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PROLOG | NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | EXAMPLES | APPLICATION USAGE | RATIONALE | FUTURE DIRECTIONS | SEE ALSO | COPYRIGHT |
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ALARM(3P) POSIX Programmer's Manual ALARM(3P)
This manual page is part of the POSIX Programmer's Manual. The
Linux implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
alarm — schedule an alarm signal
#include <unistd.h>
unsigned alarm(unsigned seconds);
The alarm() function shall cause the system to generate a SIGALRM
signal for the process after the number of realtime seconds
specified by seconds have elapsed. Processor scheduling delays may
prevent the process from handling the signal as soon as it is
generated.
If seconds is 0, a pending alarm request, if any, is canceled.
Alarm requests are not stacked; only one SIGALRM generation can be
scheduled in this manner. If the SIGALRM signal has not yet been
generated, the call shall result in rescheduling the time at which
the SIGALRM signal is generated.
Interactions between alarm() and setitimer() are unspecified.
If there is a previous alarm() request with time remaining,
alarm() shall return a non-zero value that is the number of
seconds until the previous request would have generated a SIGALRM
signal. Otherwise, alarm() shall return 0.
The alarm() function is always successful, and no return value is
reserved to indicate an error.
The following sections are informative.
None.
The fork() function clears pending alarms in the child process. A
new process image created by one of the exec functions inherits
the time left to an alarm signal in the image of the old process.
Application developers should note that the type of the argument
seconds and the return value of alarm() is unsigned. That means
that a Strictly Conforming POSIX System Interfaces Application
cannot pass a value greater than the minimum guaranteed value for
{UINT_MAX}, which the ISO C standard sets as 65535, and any
application passing a larger value is restricting its portability.
A different type was considered, but historical implementations,
including those with a 16-bit int type, consistently use either
unsigned or int.
Application developers should be aware of possible interactions
when the same process uses both the alarm() and sleep() functions.
Many historical implementations (including Version 7 and System V)
allow an alarm to occur up to a second early. Other
implementations allow alarms up to half a second or one clock tick
early or do not allow them to occur early at all. The latter is
considered most appropriate, since it gives the most predictable
behavior, especially since the signal can always be delayed for an
indefinite amount of time due to scheduling. Applications can thus
choose the seconds argument as the minimum amount of time they
wish to have elapse before the signal.
The term ``realtime'' here and elsewhere (sleep(), times()) is
intended to mean ``wall clock'' time as common English usage, and
has nothing to do with ``realtime operating systems''. It is in
contrast to virtual time, which could be misinterpreted if just
time were used.
In some implementations, including 4.3 BSD, very large values of
the seconds argument are silently rounded down to an
implementation-specific maximum value. This maximum is large
enough (to the order of several months) that the effect is not
noticeable.
There were two possible choices for alarm generation in multi-
threaded applications: generation for the calling thread or
generation for the process. The first option would not have been
particularly useful since the alarm state is maintained on a per-
process basis and the alarm that is established by the last
invocation of alarm() is the only one that would be active.
Furthermore, allowing generation of an asynchronous signal for a
thread would have introduced an exception to the overall signal
model. This requires a compelling reason in order to be justified.
None.
alarm(3p), exec(1p), fork(3p), getitimer(3p), pause(3p),
sigaction(3p), sleep(3p)
The Base Definitions volume of POSIX.1‐2017, signal.h(0p),
unistd.h(0p)
Portions of this text are reprinted and reproduced in electronic
form from IEEE Std 1003.1-2017, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The
Open Group Base Specifications Issue 7, 2018 Edition, Copyright
(C) 2018 by the Institute of Electrical and Electronics Engineers,
Inc and The Open Group. In the event of any discrepancy between
this version and the original IEEE and The Open Group Standard,
the original IEEE and The Open Group Standard is the referee
document. The original Standard can be obtained online at
http://www.opengroup.org/unix/online.html .
Any typographical or formatting errors that appear in this page
are most likely to have been introduced during the conversion of
the source files to man page format. To report such errors, see
https://www.kernel.org/doc/man-pages/reporting_bugs.html .
IEEE/The Open Group 2017 ALARM(3P)
Pages that refer to this page: signal.h(0p), unistd.h(0p), sleep(1p), alarm(3p), exec(3p), fcntl(3p), fork(3p), getitimer(3p), lockf(3p), posix_spawn(3p), pselect(3p), sleep(3p), times(3p)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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