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GNU/Linux |
CentOS 5.4 |
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signal(7) |
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signal − list of available signals
Linux supports both POSIX reliable signals (hereinafter "standard signals") and POSIX real-time signals.
Signal
Dispositions
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:
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Term |
Default action is to terminate the process. | ||
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Ign |
Default action is to ignore the signal. | ||
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Core |
Default action is to terminate the process and dump core (see core(5)). | ||
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Stop |
Default action is to stop the process. | ||
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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.
Signal Mask
and Pending Signals
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.
Standard
Signals
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.
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.
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 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.
Real-time
Signals
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:
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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. | ||
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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. | ||
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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.
POSIX.1
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.
kill(1), kill(2), killpg(2), setitimer(2), setrlimit(2), sigaction(2), signal(2), sigpending(2), sigprocmask(2), sigqueue(2), sigsuspend(2), sigwaitinfo(2), raise(3), sigvec(3), sigset(3), strsignal(3), core(5), proc(5), pthreads(7)
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signal(7) | |