Signal Concepts

Signal Generation and Delivery

A signal is said to be ``generated'' for (or sent to) a process or thread when the event that causes the signal first occurs. Examples of such events include detection of hardware faults, timer expiration, ^[RTS]  signals generated via the sigevent structure and terminal activity, as well as invocations of the kill() ^[RTS]  and sigqueue() functions. In some circumstances, the same event generates signals for multiple processes.

At the time of generation, a determination shall be made whether the signal has been generated for the process or for a specific thread within the process. Signals which are generated by some action attributable to a particular thread, such as a hardware fault, shall be generated for the thread that caused the signal to be generated. Signals that are generated in association with a process ID or process group ID or an asynchronous event, such as terminal activity, shall be generated for the process.

Each process has an action to be taken in response to each signal defined by the system (see Signal Actions ). A signal is said to be ``delivered'' to a process when the appropriate action for the process and signal is taken. A signal is said to be ``accepted'' by a process when the signal is selected and returned by one of the sigwait() functions.

During the time between the generation of a signal and its delivery or acceptance, the signal is said to be ``pending''. Ordinarily, this interval cannot be detected by an application. However, a signal can be ``blocked'' from delivery to a thread. If the action associated with a blocked signal is anything other than to ignore the signal, and if that signal is generated for the thread, the signal shall remain pending until it is unblocked, it is accepted when it is selected and returned by a call to the sigwait() function, or the action associated with it is set to ignore the signal. Signals generated for the process shall be delivered to exactly one of those threads within the process which is in a call to a sigwait() function selecting that signal or has not blocked delivery of the signal. If there are no threads in a call to a sigwait() function selecting that signal, and if all threads within the process block delivery of the signal, the signal shall remain pending on the process until a thread calls a sigwait() function selecting that signal, a thread unblocks delivery of the signal, or the action associated with the signal is set to ignore the signal. If the action associated with a blocked signal is to ignore the signal and if that signal is generated for the process, it is unspecified whether the signal is discarded immediately upon generation or remains pending.

Each thread has a ``signal mask'' that defines the set of signals currently blocked from delivery to it. The signal mask for a thread shall be initialized from that of its parent or creating thread, or from the corresponding thread in the parent process if the thread was created as the result of a call to fork(). The pthread_sigmask(), sigaction(), sigprocmask(), and sigsuspend() functions control the manipulation of the signal mask.

The determination of which action is taken in response to a signal is made at the time the signal is delivered, allowing for any changes since the time of generation. This determination is independent of the means by which the signal was originally generated. If a subsequent occurrence of a pending signal is generated, it is implementation-defined as to whether the signal is delivered or accepted more than once ^[RTS]  in circumstances other than those in which queuing is required under the Realtime Signals Extension option. The order in which multiple, simultaneously pending signals outside the range SIGRTMIN to SIGRTMAX are delivered to or accepted by a process is unspecified.

When any stop signal (SIGSTOP, SIGTSTP, SIGTTIN, SIGTTOU) is generated for a process, any pending SIGCONT signals for that process shall be discarded. Conversely, when SIGCONT is generated for a process, all pending stop signals for that process shall be discarded. When SIGCONT is generated for a process that is stopped, the process shall be continued, even if the SIGCONT signal is blocked or ignored. If SIGCONT is blocked and not ignored, it shall remain pending until it is either unblocked or a stop signal is generated for the process.

An implementation shall document any condition not specified by this volume of IEEE Std 1003.1-2001 under which the implementation generates signals.

Realtime Signal Generation and Delivery

^[RTS] This section describes extensions to support realtime signal generation and delivery. This functionality is dependent on support of the Realtime Signals Extension option (and the rest of this section is not further marked for this option).

Some signal-generating functions, such as high-resolution timer expiration, asynchronous I/O completion, interprocess message arrival, and the sigqueue() function, support the specification of an application-defined value, either explicitly as a parameter to the function or in a sigevent structure parameter. The sigevent structure is defined in <signal.h> and contains at least the following members:

The sigev_notify member specifies the notification mechanism to use when an asynchronous event occurs. This volume of IEEE Std 1003.1-2001 defines the following values for the sigev_notify member:

SIGEV_NONE

No asynchronous notification shall be delivered when the event of interest occurs.

SIGEV_SIGNAL

The signal specified in sigev_signo shall be generated for the process when the event of interest occurs. If the implementation supports the Realtime Signals Extension option and if the SA_SIGINFO flag is set for that signal number, then the signal shall be queued to the process and the value specified in sigev_value shall be the si_value component of the generated signal. If SA_SIGINFO is not set for that signal number, it is unspecified whether the signal is queued and what value, if any, is sent.

SIGEV_THREAD

A notification function shall be called to perform notification.

An implementation may define additional notification mechanisms.

The sigev_signo member specifies the signal to be generated. The sigev_value member is the application-defined value to be passed to the signal-catching function at the time of the signal delivery or to be returned at signal acceptance as the si_value member of the siginfo_t structure.

The sigval union is defined in <signal.h> and contains at least the following members:

The sival_int member shall be used when the application-defined value is of type int; the sival_ptr member shall be used when the application-defined value is a pointer.

When a signal is generated by the sigqueue() function or any signal-generating function that supports the specification of an application-defined value, the signal shall be marked pending and, if the SA_SIGINFO flag is set for that signal, the signal shall be queued to the process along with the application-specified signal value. Multiple occurrences of signals so generated are queued in FIFO order. It is unspecified whether signals so generated are queued when the SA_SIGINFO flag is not set for that signal.

Signals generated by the kill() function or other events that cause signals to occur, such as detection of hardware faults, alarm() timer expiration, or terminal activity, and for which the implementation does not support queuing, shall have no effect on signals already queued for the same signal number.

When multiple unblocked signals, all in the range SIGRTMIN to SIGRTMAX, are pending, the behavior shall be as if the implementation delivers the pending unblocked signal with the lowest signal number within that range. No other ordering of signal delivery is specified.

If, when a pending signal is delivered, there are additional signals queued to that signal number, the signal shall remain pending. Otherwise, the pending indication shall be reset.

Multi-threaded programs can use an alternate event notification mechanism. When a notification is processed, and the sigev_notify member of the sigevent structure has the value SIGEV_THREAD, the function sigev_notify_function is called with parameter sigev_value.

The function shall be executed in an environment as if it were the start_routine for a newly created thread with thread attributes specified by sigev_notify_attributes. If sigev_notify_attributes is NULL, the behavior shall be as if the thread were created with the detachstate attribute set to PTHREAD_CREATE_DETACHED. Supplying an attributes structure with a detachstate attribute of PTHREAD_CREATE_JOINABLE results in undefined behavior. The signal mask of this thread is implementation-defined.

Signal Actions

There are three types of action that can be associated with a signal: SIG_DFL, SIG_IGN, or a pointer to a function. Initially, all signals shall be set to SIG_DFL or SIG_IGN prior to entry of the main() routine (see the exec functions). The actions prescribed by these values are as follows:

SIG_DFL

Signal-specific default action.

The default actions for the signals defined in this volume of IEEE Std 1003.1-2001 are specified under <signal.h>. ^[RTS]  If the Realtime Signals Extension option is supported, the default actions for the realtime signals in the range SIGRTMIN to SIGRTMAX shall be to terminate the process abnormally.

If the default action is to stop the process, the execution of that process is temporarily suspended. When a process stops, a SIGCHLD signal shall be generated for its parent process, unless the parent process has set the SA_NOCLDSTOP flag. While a process is stopped, any additional signals that are sent to the process shall not be delivered until the process is continued, except SIGKILL which always terminates the receiving process. A process that is a member of an orphaned process group shall not be allowed to stop in response to the SIGTSTP, SIGTTIN, or SIGTTOU signals. In cases where delivery of one of these signals would stop such a process, the signal shall be discarded.

Setting a signal action to SIG_DFL for a signal that is pending, and whose default action is to ignore the signal (for example, SIGCHLD), shall cause the pending signal to be discarded, whether or not it is blocked. ^[RTS]  If the Realtime Signals Extension option is supported, any queued values pending shall be discarded and the resources used to queue them shall be released and returned to the system for other use.

The default action for SIGCONT is to resume execution at the point where the process was stopped, after first handling any pending unblocked signals.

^[XSI] When a stopped process is continued, a SIGCHLD signal may be generated for its parent process, unless the parent process has set the SA_NOCLDSTOP flag.

SIG_IGN

Ignore signal.

Delivery of the signal shall have no effect on the process. The behavior of a process is undefined after it ignores a SIGFPE, SIGILL, SIGSEGV, ^[RTS]  or SIGBUS signal that was not generated by kill(), ^[RTS] sigqueue(), or raise().

The system shall not allow the action for the signals SIGKILL or SIGSTOP to be set to SIG_IGN.

Setting a signal action to SIG_IGN for a signal that is pending shall cause the pending signal to be discarded, whether or not it is blocked.

If a process sets the action for the SIGCHLD signal to SIG_IGN, the behavior is unspecified, ^[XSI]  except as specified below.

If the action for the SIGCHLD signal is set to SIG_IGN, child processes of the calling processes shall not be transformed into zombie processes when they terminate. If the calling process subsequently waits for its children, and the process has no unwaited-for children that were transformed into zombie processes, it shall block until all of its children terminate, and wait(), waitid(), and waitpid() shall fail and set errno to [ECHILD].

^[RTS] If the Realtime Signals Extension option is supported, any queued values pending shall be discarded and the resources used to queue them shall be released and made available to queue other signals.

pointer to a function

Catch signal.

On delivery of the signal, the receiving process is to execute the signal-catching function at the specified address. After returning from the signal-catching function, the receiving process shall resume execution at the point at which it was interrupted.

If the SA_SIGINFO flag for the signal is cleared, the signal-catching function shall be entered as a C-language function call as follows:

void func(int signo);

^[XSI|RTS] If the SA_SIGINFO flag for the signal is set, the signal-catching function shall be entered as a C-language function call as follows:

void func(int signo, siginfo_t *info, void *context);

where func is the specified signal-catching function, signo is the signal number of the signal being delivered, and info is a pointer to a siginfo_t structure defined in <signal.h> containing at least the following members:

Member Type	Member Name	Description
int	si_signo	Signal number.
int	si_code	Cause of the signal.
union sigval	si_value	Signal value.

The si_signo member shall contain the signal number. This shall be the same as the signo parameter. The si_code member shall contain a code identifying the cause of the signal. The following values are defined for si_code:

SI_USER

The signal was sent by the kill() function. The implementation may set si_code to SI_USER if the signal was sent by the raise() or abort() functions or any similar functions provided as implementation extensions.

SI_QUEUE

^[RTS] The signal was sent by the sigqueue() function.

SI_TIMER

^[RTS] The signal was generated by the expiration of a timer set by timer_settime().

SI_ASYNCIO

^[RTS] The signal was generated by the completion of an asynchronous I/O request.

SI_MESGQ

^[RTS] The signal was generated by the arrival of a message on an empty message queue.

If the signal was not generated by one of the functions or events listed above, the si_code shall be set to an implementation-defined value that is not equal to any of the values defined above.

^[RTS] If the Realtime Signals Extension is supported, and si_code is one of SI_QUEUE, SI_TIMER, SI_ASYNCIO, or SI_MESGQ, then si_value shall contain the application-specified signal value. Otherwise, the contents of si_value are undefined.

The behavior of a process is undefined after it returns normally from a signal-catching function for a ^[XSI]  SIGBUS, SIGFPE, SIGILL, or SIGSEGV signal that was not generated by kill(), ^[RTS] sigqueue(), or raise().

The system shall not allow a process to catch the signals SIGKILL and SIGSTOP.

If a process establishes a signal-catching function for the SIGCHLD signal while it has a terminated child process for which it has not waited, it is unspecified whether a SIGCHLD signal is generated to indicate that child process.

When signal-catching functions are invoked asynchronously with process execution, the behavior of some of the functions defined by this volume of IEEE Std 1003.1-2001 is unspecified if they are called from a signal-catching function.

The following table defines a set of functions that shall be either reentrant or non-interruptible by signals and shall be async-signal-safe. Therefore applications may invoke them, without restriction, from signal-catching functions:

_Exit() _exit() abort() accept() access() aio_error() aio_return() aio_suspend() alarm() bind() cfgetispeed() cfgetospeed() cfsetispeed() cfsetospeed() chdir() chmod() chown() clock_gettime() close() connect() creat() dup() dup2() execle() execve() fchmod() fchown() fcntl() fdatasync() fork()

fpathconf() fstat() fsync() ftruncate() getegid() geteuid() getgid() getgroups() getpeername() getpgrp() getpid() getppid() getsockname() getsockopt() getuid() kill() link() listen() lseek() lstat() mkdir() mkfifo() open() pathconf() pause() pipe() poll() posix_trace_event() pselect() raise()

read() readlink() recv() recvfrom() recvmsg() rename() rmdir() select() sem_post() send() sendmsg() sendto() setgid() setpgid() setsid() setsockopt() setuid() shutdown() sigaction() sigaddset() sigdelset() sigemptyset() sigfillset() sigismember() sleep() signal() sigpause() sigpending() sigprocmask() sigqueue()

sigset() sigsuspend() socket() socketpair() stat() symlink() sysconf() tcdrain() tcflow() tcflush() tcgetattr() tcgetpgrp() tcsendbreak() tcsetattr() tcsetpgrp() time() timer_getoverrun() timer_gettime() timer_settime() times() umask() uname() unlink() utime() wait() waitpid() write()

All functions not in the above table are considered to be unsafe with respect to signals. In the presence of signals, all functions defined by this volume of IEEE Std 1003.1-2001 shall behave as defined when called from or interrupted by a signal-catching function, with a single exception: when a signal interrupts an unsafe function and the signal-catching function calls an unsafe function, the behavior is undefined.

When a signal is delivered to a thread, if the action of that signal specifies termination, stop, or continue, the entire process shall be terminated, stopped, or continued, respectively.

Signal Effects on Other Functions

Signals affect the behavior of certain functions defined by this volume of IEEE Std 1003.1-2001 if delivered to a process while it is executing such a function. If the action of the signal is to terminate the process, the process shall be terminated and the function shall not return. If the action of the signal is to stop the process, the process shall stop until continued or terminated. Generation of a SIGCONT signal for the process shall cause the process to be continued, and the original function shall continue at the point the process was stopped. If the action of the signal is to invoke a signal-catching function, the signal-catching function shall be invoked; in this case the original function is said to be ``interrupted'' by the signal. If the signal-catching function executes a return statement, the behavior of the interrupted function shall be as described individually for that function, except as noted for unsafe functions. Signals that are ignored shall not affect the behavior of any function; signals that are blocked shall not affect the behavior of any function until they are unblocked and then delivered, except as specified for the sigpending() and sigwait() functions.