Section: POSIX Programmer's Manual (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.
--- schedule an alarm signal
unsigned alarm(unsigned seconds);
function shall cause the system to generate a SIGALRM signal for the
process after the number of realtime seconds specified by
have elapsed. Processor scheduling delays may prevent the process from
handling the signal as soon as it is generated.
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.
If there is a previous
request with time remaining,
shall return a non-zero value that is the number of seconds until the
previous request would have generated a SIGALRM signal. Otherwise,
shall return 0.
function is always successful, and no return value is reserved to
indicate an error.
The following sections are informative.
function clears pending alarms in the child process. A new process
image created by one of the
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
and the return value of
That means that a Strictly Conforming POSIX System Interfaces
Application cannot pass a value greater than the minimum guaranteed
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
type, consistently use either
Application developers should be aware of possible interactions when
the same process uses both the
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
argument as the minimum amount of time they wish to have elapse before
The term ``realtime'' here and elsewhere (sleep(),
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
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
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.
The Base Definitions volume of POSIX.1-2008,
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
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