Manual Pages for UNIX Operating System command usage for man sem_open

Standard C Library Functions                         sem_open(3C)



NAME
     sem_open - initialize/open a named semaphore

SYNOPSIS
     #include 

     sem_t *sem_open(const char *name, int oflag,
          /* unsigned long mode, unsigned int value */ ...);


DESCRIPTION
     The sem_open() function establishes a connection  between  a
     named  semaphore and a process (or LWP or thread). Following
     a call to sem_open() with semaphore name name,  the  process
     may  reference  the semaphore associated with name using the
     address returned from the call. This semaphore may  be  used
     in   subsequent   calls  to  sem_wait(3C),  sem_trywait(3C),
     sem_post(3C),  and  sem_close(3C).  The  semaphore   remains
     usable  by  this  process until the semaphore is closed by a
     successful call to sem_close(3C), _Exit(2), or  one  of  the
     exec functions.


     The oflag argument controls whether the semaphore is created
     or  merely accessed by the call to sem_open(). The following
     flag bits may be set in oflag:

     O_CREAT    This flag is used to create  a  semaphore  if  it
                does not already exist. If O_CREAT is set and the
                semaphore already exists,  then  O_CREAT  has  no
                effect,  except as noted under O_EXCL. Otherwise,
                sem_open() creates a named semaphore. The O_CREAT
                flag  requires  a  third  and  a fourth argument:
                mode, which is of type mode_t, and  value,  which
                is of type unsigned int. The semaphore is created
                with an initial value  of  value.  Valid  initial
                values  for  semaphores are less than or equal to
                SEM_VALUE_MAX.

                The user ID of the semaphore is set to the effec-
                tive  user ID of the process; the group ID of the
                semaphore is set to a system default group ID  or
                to  the  effective  group  ID of the process. The
                permission bits of the semaphore are set  to  the
                value  of  the  mode argument except those set in
                the file mode creation mask of the  process  (see
                umask(2)).  When bits in mode other than the file
                permission bits  are  specified,  the  effect  is
                unspecified.

                After the semaphore named name has  been  created
                by   sem_open()  with  the  O_CREAT  flag,  other



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Standard C Library Functions                         sem_open(3C)



                processes can connect to the semaphore by calling
                sem_open() with the same value of name.


     O_EXCL     If O_EXCL and O_CREAT are set,  sem_open()  fails
                if  the  semaphore name exists. The check for the
                existence of the semaphore and  the  creation  of
                the  semaphore  if  it  does not exist are atomic
                with  respect  to   other   processes   executing
                sem_open() with O_EXCL and O_CREAT set. If O_EXCL
                is set and O_CREAT is  not  set,  the  effect  is
                undefined.



     If flags other than O_CREAT and O_EXCL are specified in  the
     oflag parameter, the effect is unspecified.


     The name argument points to  a  string  naming  a  semaphore
     object.  It  is  unspecified whether the name appears in the
     file system and is visible to functions that take  pathnames
     as arguments. The name argument conforms to the construction
     rules for a pathname. The first character of  name must be a
     slash   (/)  character and the remaining characters of  name
     cannot include any slash characters.  For maximum  portabil-
     ity,   name  should  include no more than 14 characters, but
     this limit is not enforced.


     If a process makes multiple successful calls  to  sem_open()
     with  the same value for name, the same semaphore address is
     returned for each such successful call, provided that  there
     have been no calls to sem_unlink(3C) for this semaphore.


     References to copies  of  the  semaphore  produce  undefined
     results.


     The sem_init(3C) function is used with unnamed semaphores.

RETURN VALUES
     Upon successful completion, the function returns the address
     of  the  semaphore.  Otherwise,  it  will  return a value of
     SEM_FAILED and set errno to indicate the error.  The  symbol
     SEM_FAILED  is  defined in the header . No suc-
     cessful  return  from  sem_open()  will  return  the   value
     SEM_FAILED.

ERRORS




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Standard C Library Functions                         sem_open(3C)



     If any of the following  conditions  occur,  the  sem_open()
     function  will  return  SEM_FAILED  and  set  errno  to  the
     corresponding value:

     EACCES          The named semaphore exists  and  the  O_RDWR
                     permissions  are  denied, or the named sema-
                     phore  does  not  exist  and  permission  to
                     create the named semaphore is denied.


     EEXIST          O_CREAT and  O_EXCL are set  and  the  named
                     semaphore already exists.


     EINTR           The sem_open() function was interrupted by a
                     signal.


     EINVAL          The sem_open() operation  is  not  supported
                     for  the  given  name, or O_CREAT was set in
                     oflag   and   value    is    greater    than
                     SEM_VALUE_MAX.


     EMFILE          The number of open semaphore descriptors  in
                     this  process exceeds  SEM_NSEMS_MAX, or the
                     number of open file descriptors in this pro-
                     cess exceeds  OPEN_MAX.


     ENAMETOOLONG    The length of name string exceeds  PATH_MAX,
                     or  a  pathname  component  is  longer  than
                     NAME_MAX while _POSIX_NO_TRUNC is in effect.


     ENFILE          Too many semaphores are  currently  open  in
                     the system.


     ENOENT          O_CREAT is not set and the  named  semaphore
                     does not exist.


     ENOSPC          There is insufficient space for the creation
                     of the new named semaphore.


     ENOSYS          The sem_open() function is not supported  by
                     the system.






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Standard C Library Functions                         sem_open(3C)



ATTRIBUTES
     See attributes(5) for descriptions of the  following  attri-
     butes:



     ____________________________________________________________
    | ATTRIBUTE TYPE              | ATTRIBUTE VALUE             |
    |_____________________________|_____________________________|
    | Interface Stability         | Committed                   |
    |_____________________________|_____________________________|
    | MT-Level                    | MT-Safe                     |
    |_____________________________|_____________________________|
    | Standard                    | See standards(5).           |
    |_____________________________|_____________________________|


SEE ALSO
     exec(2),  exit(2),  umask(2),  sem_close(3C),  sem_init(3C),
     sem_post(3C),   sem_unlink(3C),  sem_wait(3C),  sysconf(3C),
     attributes(5), standards(5)


































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