SEMACQUIRE(2) SEMACQUIRE(2)
NAME
semacquire, tsemacquire, semrelease - user level semaphores
SYNOPSIS
#include <u.h>
#include <libc.h>
int semacquire(long *addr, int block);
int tsemacquire(long *addr, ulong ms);
long semrelease(long *addr, long count);
DESCRIPTION
Semacquire, tsemacquire, and semrelease facilitate schedul-
ing between processes sharing memory. Processes arrange to
share memory by using rfork with the RFMEM flag (see
fork(2)), segattach(2), or thread(2).
The semaphore's value is the integer pointed at by addr.
Semacquire atomically waits until the semaphore has a posi-
tive value and then decrements that value. If block is zero
and the semaphore is not immediately available, semacquire
returns 0 instead of waiting. Tsemacquire only waits ms
milliseconds for the semaphore to attain a positive value
and, if available in that time, decrements that value. It
returns 0 otherwise. Both functions return 1 if the sema-
phore was acquired and -1 on error (e.g., if they were
interrupted). Semrelease adds count to the semaphore's
value and returns the new value.
Semacquire (and analogously for tsemacquire) and semrelease
can be thought of as efficient, correct replacements for:
int
semacquire(long *addr, int block)
{
while(*addr == 0){
if(!block)
return 0;
if(interrupted)
return -1;
}
--*addr;
return 1;
}
int
semrelease(long *addr, int count)
{
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SEMACQUIRE(2) SEMACQUIRE(2)
return *addr += count;
}
Like rendezvous(2), semacquire, tsemacquire, and semrelease
are not typically used directly. Instead, they are intended
to be used to coordinate scheduling in higher-level abstrac-
tions such as locks, rendezvous points, and channels (see
lock(2) and thread(2)). Also like rendezvous, semacquire,
tsemacquire, and semrelease cannot be used to coordinate
between threads in a single process. Use locks, rendezvous
points, or channels instead.
SOURCE
/sys/src/9/port/sysproc.c
SEE ALSO
fork(2), lock(2), rendezvous(2), segattach(2), thread(2)
DIAGNOSTICS
These functions set errstr.
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