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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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