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| .TH SIGWAITINFO 2 2021-03-22 "Linux" "Linux Programmer's Manual" |
| .SH NAME |
| sigwaitinfo, sigtimedwait, rt_sigtimedwait \- synchronously wait |
| for queued signals |
| .SH SYNOPSIS |
| .nf |
| .B #include <signal.h> |
| .PP |
| .BI "int sigwaitinfo(const sigset_t *restrict " set , |
| .BI " siginfo_t *restrict " info ); |
| .BI "int sigtimedwait(const sigset_t *restrict " set , |
| .BI " siginfo_t *restrict " info , |
| .BI " const struct timespec *restrict " timeout ); |
| .fi |
| .PP |
| .RS -4 |
| Feature Test Macro Requirements for glibc (see |
| .BR feature_test_macros (7)): |
| .RE |
| .PP |
| .BR sigwaitinfo (), |
| .BR sigtimedwait (): |
| .nf |
| _POSIX_C_SOURCE >= 199309L |
| .fi |
| .SH DESCRIPTION |
| .BR sigwaitinfo () |
| suspends execution of the calling thread until one of the signals in |
| .I set |
| is pending |
| (If one of the signals in |
| .I set |
| is already pending for the calling thread, |
| .BR sigwaitinfo () |
| will return immediately.) |
| .PP |
| .BR sigwaitinfo () |
| removes the signal from the set of pending |
| signals and returns the signal number as its function result. |
| If the |
| .I info |
| argument is not NULL, |
| then the buffer that it points to is used to return a structure of type |
| .I siginfo_t |
| (see |
| .BR sigaction (2)) |
| containing information about the signal. |
| .PP |
| If multiple signals in |
| .I set |
| are pending for the caller, the signal that is retrieved by |
| .BR sigwaitinfo () |
| is determined according to the usual ordering rules; see |
| .BR signal (7) |
| for further details. |
| .PP |
| .BR sigtimedwait () |
| operates in exactly the same way as |
| .BR sigwaitinfo () |
| except that it has an additional argument, |
| .IR timeout , |
| which specifies the interval for which |
| the thread is suspended waiting for a signal. |
| (This interval will be rounded up to the system clock granularity, |
| and kernel scheduling delays mean that the interval |
| may overrun by a small amount.) |
| This argument is of the following type: |
| .PP |
| .in +4n |
| .EX |
| struct timespec { |
| long tv_sec; /* seconds */ |
| long tv_nsec; /* nanoseconds */ |
| } |
| .EE |
| .in |
| .PP |
| If both fields of this structure are specified as 0, a poll is performed: |
| .BR sigtimedwait () |
| returns immediately, either with information about a signal that |
| was pending for the caller, or with an error |
| if none of the signals in |
| .I set |
| was pending. |
| .SH RETURN VALUE |
| On success, both |
| .BR sigwaitinfo () |
| and |
| .BR sigtimedwait () |
| return a signal number (i.e., a value greater than zero). |
| On failure both calls return \-1, with |
| .I errno |
| set to indicate the error. |
| .SH ERRORS |
| .TP |
| .B EAGAIN |
| No signal in |
| .I set |
| was became pending within the |
| .I timeout |
| period specified to |
| .BR sigtimedwait (). |
| .TP |
| .B EINTR |
| The wait was interrupted by a signal handler; see |
| .BR signal (7). |
| (This handler was for a signal other than one of those in |
| .IR set .) |
| .TP |
| .B EINVAL |
| .I timeout |
| was invalid. |
| .SH CONFORMING TO |
| POSIX.1-2001, POSIX.1-2008. |
| .SH NOTES |
| In normal usage, the calling program blocks the signals in |
| .I set |
| via a prior call to |
| .BR sigprocmask (2) |
| (so that the default disposition for these signals does not occur if they |
| become pending between successive calls to |
| .BR sigwaitinfo () |
| or |
| .BR sigtimedwait ()) |
| and does not establish handlers for these signals. |
| In a multithreaded program, |
| the signal should be blocked in all threads, in order to prevent |
| the signal being treated according to its default disposition in |
| a thread other than the one calling |
| .BR sigwaitinfo () |
| or |
| .BR sigtimedwait ()). |
| .PP |
| The set of signals that is pending for a given thread is the |
| union of the set of signals that is pending specifically for that thread |
| and the set of signals that is pending for the process as a whole (see |
| .BR signal (7)). |
| .PP |
| Attempts to wait for |
| .B SIGKILL |
| and |
| .B SIGSTOP |
| are silently ignored. |
| .PP |
| If multiple threads of a process are blocked |
| waiting for the same signal(s) in |
| .BR sigwaitinfo () |
| or |
| .BR sigtimedwait (), |
| then exactly one of the threads will actually receive the |
| signal if it becomes pending for the process as a whole; |
| which of the threads receives the signal is indeterminate. |
| .PP |
| .BR sigwaitinfo () |
| or |
| .BR sigtimedwait (), |
| can't be used to receive signals that |
| are synchronously generated, such as the |
| .BR SIGSEGV |
| signal that results from accessing an invalid memory address |
| or the |
| .BR SIGFPE |
| signal that results from an arithmetic error. |
| Such signals can be caught only via signal handler. |
| .PP |
| POSIX leaves the meaning of a NULL value for the |
| .I timeout |
| argument of |
| .BR sigtimedwait () |
| unspecified, permitting the possibility that this has the same meaning |
| as a call to |
| .BR sigwaitinfo (), |
| and indeed this is what is done on Linux. |
| .\" |
| .SS C library/kernel differences |
| On Linux, |
| .BR sigwaitinfo () |
| is a library function implemented on top of |
| .BR sigtimedwait (). |
| .PP |
| The glibc wrapper functions for |
| .BR sigwaitinfo () |
| and |
| .BR sigtimedwait () |
| silently ignore attempts to wait for the two real-time signals that |
| are used internally by the NPTL threading implementation. |
| See |
| .BR nptl (7) |
| for details. |
| .PP |
| The original Linux system call was named |
| .BR sigtimedwait (). |
| However, with the addition of real-time signals in Linux 2.2, |
| the fixed-size, 32-bit |
| .I sigset_t |
| type supported by that system call was no longer fit for purpose. |
| Consequently, a new system call, |
| .BR rt_sigtimedwait (), |
| was added to support an enlarged |
| .IR sigset_t |
| type. |
| The new system call takes a fourth argument, |
| .IR "size_t sigsetsize" , |
| which specifies the size in bytes of the signal set in |
| .IR set . |
| This argument is currently required to have the value |
| .IR sizeof(sigset_t) |
| (or the error |
| .B EINVAL |
| results). |
| The glibc |
| .BR sigtimedwait () |
| wrapper function hides these details from us, transparently calling |
| .BR rt_sigtimedwait () |
| when the kernel provides it. |
| .\" |
| .SH SEE ALSO |
| .BR kill (2), |
| .BR sigaction (2), |
| .BR signal (2), |
| .BR signalfd (2), |
| .BR sigpending (2), |
| .BR sigprocmask (2), |
| .BR sigqueue (3), |
| .BR sigsetops (3), |
| .BR sigwait (3), |
| .BR signal (7), |
| .BR time (7) |