term% cat index.txt SEMACQUIRE(2) System Calls Manual 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 scheduling 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 positive value and then
decrements that value. If block is zero and the semaphore is not imme‐
diately available, semacquire returns 0 instead of waiting. Tsemac‐
quire only waits ms milliseconds for the semaphore to attain a positive
value and, if available in that time, decrements that value. It re‐
turns 0 otherwise. Both functions return 1 if the semaphore was ac‐
quired 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)
{
return *addr += count;
}
Like rendezvous(2), semacquire, tsemacquire, and semrelease are not
typically used directly. Instead, they are intended to be used to co‐
ordinate scheduling in higher-level abstractions such as locks, ren‐
dezvous 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.
SEMACQUIRE(2)