sem_open — initialize and open a named semaphore


#include <fcntl.h>            /* For O_* constants */
#include <sys/stat.h>         /* For mode constants */
#include <semaphore.h>
sem_t *sem_open( const char *name,
  int oflag);
sem_t *sem_open( const char *name,
  int oflag,
  mode_t mode,
  unsigned int value);
[Note] Note

Link with −pthread.


sem_open() creates a new POSIX semaphore or opens an existing semaphore. The semaphore is identified by name. For details of the construction of name, see sem_overview(7).

The oflag argument specifies flags that control the operation of the call. (Definitions of the flags values can be obtained by including <fcntl.h> If O_CREAT is specified in oflag, then the semaphore is created if it does not already exist. The owner (user ID) of the semaphore is set to the effective user ID of the calling process. The group ownership (group ID) is set to the effective group ID of the calling process. If both O_CREAT and O_EXCL are specified in oflag, then an error is returned if a semaphore with the given name already exists.

If O_CREAT is specified in oflag, then two additional arguments must be supplied. The mode argument specifies the permissions to be placed on the new semaphore, as for open(2). (Symbolic definitions for the permissions bits can be obtained by including <sys/stat.h> The permissions settings are masked against the process umask. Both read and write permission should be granted to each class of user that will access the semaphore. The value argument specifies the initial value for the new semaphore. If O_CREAT is specified, and a semaphore with the given name already exists, then mode and value are ignored.


On success, sem_open() returns the address of the new semaphore; this address is used when calling other semaphore-related functions. On error, sem_open() returns SEM_FAILED, with errno set to indicate the error.



The semaphore exists, but the caller does not have permission to open it.


Both O_CREAT and O_EXCL were specified in oflag, but a semaphore with this name already exists.


value was greater than SEM_VALUE_MAX.


name consists of just "/", followed by no other characters.


The per-process limit on the number of open file descriptors has been reached.


name was too long.


The system-wide limit on the total number of open files has been reached.


The O_CREAT flag was not specified in oflag and no semaphore with this name exists; or, O_CREAT was specified, but name wasn't well formed.


Insufficient memory.


For an explanation of the terms used in this section, see attributes(7).

Interface Attribute Value
sem_open() Thread safety MT-Safe


POSIX.1-2001, POSIX.1-2008.


sem_close(3), sem_getvalue(3), sem_post(3), sem_unlink(3), sem_wait(3), sem_overview(7)


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  Copyright (C) 2006 Michael Kerrisk <>

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