Dispozice signálů¶

Každý signál má dispozici, která určuje, jak se proces zachová při jeho přijetí.

Údaje ve sloupci "Akce" níže uvedených tabulek určují výchozí dipozici každého signálu následujícně:

Term

Výchozí akcí je ukončení procesu.

Ign

Výchozí akcí je ignorování signálu.

Core

Výchozí akcí je ukončení procesu a výpis paměti (core dump) (viz core(5)).

Stop

Výchozí akcí je zastavení procesu.

Cont

Výchozí akcí je pokračování procesu, pokud je momentálně zastavený.

A process can change the disposition of a signal using sigaction(2) or signal(2). (The latter is less portable when establishing a signal handler; see 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 signal handler, a programmer-defined function that is automatically invoked when the signal is delivered.

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 sigaltstack(2) for a discussion of how to do this and when it might be useful.

Dispozice signálu je atribut procesu: v mnohovláknových aplikacích je dispozice určitého signálu stejná pro všechna vlákna.

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.

Sending a signal¶

The following system calls and library functions allow the caller to send a signal:

raise(3)

Sends a signal to the calling thread.

kill(2)

Sends a signal to a specified process, to all members of a specified process group, or to all processes on the system.

pidfd_send_signal(2)

Sends a signal to a process identified by a PID file descriptor.

killpg(3)

Sends a signal to all of the members of a specified process group.

pthread_kill(3)

Sends a signal to a specified POSIX thread in the same process as the caller.

tgkill(2)

Sends a signal to a specified thread within a specific process. (This is the system call used to implement pthread_kill(3).)

sigqueue(3)

Sends a real-time signal with accompanying data to a specified process.

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):

pause(2)

Suspends execution until any signal is caught.

sigsuspend(2)

Temporarily changes the signal mask (see below) and suspends execution until one of the unmasked signals is caught.

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:

•

sigwaitinfo(2), sigtimedwait(2), and 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.

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signalfd(2) returns a file descriptor that can be used to read information about signals that are delivered to the caller. Each read(2) from this file descriptor blocks until one of the signals in the set specified in the signalfd(2) call is delivered to the caller. The buffer returned by read(2) contains a structure describing the signal.

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 child created via fork(2) inherits a copy of its parent's signal mask; the signal mask is preserved across execve(2).

A signal may be process-directed or thread-directed. A process-directed signal is one that is targeted at (and thus pending for) the process as a whole. A signal may be process-directed because it was generated by the kernel for reasons other than a hardware exception, or because it was sent using kill(2) or sigqueue(3). A thread-directed signal is one that is targeted at a specific thread. A signal may be thread-directed because it was generated as a consequence of executing a specific machine-language instruction that triggered a hardware exception (e.g., SIGSEGV for an invalid memory access, or SIGFPE for a math error), or because it was targeted at a specific thread using interfaces such as tgkill(2) or 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.

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.

A child created via fork(2) initially has an empty pending signal set; the pending signal set is preserved across an execve(2).

Execution of signal handlers¶

Whenever there is a transition from kernel-mode to user-mode execution (e.g., on return from a system call or scheduling of a thread onto the CPU), the kernel checks whether there is a pending unblocked signal for which the process has established a signal handler. If there is such a pending signal, the following steps occur:

(1)

The kernel performs the necessary preparatory steps for execution of the signal handler:

(2)

The kernel constructs a frame for the signal handler on the stack. The kernel sets the program counter for the thread to point to the first instruction of the signal handler function, and configures the return address for that function to point to a piece of user-space code known as the signal trampoline (described in sigreturn(2)).

(3)

The kernel passes control back to user-space, where execution commences at the start of the signal handler function.

(4)

When the signal handler returns, control passes to the signal trampoline code.

(5)

The signal trampoline calls sigreturn(2), a system call that uses the information in the stack frame created in step 1 to restore the thread to its state before the signal handler was called. The thread's signal mask and alternate signal stack settings are restored as part of this procedure. Upon completion of the call to sigreturn(2), the kernel transfers control back to user space, and the thread recommences execution at the point where it was interrupted by the signal handler.

Note that if the signal handler does not return (e.g., control is transferred out of the handler using siglongjmp(3), or the handler executes a new program with execve(2)), then the final step is not performed. In particular, in such scenarios it is the programmer's responsibility to restore the state of the signal mask (using sigprocmask(2)), if it is desired to unblock the signals that were blocked on entry to the signal handler. (Note that siglongjmp(3) may or may not restore the signal mask, depending on the savesigs value that was specified in the corresponding call to sigsetjmp(3).)

From the kernel's point of view, execution of the signal handler code is exactly the same as the execution of any other user-space code. That is to say, the kernel does not record any special state information indicating that the thread is currently executing inside a signal handler. All necessary state information is maintained in user-space registers and the user-space stack. The depth to which nested signal handlers may be invoked is thus limited only by the user-space stack (and sensible software design!).

Standardní Signály¶

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.

Signály SIGKILL a SIGSTOP nemohou být zachyceny, blokovány ani ignorovány.

Až po Linux 2.2 včetně bylo výchozí chování pro SIGSYS, SIGXCPU, SIGXFSZ, a (na architekturách jiných než SPARC a MIPS) SIGBUS ukončit proces (bez core dump). (Na některých jiných UNIXových systémech bylo výchozí akcí pro SIGXCPU a SIGXFSZ ukončení procesu bez core dump.) Linux 2.4 splňuje požadavky POSIX.1-2001 pro tyto signály, ukončuje procesy s core dump.

SIGEMT není specifikován v POSIX.1-2001, ale stejně je přítomen na většině ostatních UNIXových systémů, kde je výchozí akcí obvykle ukončení procesu s core dump.

SIGPWR (není specifikován v POSIX.1-2001) na většině ostatních UNIXových systémů, kde se objevuje, je obvykle ignorován.

SIGIO (není specifikován v POSIX.1-2001) na některých dalších UNIXech je jako výchozí ignorován.

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.

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 siginfo_t structure (see 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.

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.

Note the following:

•

Where defined, SIGUNUSED is synonymous with SIGSYS. Since glibc 2.26, SIGUNUSED is no longer defined on any architecture.

•

Signal 29 is SIGINFO/SIGPWR (synonyms for the same value) on Alpha but SIGLOST on SPARC.

Real-time signály¶

Starting with Linux 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 SIGRTMIN and SIGRTMAX. POSIX.1-2001 requires that an implementation support at least _POSIX_RTSIG_MAX (8) real-time signals.

Linux podporuje 33 různých real-time signálů očíslovaných 32 až 64. Nicméně implementace POSIX threads v glibc používá interně dva (pro NPTL) nebo tři (pro LinuxThreads) real-time signály (viz pthreads(7)), a podle toho upravuje hodnotu SIGRTMIN (na 34 nebo 35). protože rozsah dostupných real-time signálů se liší v závislosti na implementaci vláken v glibc (může se měnit za běhu v závislosti na jádře a glibc) a navíc rozsah real-time signálů se mezi UNIXovými systémy liší, programy by nikdy neměly odkazovat na real-time signály pevně danými čísly, místo toho by měly používat notaci SIGRTMIN+n, a za běhu kontrolovat, zda SIGRTMIN+n nepřesahuje SIGRTMAX.

Na rozdíl od standardních signálů nemají real-time signály stanovený význam: Celá sada real-time signálů může být použita pro účely definované aplikací.

Výchozí akcí pro nezpracovaný real-time signál je ukončení procesu, který jej přijal.

Real-time signály se liší následujícně:

•

Vícero instancí real-time signálů může být zařazeno do fronty. Naopak pokud je doručeno vícero instancí standardního signálu, zatímco je signál blokován, je do fronty zařazen jen jeden.

•

Pokud je signál poslán pomocí sigqueue(3), může s ním být poslána doprovodná hodnota (integer nebo pointer). Pokud přijímací proces vytvoří pro tento signál handler pomocí vlajky SA_SIGINFO pro sigaction(2), tak může tato data získat v poli si_value struktury siginfo_t předané jako druhý argument handleru. Navíc mohou být pole si_pid a si_uid této struktury použita k získání PID a real user ID procesu, který signál poslal.

•

Real-time signály jsou doručeny v zaručeném pořadí. Vícero real-time signálů stejného typu je doručeno v pořadí, v jakém byly vyslány. Pokud jsou procesu poslány různé real-time signály, jsou doručeny v pořadí podle čísla, začínajíc nejnižším (tj. signály s nízkým číslem mají vyšší prioritu). Naopak, pokud na proces čeká vícero standardních signálů, není pořadí jejich doručení definováno.

Pokud má proces nevyřízené zároveň real-time a standardní signály, POSIX neurčuje, které mají být doručeny jako první. Linux, stejně jako mnoho jiných implementací, v takovém případě upřednostňí standardní signály.

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. Up to and including Linux 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.

The addition of real-time signals required the widening of the signal set structure (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:

Přerušení systémových volání a funkcí knihoven prostřednictvím "signal handlers"¶

Pokud je signal handler vyvolán v okamžiku, kdy je systémové volání nebo funkce knihovny blokována, pak:

•

je volání automaticky restartováno po návratu signal handleru, nebo

•

volání selže s chybou EINTR.

Která z těchto možností nastane, záleží na rozhraní a na tom, zda byl signal handler definován s pomocí vlajky SA_RESTART (viz sigaction(2)). Podrobnosti se mezi UNIXovými systémy liší; dále jsou uvedeny pro Linux.

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 SA_RESTART flag was used; otherwise the call fails with the error EINTR:

•

read(2), readv(2), write(2), writev(2), and 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.

•

open(2), v případě, že může blokovat (např. při otevírání FIFO; viz fifo(7)).

•

wait(2), wait3(2), wait4(2), waitid(2) a waitpid(2).

•

Socket interfaces: accept(2), connect(2), recv(2), recvfrom(2), recvmmsg(2), recvmsg(2), send(2), sendto(2), and sendmsg(2), unless a timeout has been set on the socket (see below).

•

File locking interfaces: flock(2) and the F_SETLKW and F_OFD_SETLKW operations of fcntl(2)

•

Rozhraní pro POSIXové fronty zpráv: mq_receive(3), mq_timedreceive(3), mq_send(3) a mq_timedsend(3).

•

futex(2) FUTEX_WAIT (od jádra 2.6.22; předtím vždycky selhalo s EINTR).

•

getrandom(2).

•

pthread_mutex_lock(3), pthread_cond_wait(3), and related APIs.

•

futex(2) FUTEX_WAIT_BITSET.

•

Rozhraní POSIXových semaforů: sem_wait(3) a sem_timedwait(3) (od jádra 2.6.22; předtím vždycky selhalo s EINTR).

•

read(2) from an inotify(7) file descriptor (since Linux 3.8; beforehand, always failed with EINTR).

The following interfaces are never restarted after being interrupted by a signal handler, regardless of the use of SA_RESTART; they always fail with the error EINTR when interrupted by a signal handler:

•

"Input" socket interfaces, when a timeout (SO_RCVTIMEO) has been set on the socket using setsockopt(2): accept(2), recv(2), recvfrom(2), recvmmsg(2) (also with a non-NULL timeout argument), and recvmsg(2).

•

"Output" socket interfaces, when a timeout (SO_RCVTIMEO) has been set on the socket using setsockopt(2): connect(2), send(2), sendto(2), and sendmsg(2).

•

Interfaces used to wait for signals: pause(2), sigsuspend(2), sigtimedwait(2), and sigwaitinfo(2).

•

Multiplexující rozhraní popisovačů souborů: epoll_wait(2), epoll_pwait(2), poll(2), ppoll(2), select(2) a pselect(2).

•

System V IPC rozhraní: msgrcv(2), msgsnd(2), semop(2) a semtimedop(2).

•

Rozhraní pro spánek: clock_nanosleep(2), nanosleep(2) a usleep(3).

•

io_getevents(2).

Funkce sleep(3) se také při přerušení signal handlerem nerestartuje, nýbrž vrátí úspěch: počet sekund, které zbývají ke spaní.

In certain circumstances, the seccomp(2) user-space notification feature can lead to restarting of system calls that would otherwise never be restarted by SA_RESTART; for details, see seccomp_unotify(2).

Přerušení systémovách volání a funkcí knihoven signály Stop¶

V Linuxu mohou některá blokující rozhraní selhat s chybou EINTR i bez signal handlerů, pokud je proces zastaven jedním ze stop signálů a poté obnoven pomocí SIGCONT. Toto chování neodporuje POSIX.1 a neobjevuje se v jiných systémech.

Linuxová rozhraní, v nichž se toto chování projevuje, jsou:

•

"Input" socket interfaces, when a timeout (SO_RCVTIMEO) has been set on the socket using setsockopt(2): accept(2), recv(2), recvfrom(2), recvmmsg(2) (also with a non-NULL timeout argument), and recvmsg(2).

•

"Output" socket interfaces, when a timeout (SO_RCVTIMEO) has been set on the socket using setsockopt(2): connect(2), send(2), sendto(2), and sendmsg(2), if a send timeout (SO_SNDTIMEO) has been set.

•

epoll_wait(2), epoll_pwait(2).

•

semop(2), semtimedop(2).

•

sigtimedwait(2), sigwaitinfo(2).

•

Jádro 3.7 a dřívější: read(2) z popisovače souborů inotify(7).

•

Jádro 2.6.21 a dřívější: futex(2) FUTEX_WAIT, sem_timedwait(3), sem_wait(3).

•

Jádro 2.6.8 a dřívější: msgrcv(2), msgsnd(2).

•

Jádro 2.4 a dřívější: nanosleep(2).