Each signal has a current
disposition, which determines how the process behaves when it is delivered
the signal.
The entries in the "Action" column of the tables below specify
the default disposition for each signal, as follows:
Term
Default action is to terminate the process.
Ign
Default action is to ignore the signal.
Core
Default action is to terminate the process and dump core (see
core(5)).
Stop
Default action is to stop the process.
Cont
Default action is to continue the process if it is currently stopped.
A process can change the disposition of a signal using
sigaction(2)
or (less portably)
signal(2).
Using these system calls, a process can elect one of the
following behaviours to occur on delivery of the signal:
perform the default action; ignore the signal;
or catch the signal with a
signal handler, a programmer-defined function that is automatically invoked
when the signal is delivered.
The signal disposition is a per-process attribute:
in a multithreaded application, the disposition of a
particular signal is the same for all threads.
A signal may be
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
pending.
Each thread in a process has an independent
signal mask, which indicates the set of signals that the thread is currently blocking.
A thread can manipulate its signal mask using
pthread_sigmask(3).
In a traditional single-threaded application,
sigprocmask(2)
can be used to manipulate the signal mask.
A signal may be generated (and thus pending)
for a process as a whole (e.g., when sent using
kill(2))
or for a specific thread (e.g., certain signals,
such as SIGSEGV and SIGFPE, generated as a
consequence of executing a specific machine-language instruction
are thread directed, as are signals targeted at a specific thread using
pthread_kill(2)).
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.
A thread can obtain the set of signals that it currently has pending
using
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.
Linux supports the standard signals listed below. Several signal numbers
are architecture dependent, as indicated in the "Value" column.
(Where three values are given, the first one is usually valid for
alpha and sparc, the middle one for i386, ppc and sh, and
the last one for mips.
A - denotes that a signal is absent on the corresponding architecture.)
First the signals described in the original POSIX.1-1990 standard.
Signal
Value
Action
Comment
or death of controlling process
SIGINT
2
Term
Interrupt from keyboard
SIGQUIT
3
Core
Quit from keyboard
SIGILL
4
Core
Illegal Instruction
SIGABRT
6
Core
Abort signal from abort(3)
SIGFPE
8
Core
Floating point exception
SIGKILL
9
Term
Kill signal
SIGSEGV
11
Core
Invalid memory reference
SIGPIPE
13
Term
Broken pipe: write to pipe with no readers
SIGALRM
14
Term
Timer signal from alarm(2)
SIGTERM
15
Term
Termination signal
SIGUSR1
30,10,16
Term
User-defined signal 1
SIGUSR2
31,12,17
Term
User-defined signal 2
SIGCHLD
20,17,18
Ign
Child stopped or terminated
SIGCONT
19,18,25
Cont
Continue if stopped
SIGSTOP
17,19,23
Stop
Stop process
SIGTSTP
18,20,24
Stop
Stop typed at tty
SIGTTIN
21,21,26
Stop
tty input for background process
SIGTTOU
22,22,27
Stop
tty output for background process
The signals
SIGKILL and
SIGSTOP cannot be caught, blocked, or ignored.
Next the signals not in the POSIX.1-1990 standard but described in
SUSv2 and POSIX.1-2001.
Signal
Value
Action
Comment
SIGPOLL
Term
Pollable event (Sys V). Synonym of SIGIO
SIGPROF
27,27,29
Term
Profiling timer expired
SIGSYS
12,-,12
Core
Bad argument to routine (SVr4)
SIGTRAP
5
Core
Trace/breakpoint trap
SIGURG
16,23,21
Ign
Urgent condition on socket (4.2BSD)
SIGVTALRM
26,26,28
Term
Virtual alarm clock (4.2BSD)
SIGXCPU
24,24,30
Core
CPU time limit exceeded (4.2BSD)
SIGXFSZ
25,25,31
Core
File size limit exceeded (4.2BSD)
Up to and including Linux 2.2, the default behaviour for
SIGSYS, SIGXCPU, SIGXFSZ, and (on architectures other than SPARC and MIPS)
SIGBUS was to terminate the process (without a core dump).
(On some other Unices the default action for
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.
Next various other signals.
Signal
Value
Action
Comment
SIGEMT
7,-,7
Term
SIGSTKFLT
-,16,-
Term
Stack fault on coprocessor (unused)
SIGIO
23,29,22
Term
I/O now possible (4.2BSD)
SIGCLD
-,-,18
Ign
A synonym for SIGCHLD
SIGPWR
29,30,19
Term
Power failure (System V)
SIGINFO
29,-,-
A synonym for SIGPWR
SIGLOST
-,-,-
Term
File lock lost
SIGWINCH
28,28,20
Ign
Window resize signal (4.3BSD, Sun)
SIGUNUSED
-,31,-
Term
Unused signal (will be SIGSYS)
(Signal 29 is
SIGINFO /
SIGPWR on an alpha but
SIGLOST on a sparc.)
SIGEMT is not specified in POSIX.1-2001, but nevertheless appears
on most other Unices, where its default action is typically to terminate
the process with a core dump.
SIGPWR (which is not specified in POSIX.1-2001) is typically ignored
by default on those other Unices where it appears.
SIGIO (which is not specified in POSIX.1-2001) is ignored by default
on several other Unices.
Linux supports real-time signals as originally defined in the POSIX.1b
real-time extensions (and now included in POSIX.1-2001).
Linux supports 32 real-time signals, numbered from 32
(SIGRTMIN) to 63
(SIGRTMAX). (Programs should always refer to real-time signals using notation
SIGRTMIN+n, since the range of real-time signal numbers varies across Unices.)
Unlike standard signals, real-time signals have no predefined meanings:
the entire set of real-time signals can be used for application-defined
purposes.
(Note, however, that the LinuxThreads implementation uses the first
three real-time signals.)
The default action for an unhandled real-time signal is to terminate the
receiving process.
Real-time signals are distinguished by the following:
1.
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.
2.
If the signal is sent using
sigqueue(2),
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
SA_SIGINFO flag to
sigaction(2)
then it can obtain this data via the
si_value field of the
siginfo_t structure passed as the second argument to the handler.
Furthermore, the
si_pid and
si_uid fields of this structure can be used to obtain the PID
and real user ID of the process sending the signal.
3.
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.)
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.
According to POSIX, an implementation should permit at least
_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
/proc/sys/kernel/rtsig-max file.
A related file,
/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
/proc interfaces were replaced by the
RLIMIT_SIGPENDING resource limit, which specifies a per-user limit for queued
signals; see
setrlimit(2)
for further details.
SIGIO and
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
SIGLOST.
