trusty (3) alarm.3posix.gz

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 .