a
.B SIGILL
signal:
.RS 4
.TP
.B ILL_ILLOPC
Illegal opcode.
.TP
.B ILL_ILLOPN
Illegal operand.
.TP
.B ILL_ILLADR
Illegal addressing mode.
.TP
.B ILL_ILLTRP
Illegal trap.
.TP
.B ILL_PRVOPC
Privileged opcode.
.TP
.B ILL_PRVREG
Privileged register.
.TP
.B ILL_COPROC
Coprocessor error.
.TP
.B ILL_BADSTK
Internal stack error.
.RE
.PP
The following values can be placed in
.I si_code
for a
.B SIGFPE
signal:
.RS 4
.TP
.B FPE_INTDIV
Integer divide by zero.
.TP
.B FPE_INTOVF
Integer overflow.
.TP
.B FPE_FLTDIV
Floating-point divide by zero.
.TP
.B FPE_FLTOVF
Floating-point overflow.
.TP
.B FPE_FLTUND
Floating-point underflow.
.TP
.B FPE_FLTRES
Floating-point inexact result.
.TP
.B FPE_FLTINV
Floating-point invalid operation.
.TP
.B FPE_FLTSUB
Subscript out of range.
.RE
.PP
The following values can be placed in
.I si_code
for a
.B SIGSEGV
signal:
.RS 4
.TP
.B SEGV_MAPERR
Address not mapped to object.
.TP
.B SEGV_ACCERR
Invalid permissions for mapped object.
.TP
.BR SEGV_BNDERR " (since Linux 3.19)"
.\" commit ee1b58d36aa1b5a79eaba11f5c3633c88231da83
Failed address bound checks.
.TP
.BR SEGV_PKUERR " (since Linux 4.6)"
.\" commit cd0ea35ff5511cde299a61c21a95889b4a71464e
Access was denied by memory protection keys.
See
.BR pkeys (7).
The protection key which applied to this access is available via
.IR si_pkey .
.RE
.PP
The following values can be placed in
.I si_code
for a
.B SIGBUS
signal:
.RS 4
.TP
.B BUS_ADRALN
Invalid address alignment.
.TP
.B BUS_ADRERR
Nonexistent physical address.
.TP
.B BUS_OBJERR
Object-specific hardware error.
.TP
.BR BUS_MCEERR_AR " (since Linux 2.6.32)"
Hardware memory error consumed on a machine check; action required.
.TP
.BR BUS_MCEERR_AO " (since Linux 2.6.32)"
Hardware memory error detected in process but not consumed; action optional.
.RE
.PP
The following values can be placed in
.I si_code
for a
.B SIGTRAP
signal:
.RS 4
.TP
.B TRAP_BRKPT
Process breakpoint.
.TP
.B TRAP_TRACE
Process trace trap.
.TP
.BR TRAP_BRANCH " (since Linux 2.4, IA64 only)"
Process taken branch trap.
.TP
.BR TRAP_HWBKPT " (since Linux 2.4, IA64 only)"
Hardware breakpoint/watchpoint.
.RE
.PP
The following values can be placed in
.I si_code
for a
.B SIGCHLD
signal:
.RS 4
.TP
.B CLD_EXITED
Child has exited.
.TP
.B CLD_KILLED
Child was killed.
.TP
.B CLD_DUMPED
Child terminated abnormally.
.TP
.B CLD_TRAPPED
Traced child has trapped.
.TP
.B CLD_STOPPED
Child has stopped.
.TP
.BR CLD_CONTINUED " (since Linux 2.6.9)"
Stopped child has continued.
.RE
.PP
The following values can be placed in
.I si_code
for a
.BR SIGIO / SIGPOLL
signal:
.RS 4
.TP
.B POLL_IN
Data input available.
.TP
.B POLL_OUT
Output buffers available.
.TP
.B POLL_MSG
Input message available.
.TP
.B POLL_ERR
I/O error.
.TP
.B POLL_PRI
High priority input available.
.TP
.B POLL_HUP
Device disconnected.
.RE
.PP
The following value can be placed in
.I si_code
for a
.B SIGSYS
signal:
.RS 4
.TP
.BR SYS_SECCOMP " (since Linux 3.5)"
Triggered by a
.BR seccomp (2)
filter rule.
.RE
.SS Dynamically probing for flag bit support
The
.BR sigaction ()
call on Linux accepts unknown bits set in
.I act\->sa_flags
without error.
The behavior of the kernel starting with Linux 5.11 is that a second
.BR sigaction ()
will clear unknown bits from
.IR oldact\->sa_flags .
However, historically, a second
.BR sigaction ()
call would typically leave those bits set in
.IR oldact\->sa_flags .
.PP
This means that support for new flags cannot be detected
simply by testing for a flag in
.IR sa_flags ,
and a program must test that
.B SA_UNSUPPORTED
has been cleared before relying on the contents of
.IR sa_flags .
.PP
Since the behavior of the signal handler cannot be guaranteed
unless the check passes,
it is wise to either block the affected signal
while registering the handler and performing the check in this case,
or where this is not possible,
for example if the signal is synchronous, to issue the second
.BR sigaction ()
in the signal handler itself.
.PP
In kernels that do not support a specific flag,
the kernel's behavior is as if the flag was not set,
even if the flag was set in
.IR act\->sa_flags .
.PP
The flags
.BR SA_NOCLDSTOP ,
.BR SA_NOCLDWAIT ,
.BR SA_SIGINFO ,
.BR SA_ONSTACK ,
.BR SA_RESTART ,
.BR SA_NODEFER ,
.BR SA_RESETHAND ,
and, if defined by the architecture,
.B SA_RESTORER
may not be reliably probed for using this mechanism,
because they were introduced before Linux 5.11.
However, in general, programs may assume that these flags are supported,
since they have all been supported since Linux 2.6,
which was released in the year 2003.
.PP
See EXAMPLES below for a demonstration of the use of
.BR SA_UNSUPPORTED .
.SH RETURN VALUE
.BR sigaction ()
returns 0 on success; on error, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EFAULT
.IR act " or " oldact
points to memory which is not a valid part of the process address space.
.TP
.B EINVAL
An invalid signal was specified.
This will also be generated if an attempt
is made to change the action for
.BR SIGKILL " or " SIGSTOP ,
which cannot be caught or ignored.
.SH VERSIONS
.SS C library/kernel differences
The glibc wrapper function for
.BR sigaction ()
gives an error
.RB ( EINVAL )
on attempts to change the disposition of the two real-time signals
used internally by the NPTL threading implementation.
See
.BR nptl (7)
for details.
.PP
On architectures where the signal trampoline resides in the C library,
the glibc wrapper function for
.BR sigaction ()
places the address of the trampoline code in the
.I act.sa_restorer
field and sets the
.B SA_RESTORER
flag in the
.I act.sa_flags
field.
See
.BR sigreturn (2).
.PP
The original Linux system call was named
.BR sigaction ().
However, with the addition of real-time signals in Linux 2.2,
the fixed-size, 32-bit
.I sigset_t
type supported by that system call was no longer fit for purpose.
Consequently, a new system call,
.BR rt_sigaction (),
was added to support an enlarged
.I sigset_t
type.
The new system call takes a fourth argument,
.IR "size_t sigsetsize" ,
which specifies the size in bytes of the signal sets in
.I act.sa_mask
and
.IR oldact.sa_mask .
This argument is currently required to have the value
.I sizeof(sigset_t)
(or the error
.B EINVAL
results).
The glibc
.BR sigaction ()
wrapper function hides these details from us, transparently calling
.BR rt_sigaction ()
when the kernel provides it.
.SH STANDARDS
POSIX.1-2008.
.SH HISTORY
POSIX.1-2001, SVr4.
.\" SVr4 does not document the EINTR condition.
.PP
POSIX.1-1990 disallowed setting the action for
.B SIGCHLD
to
.BR SIG_IGN .
POSIX.1-2001 and later allow this possibility, so that ignoring
.B SIGCHLD
can be used to prevent the creation of zombies (see
.BR wait (2)).
Nevertheless, the historical BSD and System\ V behaviors for ignoring
.B SIGCHLD
differ, so that the only completely portable method of ensuring that
terminated children do not become zombies is to catch the
.B SIGCHLD
signal and perform a
.BR wait (2)
or similar.
.PP
POSIX.1-1990 specified only
.BR SA_NOCLDSTOP .
POSIX.1-2001 added
.BR SA_NOCLDSTOP ,
.BR SA_NOCLDWAIT ,
.BR SA_NODEFER ,
.BR SA_ONSTACK ,
.BR SA_RESETHAND ,
.BR SA_RESTART ,
and
.BR SA_SIGINFO .
Use of these latter values in
.I sa_flags
may be less portable in applications intended for older
UNIX implementations.
.PP
The
.B SA_RESETHAND
flag is compatible with the SVr4 flag of the same name.
.PP
The
.B SA_NODEFER
flag is compatible with the SVr4 flag of the same name under kernels
1.3.9 and later.
On older kernels the Linux implementation
allowed the receipt of any signal, not just the one we are installing
(effectively overriding any
.I sa_mask
settings).
.SH NOTES
A child created via
.BR fork (2)
inherits a copy of its parent's signal dispositions.
During an
.BR execve (2),
the dispositions of handled signals are reset to the default;
the dispositions of ignored signals are left unchanged.
.PP
According to POSIX, the behavior of a process is undefined after it
ignores a
.BR SIGFPE ,
.BR SIGILL ,
or
.B SIGSEGV
signal that was not generated by
.BR kill (2)
or
.BR raise (3).
Integer division by zero has undefined result.
On some architectures it will generate a
.B SIGFPE
signal.
(Also dividing the most negative integer by \-1 may generate
.BR SIGFPE .)
Ignoring this signal might lead to an endless loop.
.PP
.BR sigaction ()
can be called with a NULL second argument to query the current signal
handler.
It can also be used to check whether a given signal is valid for
the current machine by calling it with NULL second and third arguments.
.PP
It is not possible to block
.BR SIGKILL " or " SIGSTOP
(by specifying them in
.IR sa_mask ).
Attempts to do so are silently ignored.
.PP
See
.BR sigsetops (3)
for details on manipulating signal sets.
.PP
See
.BR signal\-safety (7)
for a list of the async-signal-safe functions that can be
safely called inside from inside a signal handler.
.\"
.SS Undocumented
Before the introduction of
.BR SA_SIGINFO ,
it was also possible to get some additional information about the signal.
This was done by providing an
.I sa_handler
signal handler with a second argument of type
.IR "struct sigcontext" ,
which is the same structure as the one that is passed in the
.I uc_mcontext
field of the
.I ucontext
structure that is passed (via a pointer) in the third argument of the
.I sa_sigaction
handler.
See the relevant Linux kernel sources for details.
This use is obsolete now.
.SH BUGS
When delivering a signal with a
.B SA_SIGINFO
handler,
the kernel does not always provide meaningful values
for all of the fields of the
.I siginfo_t
that are relevant for that signal.
.PP
Up to and including Linux 2.6.13, specifying
.B SA_NODEFER
in
.I sa_flags
prevents not only the delivered signal from being masked during
execution of the handler, but also the signals specified in
.IR sa_mask .
This bug was fixed in Linux 2.6.14.
.\" commit 69be8f189653cd81aae5a74e26615b12871bb72e
.SH EXAMPLES
See
.BR mprotect (2).
.SS Probing for flag support
The following example program exits with status
.B EXIT_SUCCESS
if
.B SA_EXPOSE_TAGBITS
is determined to be supported, and
.B EXIT_FAILURE
otherwise.
.PP
.\" SRC BEGIN (sigaction.c)
.EX
#include <signal.h>
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
\&
void
handler(int signo, siginfo_t *info, void *context)
{
    struct sigaction oldact;
\&
    if (sigaction(SIGSEGV, NULL, &oldact) == \-1
        || (oldact.sa_flags & SA_UNSUPPORTED)
        || !(oldact.sa_flags & SA_EXPOSE_TAGBITS))
    {
        _exit(EXIT_FAILURE);
    }
    _exit(EXIT_SUCCESS);
}
\&
int
main(void)
{
    struct sigaction act = { 0 };
\&
    act.sa_flags = SA_SIGINFO | SA_UNSUPPORTED | SA_EXPOSE_TAGBITS;
    act.sa_sigaction = &handler;
    if (sigaction(SIGSEGV, &act, NULL) == \-1) {
        perror("sigaction");
        exit(EXIT_FAILURE);
    }
\&
    raise(SIGSEGV);
}
.EE
.\" SRC END
.SH SEE ALSO
.BR kill (1),
.BR kill (2),
.BR pause (2),
.BR pidfd_send_signal (2),
.BR restart_syscall (2),
.BR seccomp (2),
.BR sigaltstack (2),
.BR signal (2),
.BR signalfd (2),
.BR sigpending (2),
.BR sigprocmask (2),
.BR sigreturn (2),
.BR sigsuspend (2),
.BR wait (2),
.BR killpg (3),
.BR raise (3),
.BR siginterrupt (3),
.BR sigqueue (3),
.BR sigsetops (3),
.BR sigvec (3),
.BR core (5),
.BR signal (7)