semacquire, tsemacquire, semrelease – user level semaphores

#include <u.h>
#include <libc.h>

int semacquire(long *addr, int block);

int tsemacquire(long *addr, ulong ms);

long semrelease(long *addr, long count);

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 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 semaphore 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:
semacquire(long *addr, int block)
while(*addr == 0){
return 0;
return –1;
return 1;
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 coordinate scheduling in higher–level abstractions 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.


fork(2), lock(2), rendezvous(2), segattach(2), thread(2)

These functions set errstr.