sigaction, rt_sigaction — examine and change a signal action
#include <signal.h>
int
sigaction( |
int signum, |
| const struct sigaction *act, | |
struct sigaction *oldact); |
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Note | |||
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The sigaction() system call
is used to change the action taken by a process on receipt of
a specific signal. (See signal(7) for an overview
of signals.)
signum specifies
the signal and can be any valid signal except SIGKILL and SIGSTOP.
If act is
non-NULL, the new action for signal signum is installed from
act. If oldact is non-NULL, the
previous action is saved in oldact.
The sigaction structure is defined as something like:
struct sigaction { void (* sa_handler)(int);void (* sa_sigaction)(int, siginfo_t *, void *);sigset_t sa_mask;int sa_flags;void (* sa_restorer)(void);};
On some architectures a union is involved: do not assign
to both sa_handler
and sa_sigaction.
The sa_restorer
field is not intended for application use. (POSIX does not
specify a sa_restorer
field.) Some further details of purpose of this field can be
found in sigreturn(2).
sa_handler
specifies the action to be associated with signum and may be SIG_DFL for the default action,
SIG_IGN to ignore this signal,
or a pointer to a signal handling function. This function
receives the signal number as its only argument.
If SA_SIGINFO is specified
in sa_flags, then
sa_sigaction (instead
of sa_handler)
specifies the signal-handling function for signum. This function receives
the signal number as its first argument, a pointer to a
siginfo_t as its second argument
and a pointer to a ucontext_t (cast
to void *) as its
third argument. (Commonly, the handler function doesn't make
any use of the third argument. See getcontext(3) for further
information about ucontext_t.)
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 SA_NODEFER flag is
used.
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:
SA_NOCLDSTOPIf
signumisSIGCHLD, do not receive notification when child processes stop (i.e., when they receive one ofSIGSTOP,SIGTSTP,SIGTTIN, orSIGTTOU) or resume (i.e., they receiveSIGCONT) (see wait(2)). This flag is meaningful only when establishing a handler forSIGCHLD.SA_NOCLDWAIT(since Linux 2.6)If
signumisSIGCHLD, do not transform children into zombies when they terminate. See also waitpid(2). This flag is meaningful only when establishing a handler forSIGCHLD, or when setting that signal's disposition toSIG_DFL.If the
SA_NOCLDWAITflag is set when establishing a handler forSIGCHLD, POSIX.1 leaves it unspecified whether aSIGCHLDsignal is generated when a child process terminates. On Linux, aSIGCHLDsignal is generated in this case; on some other implementations, it is not.SA_NODEFERDo not prevent the signal from being received from within its own signal handler. This flag is meaningful only when establishing a signal handler.
SA_NOMASKis an obsolete, nonstandard synonym for this flag.SA_ONSTACKCall the signal handler on an alternate signal stack provided by 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.
SA_RESETHANDRestore the signal action to the default upon entry to the signal handler. This flag is meaningful only when establishing a signal handler.
SA_ONESHOTis an obsolete, nonstandard synonym for this flag.SA_RESTARTProvide 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 signal(7) for a discussion of system call restarting.
SA_RESTORERNot intended for application use. This flag is used by C libraries to indicate that the
sa_restorerfield contains the address of a "signal trampoline". See sigreturn(2) for more details.SA_SIGINFO(since Linux 2.2)The signal handler takes three arguments, not one. In this case,
sa_sigactionshould be set instead ofsa_handler. This flag is meaningful only when establishing a signal handler.
The siginfo_t argument to
sa_sigaction is a
struct with the following fields:
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) */ 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 */ sigval_t 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 */ void *si_addr; /* Memory location which caused fault */ long si_band; /* Band event (was int 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_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) */ }
si_signo,
si_errno and
si_code are defined
for all signals. (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:
Signals sent with kill(2) and sigqueue(3) fill in
si_pid and
si_uid. In
addition, signals sent with sigqueue(3) fill in
si_int and
si_ptr with
the values specified by the sender of the signal; see
sigqueue(3) for more
details.
Signals sent by POSIX.1b timers (since Linux 2.6)
fill in si_overrun and
si_timerid.
The 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
timer_create(2). The
si_overrun
field is the timer overrun count; this is the same
information as is obtained by a call to timer_getoverrun(2).
These fields are nonstandard Linux extensions.
Signals sent for message queue notification (see the
description of SIGEV_SIGNAL in mq_notify(3)) fill in
si_int/si_ptr, with the
sigev_value
supplied to mq_notify(3);
si_pid, with
the process ID of the message sender; and si_uid, with the real
user ID of the message sender.
SIGCHLD fills in
si_pid,
si_uid,
si_status,
si_utime, and
si_stime,
providing information about the child. The si_pid field is the
process ID of the child; si_uid is the child's
real user ID. The si_status field
contains the exit status of the child (if si_code is CLD_EXITED), or the signal number
that caused the process to change state. The si_utime and 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 getrusage(2) and
times(2)). In kernels
up to 2.6, and since 2.6.27, these fields report CPU
time in units of sysconf(_SC_CLK_TCK).
In 2.6 kernels before 2.6.27, a bug meant that these
fields reported time in units of the (configurable)
system jiffy (see time(7)).
SIGILL, SIGFPE, SIGSEGV, SIGBUS, and SIGTRAP fill in si_addr with the
address of the fault. On some architectures, these
signals also fill in the si_trapno field. Some
suberrors of SIGBUS, in
particular BUS_MCEERR_AO
and BUS_MCEERR_AR, also
fill in 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, si_addr_lsb contains
log2(sysconf(_SC_PAGESIZE)).
When SIGTRAP is delivered
in response to a ptrace(2) event
(PTRACE_EVENT_foo), si_addr is not
populated, but si_pid and si_uid are populated
with the respective process ID and user ID responsible
for delivering the trap. In the case of seccomp(2), the
tracee will be shown as delivering the event.
BUS_MCEERR_*
and si_addr_lsb are
Linux-specific extensions.
SIGIO/SIGPOLL (the two names are synonyms
on Linux) fills in si_band and si_fd. The si_band event is a bit
mask containing the same values as are filled in the
revents field
by poll(2). The
si_fd field
indicates the file descriptor for which the I/O event
occurred; for further details, see the description of
F_SETSIG in fcntl(2).
SIGSYS, generated
(since Linux 3.5) when a seccomp filter returns
SECCOMP_RET_TRAP, fills
in si_call_addr,
si_syscall,
si_arch,
si_errno, and
other fields as described in seccomp(2).
si_code is a
value (not a bit mask) indicating why this signal was sent.
For a ptrace(2) event, si_code will contain
SIGTRAP and have the ptrace
event in the high byte:
(SIGTRAP | PTRACE_EVENT_foo << 8).
For a regular signal, the following list shows the values
which can be placed in si_code for any signal, along
with reason that the signal was generated.
SI_USERSI_KERNELSent by the kernel.
SI_QUEUESI_TIMERPOSIX timer expired.
SI_MESGQ(since Linux 2.6.6)POSIX message queue state changed; see mq_notify(3).
SI_ASYNCIOAIO completed.
SI_SIGIOQueued
SIGIO(only in kernels up to Linux 2.2; from Linux 2.4 onwardSIGIO/SIGPOLLfills insi_codeas described below).SI_TKILL(since Linux 2.4.19)
The following values can be placed in si_code for a SIGILL signal:
ILL_ILLOPCIllegal opcode.
ILL_ILLOPNIllegal operand.
ILL_ILLADRIllegal addressing mode.
ILL_ILLTRPIllegal trap.
ILL_PRVOPCPrivileged opcode.
ILL_PRVREGPrivileged register.
ILL_COPROCCoprocessor error.
ILL_BADSTKInternal stack error.
The following values can be placed in si_code for a SIGFPE signal:
FPE_INTDIVInteger divide by zero.
FPE_INTOVFInteger overflow.
FPE_FLTDIVFloating-point divide by zero.
FPE_FLTOVFFloating-point overflow.
FPE_FLTUNDFloating-point underflow.
FPE_FLTRESFloating-point inexact result.
FPE_FLTINVFloating-point invalid operation.
FPE_FLTSUBSubscript out of range.
The following values can be placed in si_code for a SIGSEGV signal:
SEGV_MAPERRAddress not mapped to object.
SEGV_ACCERRInvalid permissions for mapped object.
The following values can be placed in si_code for a SIGBUS signal:
BUS_ADRALNInvalid address alignment.
BUS_ADRERRNonexistent physical address.
BUS_OBJERRObject-specific hardware error.
BUS_MCEERR_AR(since Linux 2.6.32)Hardware memory error consumed on a machine check; action required.
BUS_MCEERR_AO(since Linux 2.6.32)Hardware memory error detected in process but not consumed; action optional.
The following values can be placed in si_code for a SIGTRAP signal:
TRAP_BRKPTProcess breakpoint.
TRAP_TRACEProcess trace trap.
TRAP_BRANCH(since Linux 2.4)Process taken branch trap.
TRAP_HWBKPT(since Linux 2.4)Hardware breakpoint/watchpoint.
The following values can be placed in si_code for a SIGCHLD signal:
CLD_EXITEDChild has exited.
CLD_KILLEDChild was killed.
CLD_DUMPEDChild terminated abnormally.
CLD_TRAPPEDTraced child has trapped.
CLD_STOPPEDChild has stopped.
CLD_CONTINUED(since Linux 2.6.9)Stopped child has continued.
The following values can be placed in si_code for a SIGIO/SIGPOLL
signal:
POLL_INData input available.
POLL_OUTOutput buffers available.
POLL_MSGInput message available.
POLL_ERRI/O error.
POLL_PRIHigh priority input available.
POLL_HUPDevice disconnected.
The following value can be placed in si_code for a SIGSYS signal:
SYS_SECCOMP(since Linux 3.5)Triggered by a seccomp(2) filter rule.
sigaction() returns 0 on
success; on error, −1 is returned, and errno is set to indicate the error.
act or
oldact points
to memory which is not a valid part of the process
address space.
An invalid signal was specified. This will also be
generated if an attempt is made to change the action
for SIGKILL or
SIGSTOP, which cannot be
caught or ignored.
A child created via fork(2) inherits a copy of its parent's signal dispositions. During an execve(2), the dispositions of handled signals are reset to the default; the dispositions of ignored signals are left unchanged.
According to POSIX, the behavior of a process is undefined
after it ignores a SIGFPE,
SIGILL, or SIGSEGV signal that was not generated by
kill(2) or raise(3). Integer division
by zero has undefined result. On some architectures it will
generate a SIGFPE signal. (Also
dividing the most negative integer by −1 may generate
SIGFPE.) Ignoring this signal
might lead to an endless loop.
POSIX.1-1990 disallowed setting the action for
SIGCHLD to SIG_IGN. POSIX.1-2001 and later allow this
possibility, so that ignoring SIGCHLD can be used to prevent the creation
of zombies (see wait(2)). Nevertheless, the
historical BSD and System V behaviors for ignoring
SIGCHLD differ, so that the
only completely portable method of ensuring that terminated
children do not become zombies is to catch the SIGCHLD signal and perform a wait(2) or similar.
POSIX.1-1990 specified only SA_NOCLDSTOP. POSIX.1-2001 added
SA_NOCLDSTOP, SA_NOCLDWAIT, SA_NODEFER, SA_ONSTACK, SA_RESETHAND, SA_RESTART, and SA_SIGINFO. Use of these latter values in
sa_flags may be less
portable in applications intended for older UNIX
implementations.
The SA_RESETHAND flag is
compatible with the SVr4 flag of the same name.
The SA_NODEFER flag is
compatible with the SVr4 flag of the same name under kernels
1.3.9 and newer. On older kernels the Linux implementation
allowed the receipt of any signal, not just the one we are
installing (effectively overriding any sa_mask settings).
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.
It is not possible to block SIGKILL or SIGSTOP (by specifying them in sa_mask). Attempts to do so are
silently ignored.
See sigsetops(3) for details on manipulating signal sets.
See signal(7) for a list of the async-signal-safe functions that can be safely called inside from inside a signal handler.
The glibc wrapper function for sigaction() gives an error (EINVAL) on attempts to change the
disposition of the two real-time signals used internally by
the NPTL threading implementation. See nptl(7) for details.
The original Linux system call was named sigaction(). However, with the addition
of real-time signals in Linux 2.2, the fixed-size, 32-bit
sigset_t type supported by that
system call was no longer fit for purpose. Consequently, a
new system call, rt_sigaction(), was added to support an
enlarged sigset_t type. The new
system call takes a fourth argument, size_t sigsetsize, which
specifies the size in bytes of the signal sets in
act.sa_mask and
oldact.sa_mask.
This argument is currently required to have the value
sizeof(sigset_t)
(or the error EINVAL
results). The glibc sigaction() wrapper function hides these
details from us, transparently calling rt_sigaction() when the kernel provides
it.
In kernels up to and including 2.6.13, specifying
SA_NODEFER in sa_flags prevents not only the
delivered signal from being masked during execution of the
handler, but also the signals specified in sa_mask. This bug was fixed in
kernel 2.6.14.
kill(1), kill(2), killpg(2), pause(2), restart_syscall(2), seccomp(2) sigaltstack(2), signal(2), signalfd(2), sigpending(2), sigreturn(2), sigprocmask(2), sigsuspend(2), wait(2), raise(3), siginterrupt(3), sigqueue(3), sigsetops(3), sigvec(3), core(5), signal(7)
This page is part of release 4.04 of the Linux man-pages project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
http://www.kernel.org/doc/man−pages/.
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t Copyright (c) 1994,1995 Mike Battersby <mibdeakin.edu.au> and Copyright 2004, 2005 Michael Kerrisk <mtk.manpagesgmail.com> based on work by faithcs.unc.edu %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END Modified, aeb, 960424 Modified Fri Jan 31 17:31:20 1997 by Eric S. Raymond <esrthyrsus.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 <ejonesuwaterloo.ca> SA_ONSTACK Modified 2004-11-11 by Michael Kerrisk <mtk.manpagesgmail.com> Added mention of SIGCONT under SA_NOCLDSTOP Added SA_NOCLDWAIT Modified 2004-11-17 by Michael Kerrisk <mtk.manpagesgmail.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 <keescookchromium.org> Added notes on ptrace SIGTRAP and SYS_SECCOMP. |