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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
       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:

              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
       coordinate 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)