NAME
alarm - schedule an alarm signal
SYNOPSIS
#include <unistd.h>
unsigned alarm(unsigned seconds);
DESCRIPTION
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 any of setitimer(), ualarm(), or
usleep() are unspecified.
RETURN VALUE
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.
ERRORS
The alarm() function is always successful, and no return value is
reserved to indicate an error.
The following sections are informative.
EXAMPLES
None.
APPLICATION USAGE
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 old process’ image.
Application writers 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 writers should be aware of possible interactions when the
same process uses both the alarm() and sleep() functions.
RATIONALE
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-defined
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.
FUTURE DIRECTIONS
None.
SEE ALSO
alarm , exec() , fork() , getitimer() , pause() , sigaction() , sleep()
, ualarm() , usleep() , the Base Definitions volume of
IEEE Std 1003.1-2001, <signal.h>, <unistd.h>
COPYRIGHT
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2003 Edition, Standard for Information Technology
-- Portable Operating System Interface (POSIX), The Open Group Base
Specifications Issue 6, Copyright (C) 2001-2003 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 .