sem_getvalue(3)

1sem_getvalue(3)            Library Functions Manual            sem_getvalue(3)
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NAME

6       sem_getvalue - get the value of a semaphore
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LIBRARY

9       POSIX threads library (libpthread, -lpthread)
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SYNOPSIS

12       #include <semaphore.h>
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14       int sem_getvalue(sem_t *restrict sem, int *restrict sval);
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DESCRIPTION

17       sem_getvalue() places the current value of the semaphore pointed to sem
18       into the integer pointed to by sval.
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20       If one or more processes or threads are blocked  waiting  to  lock  the
21       semaphore  with  sem_wait(3), POSIX.1 permits two possibilities for the
22       value returned in sval: either 0 is  returned;  or  a  negative  number
23       whose  absolute  value  is  the  count  of  the number of processes and
24       threads currently blocked in sem_wait(3).  Linux adopts the former  be‐
25       havior.
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RETURN VALUE

28       sem_getvalue() returns 0 on success; on error, -1 is returned and errno
29       is set to indicate the error.
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ERRORS

32       EINVAL sem is not a valid semaphore.  (The  glibc  implementation  cur‐
33              rently does not check whether sem is valid.)
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ATTRIBUTES

36       For  an  explanation  of  the  terms  used  in  this  section,  see at‐
37       tributes(7).
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39       ┌────────────────────────────────────────────┬───────────────┬─────────┐
40       │Interface                                   │ Attribute     │ Value   │
41       ├────────────────────────────────────────────┼───────────────┼─────────┤
42       │sem_getvalue()                              │ Thread safety │ MT-Safe │
43       └────────────────────────────────────────────┴───────────────┴─────────┘
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STANDARDS

46       POSIX.1-2008.
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HISTORY

49       POSIX.1-2001.
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NOTES

52       The value of the  semaphore  may  already  have  changed  by  the  time
53       sem_getvalue() returns.
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