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SEM_OPEN(3) Linux Programmer's Manual SEM_OPEN(3)


     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);
     Link with -pthread.


     sem_open()  creates  a  new  POSIX semaphore or opens an existing sema-
     phore.  The semaphore is identified by name.  For details of  the  con-
     struction 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 sema-
     phore  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
     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 per-
     missions bits can be obtained by including <sys/stat.h>.)  The  permis-
     sions  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


     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 per-process limit on the number of open file descriptors has
            been reached.
            name was too long.
     ENFILE The system-wide limit on the total number of open files has been
     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.


     For  an  explanation  of  the  terms  used   in   this   section,   see
     |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),


     This page is part of release 4.16 of the Linux  man-pages  project.   A
     description  of  the project, information about reporting bugs, and the
     latest    version    of    this    page,    can     be     found     at

Linux 2017-09-15 SEM_OPEN(3)

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