useconds_t ualarm(useconds_t usecs, useconds_t interval);
Feature Test Macro Requirements for glibc (see feature_test_macros(7)):
Since glibc 2.12:
(_XOPEN_SOURCE >= 500 ||
_XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED) &&
!(_POSIX_C_SOURCE >= 200809L || _XOPEN_SOURCE >= 700)
Before glibc 2.12:
_BSD_SOURCE || _XOPEN_SOURCE >= 500 ||
_XOPEN_SOURCE && _XOPEN_SOURCE_EXTENDED
The ualarm() function causes the signal SIGALRM to be sent to the
invoking process after (not less than) usecs microseconds. The delay
may be lengthened slightly by any system activity or by the time spent
processing the call or by the granularity of system timers.
Unless caught or ignored, the SIGALRM signal will terminate the
If the interval argument is nonzero, further SIGALRM signals will be
sent every interval microseconds after the first.
This function returns the number of microseconds remaining for any
alarm that was previously set, or 0 if no alarm was pending.
EINTR Interrupted by a signal.
EINVAL usecs or interval is not smaller than 1000000. (On systems
where that is considered an error.)
4.3BSD, POSIX.1-2001. POSIX.1-2001 marks ualarm() as obsolete.
POSIX.1-2008 removes the specification of ualarm(). 4.3BSD, SUSv2, and
POSIX do not define any errors.
POSIX.1-2001 does not specify what happens if the usecs argument is 0.
On Linux (and probably most other systems), the effect is to cancel any
The type useconds_t is an unsigned integer type capable of holding
integers in the range [0,1000000]. On the original BSD implementation,
and in glibc before version 2.1, the arguments to ualarm() were instead
typed as unsigned int. Programs will be more portable if they never
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