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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.1‐2008, signal.h(0p),
unistd.h(0p)
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/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 2013 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)