[thirdparty/man-pages.git] / man7 / signal.7

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.\" Copyright (c) 1993 by Thomas Koenig (ig25@rz.uni-karlsruhe.de)
.\" and Copyright (c) 2002, 2006 by Michael Kerrisk <mtk.manpages@gmail.com>
.\" and Copyright (c) 2008 Linux Foundation, written by Michael Kerrisk
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.\" Modified Sat Jul 24 17:34:08 1993 by Rik Faith (faith@cs.unc.edu)
.\" Modified Sun Jan  7 01:41:27 1996 by Andries Brouwer (aeb@cwi.nl)
.\" Modified Sun Apr 14 12:02:29 1996 by Andries Brouwer (aeb@cwi.nl)
.\" Modified Sat Nov 13 16:28:23 1999 by Andries Brouwer (aeb@cwi.nl)
.\" Modified 10 Apr 2002, by Michael Kerrisk <mtk.manpages@gmail.com>
.\" Modified  7 Jun 2002, by Michael Kerrisk <mtk.manpages@gmail.com>
.\"     Added information on real-time signals
.\" Modified 13 Jun 2002, by Michael Kerrisk <mtk.manpages@gmail.com>
.\"     Noted that SIGSTKFLT is in fact unused
.\" 2004-12-03, Modified mtk, added notes on RLIMIT_SIGPENDING
.\" 2006-04-24, mtk, Added text on changing signal dispositions,
.\"             signal mask, and pending signals.
.\" 2008-07-04, mtk:
.\"     Added section on system call restarting (SA_RESTART)
.\"     Added section on stop/cont signals interrupting syscalls.
.\" 2008-10-05, mtk: various additions
.\"
.TH SIGNAL 7  2019-03-06 "Linux" "Linux Programmer's Manual"
.SH NAME
signal \- overview of signals
.SH DESCRIPTION
Linux supports both POSIX reliable signals (hereinafter
"standard signals") and POSIX real-time signals.
.SS Signal dispositions
Each signal has a current
.IR disposition ,
which determines how the process behaves when it is delivered
the signal.
.PP
The entries in the "Action" column of the table below specify
the default disposition for each signal, as follows:
.IP Term
Default action is to terminate the process.
.IP Ign
Default action is to ignore the signal.
.IP Core
Default action is to terminate the process and dump core (see
.BR core (5)).
.IP Stop
Default action is to stop the process.
.IP Cont
Default action is to continue the process if it is currently stopped.
.PP
A process can change the disposition of a signal using
.BR sigaction (2)
or
.BR signal (2).
(The latter is less portable when establishing a signal handler;
see
.BR signal (2)
for details.)
Using these system calls, a process can elect one of the
following behaviors to occur on delivery of the signal:
perform the default action; ignore the signal;
or catch the signal with a
.IR "signal handler" ,
a programmer-defined function that is automatically invoked
when the signal is delivered.
.PP
By default, a signal handler is invoked on the
normal process stack.
It is possible to arrange that the signal handler
uses an alternate stack; see
.BR sigaltstack (2)
for a discussion of how to do this and when it might be useful.
.PP
The signal disposition is a per-process attribute:
in a multithreaded application, the disposition of a
particular signal is the same for all threads.
.PP
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.
.SS Sending a signal
The following system calls and library functions allow
the caller to send a signal:
.TP 16
.BR raise (3)
Sends a signal to the calling thread.
.TP
.BR kill (2)
Sends a signal to a specified process,
to all members of a specified process group,
or to all processes on the system.
.TP
.BR killpg (3)
Sends a signal to all of the members of a specified process group.
.TP
.BR pthread_kill (3)
Sends a signal to a specified POSIX thread in the same process as
the caller.
.TP
.BR tgkill (2)
Sends a signal to a specified thread within a specific process.
(This is the system call used to implement
.BR pthread_kill (3).)
.TP
.BR sigqueue (3)
Sends a real-time signal with accompanying data to a specified process.
.SS Waiting for a signal to be caught
The following system calls suspend execution of the calling
thread until a signal is caught
(or an unhandled signal terminates the process):
.TP 16
.BR pause (2)
Suspends execution until any signal is caught.
.TP
.BR sigsuspend (2)
Temporarily changes the signal mask (see below) and suspends
execution until one of the unmasked signals is caught.
.SS Synchronously accepting a signal
Rather than asynchronously catching a signal via a signal handler,
it is possible to synchronously accept the signal, that is,
to block execution until the signal is delivered,
at which point the kernel returns information about the
signal to the caller.
There are two general ways to do this:
.IP * 2
.BR sigwaitinfo (2),
.BR sigtimedwait (2),
and
.BR sigwait (3)
suspend execution until one of the signals in a specified
set is delivered.
Each of these calls returns information about the delivered signal.
.IP *
.BR signalfd (2)
returns a file descriptor that can be used to read information
about signals that are delivered to the caller.
Each
.BR read (2)
from this file descriptor blocks until one of the signals
in the set specified in the
.BR signalfd (2)
call is delivered to the caller.
The buffer returned by
.BR read (2)
contains a structure describing the signal.
.SS Signal mask and pending signals
A signal may be
.IR blocked ,
which means that it will not be delivered until it is later unblocked.
Between the time when it is generated and when it is delivered
a signal is said to be
.IR pending .
.PP
Each thread in a process has an independent
.IR "signal mask" ,
which indicates the set of signals that the thread is currently blocking.
A thread can manipulate its signal mask using
.BR pthread_sigmask (3).
In a traditional single-threaded application,
.BR sigprocmask (2)
can be used to manipulate the signal mask.
.PP
A child created via
.BR fork (2)
inherits a copy of its parent's signal mask;
the signal mask is preserved across
.BR execve (2).
.PP
A signal may be generated (and thus pending)
for a process as a whole (e.g., when sent using
.BR kill (2))
or for a specific thread (e.g., certain signals,
such as
.B SIGSEGV
and
.BR SIGFPE ,
generated as a
consequence of executing a specific machine-language instruction
are thread directed, as are signals targeted at a specific thread using
.BR pthread_kill (3)).
A process-directed signal may be delivered to any one of the
threads that does not currently have the signal blocked.
If more than one of the threads has the signal unblocked, then the
kernel chooses an arbitrary thread to which to deliver the signal.
.PP
A thread can obtain the set of signals that it currently has pending
using
.BR sigpending (2).
This set will consist of the union of the set of pending
process-directed signals and the set of signals pending for
the calling thread.
.PP
A child created via
.BR fork (2)
initially has an empty pending signal set;
the pending signal set is preserved across an
.BR execve (2).
.SS Standard signals
Linux supports the standard signals listed below.
The second column of the table indicates which standard (if any)
specified the signal: "P1990" indicates that the signal is described
in the original POSIX.1-1990 standard;
"P2001" indicates that the signal was added in SUSv2 and POSIX.1-2001.
.TS
l c c l
____
lB c c l.
Signal  Standard        Action  Comment
SIGABRT P1990   Core    Abort signal from \fBabort\fP(3)
SIGALRM P1990   Term    Timer signal from \fBalarm\fP(2)
SIGBUS  P2001   Core    Bus error (bad memory access)
SIGCHLD P1990   Ign     Child stopped or terminated
SIGCLD  \-      Ign     A synonym for \fBSIGCHLD\fP
SIGCONT P1990   Cont    Continue if stopped
SIGEMT  \-      Term    Emulator trap
SIGFPE  P1990   Core    Floating-point exception
SIGHUP  P1990   Term    Hangup detected on controlling terminal
                        or death of controlling process
SIGILL  P1990   Core    Illegal Instruction
SIGINFO \-              A synonym for \fBSIGPWR\fP
SIGINT  P1990   Term    Interrupt from keyboard
SIGIO   \-      Term    I/O now possible (4.2BSD)
SIGIOT  \-      Core    IOT trap. A synonym for \fBSIGABRT\fP
SIGKILL P1990   Term    Kill signal
SIGLOST \-      Term    File lock lost (unused)
SIGPIPE P1990   Term    Broken pipe: write to pipe with no
                        readers; see \fBpipe\fP(7)
SIGPOLL P2001   Term    Pollable event (Sys V).
                        Synonym for \fBSIGIO\fP
SIGPROF P2001   Term    Profiling timer expired
SIGPWR  \-      Term    Power failure (System V)
SIGQUIT P1990   Core    Quit from keyboard
SIGSEGV P1990   Core    Invalid memory reference
SIGSTKFLT       \-      Term    Stack fault on coprocessor (unused)
SIGSTOP P1990   Stop    Stop process
SIGTSTP P1990   Stop    Stop typed at terminal
SIGSYS  P2001   Core    Bad system call (SVr4);
                        see also \fBseccomp\fP(2)
SIGTERM P1990   Term    Termination signal
SIGTRAP P2001   Core    Trace/breakpoint trap
SIGTTIN P1990   Stop    Terminal input for background process
SIGTTOU P1990   Stop    Terminal output for background process
SIGUNUSED       \-      Core    Synonymous with \fBSIGSYS\fP
SIGURG  P2001   Ign     Urgent condition on socket (4.2BSD)
SIGUSR1 P1990   Term    User-defined signal 1
SIGUSR2 P1990   Term    User-defined signal 2
SIGVTALRM       P2001   Term    Virtual alarm clock (4.2BSD)
SIGXCPU P2001   Core    CPU time limit exceeded (4.2BSD);
                        see \fBsetrlimit\fP(2)
SIGXFSZ P2001   Core    File size limit exceeded (4.2BSD);
                        see \fBsetrlimit\fP(2)
SIGWINCH        \-      Ign     Window resize signal (4.3BSD, Sun)
.TE
.PP
The signals
.B SIGKILL
and
.B SIGSTOP
cannot be caught, blocked, or ignored.
.PP
Up to and including Linux 2.2, the default behavior for
.BR SIGSYS ", " SIGXCPU ", " SIGXFSZ ", "
and (on architectures other than SPARC and MIPS)
.B SIGBUS
was to terminate the process (without a core dump).
(On some other UNIX systems the default action for
.BR SIGXCPU " and " SIGXFSZ
is to terminate the process without a core dump.)
Linux 2.4 conforms to the POSIX.1-2001 requirements for these signals,
terminating the process with a core dump.
.PP
.PP
.B SIGEMT
is not specified in POSIX.1-2001, but nevertheless appears
on most other UNIX systems,
where its default action is typically to terminate
the process with a core dump.
.PP
.B SIGPWR
(which is not specified in POSIX.1-2001) is typically ignored
by default on those other UNIX systems where it appears.
.PP
.B SIGIO
(which is not specified in POSIX.1-2001) is ignored by default
on several other UNIX systems.
.\"
.SS Queueing and delivery semantics for standard signals
If multiple standard signals are pending for a process,
the order in which the signals are delivered is unspecified.
.PP
Standard signals do not queue.
If multiple instances of a standard signal are generated while
that signal is blocked,
then only one instance of the signal is marked as pending
(and the signal will be delivered just once when it is unblocked).
In the case where a standard signal is already pending, the
.I siginfo_t
structure (see
.BR sigaction (2))
associated with that signal is not overwritten
on arrival of subsequent instances of the same signal.
Thus, the process will receive the information
associated with the first instance of the signal.
.\"
.SS Signal numbering for standard signals
The numeric value for each signal is given in the table below.
As shown in the table, many signals have different numeric values
on different architectures.
The first numeric value in each table row shows the signal number
on x86, ARM, and most other architectures;
the second value is for Alpha and SPARC; the third is for MIPS;
and the last is for PARISC.
A dash (\-) denotes that a signal is absent on the corresponding architecture.
.TS
l c c c c l
l c c c c l
______
lB c c c c l.
Signal  x86/ARM Alpha/  MIPS    PARISC  Notes
        most others     SPARC
SIGHUP  \01     \01     \01     \01
SIGINT  \02     \02     \02     \02
SIGQUIT \03     \03     \03     \03
SIGILL  \04     \04     \04     \04
SIGTRAP \05     \05     \05     \05
SIGABRT \06     \06     \06     \06
SIGIOT  \06     \06     \06     \06
SIGBUS  \07     10      10      10
SIGEMT  \-      \07     \07     -
SIGFPE  \08     \08     \08     \08
SIGKILL \09     \09     \09     \09
SIGUSR1 10      30      16      16
SIGSEGV 11      11      11      11
SIGUSR2 12      31      17      17
SIGPIPE 13      13      13      13
SIGALRM 14      14      14      14
SIGTERM 15      15      15      15
SIGSTKFLT       16      \-      \-      \07
SIGCHLD 17      20      18      18
SIGCLD  \-      \-      18      \-
SIGCONT 18      19      25      26
SIGSTOP 19      17      23      24
SIGTSTP 20      18      24      25
SIGTTIN 21      21      26      27
SIGTTOU 22      22      27      28
SIGURG  23      16      21      29
SIGXCPU 24      24      30      12
SIGXFSZ 25      25      31      30
SIGVTALRM       26      26      28      20
SIGPROF 27      27      29      21
SIGWINCH        28      28      20      23
SIGIO   29      23      22      22
SIGPOLL                                 Same as SIGIO
SIGPWR  30      29/\-   19      19
SIGINFO \-      29/\-   \-      \-
SIGLOST \-      \-/29   \-      \-
SIGSYS  31      12      12      31
SIGUNUSED       31      \-      \-      31
.TE
.PP
Note the following:
.IP * 3
Where defined,
.B SIGUNUSED
is synonymous with
.BR SIGSYS .
Since glibc 2.26,
.B SIGUNUSED
is no longer defined on any architecture.
.IP *
Signal 29 is
.BR SIGINFO / SIGPWR
(synonyms for the same value) on Alpha but
.B SIGLOST
on SPARC.
.\"
.SS Real-time signals
Starting with version 2.2,
Linux supports real-time signals as originally defined in the POSIX.1b
real-time extensions (and now included in POSIX.1-2001).
The range of supported real-time signals is defined by the macros
.B SIGRTMIN
and
.BR SIGRTMAX .
POSIX.1-2001 requires that an implementation support at least
.B _POSIX_RTSIG_MAX
(8) real-time signals.
.PP
The Linux kernel supports a range of 33 different real-time
signals, numbered 32 to 64.
However, the glibc POSIX threads implementation internally uses
two (for NPTL) or three (for LinuxThreads) real-time signals
(see
.BR pthreads (7)),
and adjusts the value of
.B SIGRTMIN
suitably (to 34 or 35).
Because the range of available real-time signals varies according
to the glibc threading implementation (and this variation can occur
at run time according to the available kernel and glibc),
and indeed the range of real-time signals varies across UNIX systems,
programs should
.IR "never refer to real-time signals using hard-coded numbers" ,
but instead should always refer to real-time signals using the notation
.BR SIGRTMIN +n,
and include suitable (run-time) checks that
.BR SIGRTMIN +n
does not exceed
.BR SIGRTMAX .
.PP
Unlike standard signals, real-time signals have no predefined meanings:
the entire set of real-time signals can be used for application-defined
purposes.
.PP
The default action for an unhandled real-time signal is to terminate the
receiving process.
.PP
Real-time signals are distinguished by the following:
.IP 1. 4
Multiple instances of real-time signals can be queued.
By contrast, if multiple instances of a standard signal are delivered
while that signal is currently blocked, then only one instance is queued.
.IP 2. 4
If the signal is sent using
.BR sigqueue (3),
an accompanying value (either an integer or a pointer) can be sent
with the signal.
If the receiving process establishes a handler for this signal using the
.B SA_SIGINFO
flag to
.BR sigaction (2),
then it can obtain this data via the
.I si_value
field of the
.I siginfo_t
structure passed as the second argument to the handler.
Furthermore, the
.I si_pid
and
.I si_uid
fields of this structure can be used to obtain the PID
and real user ID of the process sending the signal.
.IP 3. 4
Real-time signals are delivered in a guaranteed order.
Multiple real-time signals of the same type are delivered in the order
they were sent.
If different real-time signals are sent to a process, they are delivered
starting with the lowest-numbered signal.
(I.e., low-numbered signals have highest priority.)
By contrast, if multiple standard signals are pending for a process,
the order in which they are delivered is unspecified.
.PP
If both standard and real-time signals are pending for a process,
POSIX leaves it unspecified which is delivered first.
Linux, like many other implementations, gives priority
to standard signals in this case.
.PP
According to POSIX, an implementation should permit at least
.B _POSIX_SIGQUEUE_MAX
(32) real-time signals to be queued to
a process.
However, Linux does things differently.
In kernels up to and including 2.6.7, Linux imposes
a system-wide limit on the number of queued real-time signals
for all processes.
This limit can be viewed and (with privilege) changed via the
.I /proc/sys/kernel/rtsig-max
file.
A related file,
.IR /proc/sys/kernel/rtsig-nr ,
can be used to find out how many real-time signals are currently queued.
In Linux 2.6.8, these
.I /proc
interfaces were replaced by the
.B RLIMIT_SIGPENDING
resource limit, which specifies a per-user limit for queued
signals; see
.BR setrlimit (2)
for further details.
.PP
The addition of real-time signals required the widening
of the signal set structure
.RI ( sigset_t )
from 32 to 64 bits.
Consequently, various system calls were superseded by new system calls
that supported the larger signal sets.
The old and new system calls are as follows:
.TS
lb lb
l l.
Linux 2.0 and earlier   Linux 2.2 and later
\fBsigaction\fP(2)      \fBrt_sigaction\fP(2)
\fBsigpending\fP(2)     \fBrt_sigpending\fP(2)
\fBsigprocmask\fP(2)    \fBrt_sigprocmask\fP(2)
\fBsigreturn\fP(2)      \fBrt_sigreturn\fP(2)
\fBsigsuspend\fP(2)     \fBrt_sigsuspend\fP(2)
\fBsigtimedwait\fP(2)   \fBrt_sigtimedwait\fP(2)
.TE
.\"
.SS Interruption of system calls and library functions by signal handlers
If a signal handler is invoked while a system call or library
function call is blocked, then either:
.IP * 2
the call is automatically restarted after the signal handler returns; or
.IP *
the call fails with the error
.BR EINTR .
.PP
Which of these two behaviors occurs depends on the interface and
whether or not the signal handler was established using the
.BR SA_RESTART
flag (see
.BR sigaction (2)).
The details vary across UNIX systems;
below, the details for Linux.
.PP
If a blocked call to one of the following interfaces is interrupted
by a signal handler, then the call is automatically restarted
after the signal handler returns if the
.BR SA_RESTART
flag was used; otherwise the call fails with the error
.BR EINTR :
.\" The following system calls use ERESTARTSYS,
.\" so that they are restartable
.IP * 2
.BR read (2),
.BR readv (2),
.BR write (2),
.BR writev (2),
and
.BR ioctl (2)
calls on "slow" devices.
A "slow" device is one where the I/O call may block for an
indefinite time, for example, a terminal, pipe, or socket.
If an I/O call on a slow device has already transferred some
data by the time it is interrupted by a signal handler,
then the call will return a success status
(normally, the number of bytes transferred).
Note that a (local) disk is not a slow device according to this definition;
I/O operations on disk devices are not interrupted by signals.
.IP *
.BR open (2),
if it can block (e.g., when opening a FIFO; see
.BR fifo (7)).
.IP *
.BR wait (2),
.BR wait3 (2),
.BR wait4 (2),
.BR waitid (2),
and
.BR waitpid (2).
.IP *
Socket interfaces:
.\" If a timeout (setsockopt()) is in effect on the socket, then these
.\" system calls switch to using EINTR.  Consequently, they and are not
.\" automatically restarted, and they show the stop/cont behavior
.\" described below.  (Verified from 2.6.26 source, and by experiment; mtk)
.BR accept (2),
.BR connect (2),
.BR recv (2),
.BR recvfrom (2),
.BR recvmmsg (2),
.BR recvmsg (2),
.BR send (2),
.BR sendto (2),
and
.BR sendmsg (2),
.\" FIXME What about sendmmsg()?
unless a timeout has been set on the socket (see below).
.IP *
File locking interfaces:
.BR flock (2)
and
the
.BR F_SETLKW
and
.BR F_OFD_SETLKW
operations of
.BR fcntl (2)
.IP *
POSIX message queue interfaces:
.BR mq_receive (3),
.BR mq_timedreceive (3),
.BR mq_send (3),
and
.BR mq_timedsend (3).
.IP *
.BR futex (2)
.B FUTEX_WAIT
(since Linux 2.6.22;
.\" commit 72c1bbf308c75a136803d2d76d0e18258be14c7a
beforehand, always failed with
.BR EINTR ).
.IP *
.BR getrandom (2).
.IP *
.BR pthread_mutex_lock (3),
.BR pthread_cond_wait (3),
and related APIs.
.IP *
.BR futex (2)
.BR FUTEX_WAIT_BITSET .
.IP *
POSIX semaphore interfaces:
.BR sem_wait (3)
and
.BR sem_timedwait (3)
(since Linux 2.6.22;
.\" as a consequence of the 2.6.22 changes in the futex() implementation
beforehand, always failed with
.BR EINTR ).
.IP *
.BR read (2)
from an
.BR inotify (7)
file descriptor
(since Linux 3.8;
.\" commit 1ca39ab9d21ac93f94b9e3eb364ea9a5cf2aba06
beforehand, always failed with
.BR EINTR ).
.PP
The following interfaces are never restarted after
being interrupted by a signal handler,
regardless of the use of
.BR SA_RESTART ;
they always fail with the error
.B EINTR
when interrupted by a signal handler:
.\" These are the system calls that give EINTR or ERESTARTNOHAND
.\" on interruption by a signal handler.
.IP * 2
"Input" socket interfaces, when a timeout
.RB ( SO_RCVTIMEO )
has been set on the socket using
.BR setsockopt (2):
.BR accept (2),
.BR recv (2),
.BR recvfrom (2),
.BR recvmmsg (2)
(also with a non-NULL
.IR timeout
argument),
and
.BR recvmsg (2).
.IP *
"Output" socket interfaces, when a timeout
.RB ( SO_RCVTIMEO )
has been set on the socket using
.BR setsockopt (2):
.BR connect (2),
.BR send (2),
.BR sendto (2),
and
.BR sendmsg (2).
.\" FIXME What about sendmmsg()?
.IP *
Interfaces used to wait for signals:
.BR pause (2),
.BR sigsuspend (2),
.BR sigtimedwait (2),
and
.BR sigwaitinfo (2).
.IP *
File descriptor multiplexing interfaces:
.BR epoll_wait (2),
.BR epoll_pwait (2),
.BR poll (2),
.BR ppoll (2),
.BR select (2),
and
.BR pselect (2).
.IP *
System V IPC interfaces:
.\" On some other systems, SA_RESTART does restart these system calls
.BR msgrcv (2),
.BR msgsnd (2),
.BR semop (2),
and
.BR semtimedop (2).
.IP *
Sleep interfaces:
.BR clock_nanosleep (2),
.BR nanosleep (2),
and
.BR usleep (3).
.IP *
.BR io_getevents (2).
.PP
The
.BR sleep (3)
function is also never restarted if interrupted by a handler,
but gives a success return: the number of seconds remaining to sleep.
.SS Interruption of system calls and library functions by stop signals
On Linux, even in the absence of signal handlers,
certain blocking interfaces can fail with the error
.BR EINTR
after the process is stopped by one of the stop signals
and then resumed via
.BR SIGCONT .
This behavior is not sanctioned by POSIX.1, and doesn't occur
on other systems.
.PP
The Linux interfaces that display this behavior are:
.IP * 2
"Input" socket interfaces, when a timeout
.RB ( SO_RCVTIMEO )
has been set on the socket using
.BR setsockopt (2):
.BR accept (2),
.BR recv (2),
.BR recvfrom (2),
.BR recvmmsg (2)
(also with a non-NULL
.IR timeout
argument),
and
.BR recvmsg (2).
.IP *
"Output" socket interfaces, when a timeout
.RB ( SO_RCVTIMEO )
has been set on the socket using
.BR setsockopt (2):
.BR connect (2),
.BR send (2),
.BR sendto (2),
and
.\" FIXME What about sendmmsg()?
.BR sendmsg (2),
if a send timeout
.RB ( SO_SNDTIMEO )
has been set.
.IP * 2
.BR epoll_wait (2),
.BR epoll_pwait (2).
.IP *
.BR semop (2),
.BR semtimedop (2).
.IP *
.BR sigtimedwait (2),
.BR sigwaitinfo (2).
.IP *
Linux 3.7 and earlier:
.BR read (2)
from an
.BR inotify (7)
file descriptor
.\" commit 1ca39ab9d21ac93f94b9e3eb364ea9a5cf2aba06
.IP *
Linux 2.6.21 and earlier:
.BR futex (2)
.BR FUTEX_WAIT ,
.BR sem_timedwait (3),
.BR sem_wait (3).
.IP *
Linux 2.6.8 and earlier:
.BR msgrcv (2),
.BR msgsnd (2).
.IP *
Linux 2.4 and earlier:
.BR nanosleep (2).
.SH CONFORMING TO
POSIX.1, except as noted.
.\" It must be a *very* long time since this was true:
.\" .SH BUGS
.\" .B SIGIO
.\" and
.\" .B SIGLOST
.\" have the same value.
.\" The latter is commented out in the kernel source, but
.\" the build process of some software still thinks that
.\" signal 29 is
.\" .BR SIGLOST .
.SH NOTES
For a discussion of async-signal-safe functions, see
.BR signal-safety (7).
.PP
The
.I /proc/[pid]/task/[tid]/status
file contains various fields that show the signals
that a thread is blocking
.RI ( SigBlk ),
catching
.RI ( SigCgt ),
or ignoring
.RI ( SigIgn ).
(The set of signals that are caught or ignored will be the same
across all threads in a process.)
Other fields show the set of pending signals that are directed to the thread
.RI ( SigPnd )
as well as the set of pending signals that are directed
to the process as a whole
.RI ( ShdPnd ).
The corresponding fields in
.I /proc/[pid]/status
show the information for the main thread.
See
.BR proc (5)
for further details.
.SH SEE ALSO
.BR kill (1),
.BR clone (2),
.BR getrlimit (2),
.BR kill (2),
.BR restart_syscall (2),
.BR rt_sigqueueinfo (2),
.BR setitimer (2),
.BR setrlimit (2),
.BR sgetmask (2),
.BR sigaction (2),
.BR sigaltstack (2),
.BR signal (2),
.BR signalfd (2),
.BR sigpending (2),
.BR sigprocmask (2),
.BR sigreturn (2),
.BR sigsuspend (2),
.BR sigwaitinfo (2),
.BR abort (3),
.BR bsd_signal (3),
.BR killpg (3),
.BR longjmp (3),
.BR pthread_sigqueue (3),
.BR raise (3),
.BR sigqueue (3),
.BR sigset (3),
.BR sigsetops (3),
.BR sigvec (3),
.BR sigwait (3),
.BR strsignal (3),
.BR sysv_signal (3),
.BR core (5),
.BR proc (5),
.BR nptl (7),
.BR pthreads (7),
.BR sigevent (7)