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NAME

       sem_open - initialize and open a named semaphore

SYNOPSIS

       #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);

       Link with -lrt or -pthread.

DESCRIPTION

       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.

RETURN VALUE

       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.

ERRORS

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

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

       EINVAL value was greater than SEM_VALUE_MAX.

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

       EMFILE The process already has the maximum number of files and open.

       ENAMETOOLONG
              name was too long.

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

       ENOENT 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.

       ENOMEM Insufficient memory.

CONFORMING TO

       POSIX.1-2001.

SEE ALSO

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

COLOPHON

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       project, and information about reporting bugs, can be found at  http://man7.org/linux/man-
       pages/.