Index for Section 3

Alphabetical listing for S

Bottom of page

sem_init(3)
NAME
  sem_init - Initializes an unnamed semaphore (P1003.1b)

LIBRARY
  Realtime Library (librt.so, librt.a)

SYNOPSIS
  #include <semaphore.h>

  int sem_init (
  sem_t *sem,
  int pshared,
  unsigned int value);

PARAMETERS
  sem	      Specifies a location to receive the descriptor of the
	      initialized semaphore.

  pshared Specifies a value indicating whether the semaphore should be
	  sharable between processes (non-zero value) or not (zero).

  value	  Specifies the initial value to be given to the semaphore.

DESCRIPTION
  The sem_init function creates a new counting semaphore with a specific
  value.  A semaphore is used to limit access to a critical resource.  When a
  process requires access to the resource without interference from other
  processes, it attempts to establish a connection with the associated
  semaphore.  If the semaphore value is greater than zero, the connection is
  established and the semaphore value is decremented by one.  If the
  semaphore value is less than or equal to zero, the process attempting to
  access the resource is blocked and must wait for another process to release
  the semaphore and increment the semaphore value.

  The sem_init function establishes a connection between an unnamed semaphore
  and a process; the sem_wait and sem_trywait functions lock the semaphore;
  and the sem_post function unlocks the semaphore.  Use the sem_destroy
  function to deallocate system resources allocated to the process for use
  with the semaphore.  You can use the sem_getvalue function to obtain the
  value of a semaphore.

  A semaphore created by a call to the sem_init function remains valid until
  the semaphore is removed by a call to the sem_destroy function.

RETURN VALUES
  On successful completion, the function returns the value 0 (zero);
  otherwise, the function returns the value -1 and sets errno to indicate the
  error.

ERRORS
  The sem_init function fails under the following conditions:

  [EINVAL]	      The value argument exceeds {SEM_VALUE_MAX}.

  [ENOSPC]	      A resource required to initialize the semaphore has
		      been exhausted.  The limit on semaphores
		      ({SEM_NSEMS_MAX}) has been reached.

  [EPERM]	      The process lacks the appropriate privilege to
		      initialize the semaphore.

RELATED INFORMATION
  Functions: sem_destroy(3), sem_post(3), sem_trywait(3),sem_wait(3)

  Guide to Realtime Programming

Index for Section 3

Alphabetical listing for S

Top of page