👤
Home Man
Search
Today's Posts
Register

Linux & Unix Commands - Search Man Pages
Man Page or Keyword Search:
Select Section of Man Page:
Select Man Page Repository:

RedHat 9 (Linux i386) - man page for semctl (redhat section 2)

SEMCTL(2)			    Linux Programmer's Manual				SEMCTL(2)

NAME
       semctl - semaphore control operations

SYNOPSIS
       #include <sys/types.h>
       #include <sys/ipc.h>
       #include <sys/sem.h>

       int semctl(int semid, int semnum, int cmd, ...);

DESCRIPTION
       The  function  semctl performs the control operation specified by cmd on the semaphore set
       identified by semid, or on the semnum-th semaphore of that set.	(Semaphores are  numbered
       starting at 0.)

       This  function  has  three  or  four  arguments.  When  there  are  four, the call is sem-
       ctl(semid,semnum,cmd,arg); where the fourth argument arg has a type union semun defined as
       follows:

       #if defined(__GNU_LIBRARY__) && !defined(_SEM_SEMUN_UNDEFINED)
       /* union semun is defined by including <sys/sem.h> */
       #else
       /* according to X/OPEN we have to define it ourselves */
       union semun {
	     int val;		       /* value for SETVAL */
	     struct semid_ds *buf;     /* buffer for IPC_STAT, IPC_SET */
	     unsigned short *array;    /* array for GETALL, SETALL */
				       /* Linux specific part: */
	     struct seminfo *__buf;    /* buffer for IPC_INFO */
       };
       #endif

       Legal values for cmd are:

       IPC_STAT    Copy  info from the semaphore set data structure into the structure pointed to
		   by arg.buf.	The argument semnum is ignored.  The calling  process  must  have
		   read access privileges on the semaphore set.

       IPC_SET	   Write  the  values  of  some  members  of the semid_ds structure pointed to by
		   arg.buf to the semaphore set data structure, updating also its sem_ctime  mem-
		   ber.   Considered members from the user supplied struct semid_ds pointed to by
		   arg.buf are

			sem_perm.uid
			sem_perm.gid
			sem_perm.mode  /* only lowest 9-bits */

		   The effective user-ID of the calling process must be that of  the  super-user,
		   or  match  the  creator or owner of the semaphore set.  The argument semnum is
		   ignored.

       IPC_RMID    Immediately remove the semaphore set and its  data  structures  awakening  all
		   waiting  processes  (with an error return and errno set to EIDRM).  The effec-
		   tive user-ID of the calling process must be that of the super-user,	or  match
		   the creator or owner of the semaphore set.  The argument semnum is ignored.

       GETALL	   Return semval for all semaphores of the set into arg.array.	The argument sem-
		   num is ignored.  The calling process must have read access privileges  on  the
		   semaphore set.

       GETNCNT	   The	system	call  returns the value of semncnt for the semnum-th semaphore of
		   the set (i.e. the number of processes waiting for an increase  of  semval  for
		   the	semnum-th  semaphore  of  the  set).   The calling process must have read
		   access privileges on the semaphore set.

       GETPID	   The system call returns the value of sempid for the semnum-th semaphore of the
		   set	(i.e.  the  pid  of the process that executed the last semop call for the
		   semnum-th semaphore of the set).  The calling process must  have  read  access
		   privileges on the semaphore set.

       GETVAL	   The system call returns the value of semval for the semnum-th semaphore of the
		   set.  The calling process must have read access privileges  on  the	semaphore
		   set.

       GETZCNT	   The	system	call  returns the value of semzcnt for the semnum-th semaphore of
		   the set (i.e. the number of processes waiting for semval of the semnum-th sem-
		   aphore  of  the  set  to become 0).	The calling process must have read access
		   privileges on the semaphore set.

       SETALL	   Set semval for all semaphores of the set using arg.array,  updating	also  the
		   sem_ctime  member  of  the  semid_ds  structure  associated	to the set.  Undo
		   entries are cleared for altered semaphores in all processes.  Processes sleep-
		   ing on the wait queue are awakened if some semval becomes 0 or increases.  The
		   argument semnum is ignored.	The calling process must have alter access privi-
		   leges on the semaphore set.

       SETVAL	   Set	the  value  of	semval to arg.val for the semnum-th semaphore of the set,
		   updating also the sem_ctime member of the semid_ds structure associated to the
		   set.   Undo entries are cleared for altered semaphores in all processes.  Pro-
		   cesses sleeping on the  wait  queue	are  awakened  if  semval  becomes  0  or
		   increases.  The calling process must have alter access privileges on the sema-
		   phore set.

RETURN VALUE
       On failure semctl returns -1 with errno indicating the error.  Otherwise the  system  call
       returns a nonnegative value depending on cmd as follows:

       GETNCNT	  the value of semncnt.

       GETPID	  the value of sempid.

       GETVAL	  the value of semval.

       GETZCNT	  the value of semzcnt.

       All other cmd values return 0 on success.

ERRORS
       On failure, errno will be set to one of the following:

       EACCES	  The calling process has no access permissions needed to execute cmd.

       EFAULT	  The address pointed to by arg.buf or arg.array isn't accessible.

       EIDRM	  The semaphore set was removed.

       EINVAL	  Invalid value for cmd or semid.

       EPERM	  The  argument  cmd  has  value  IPC_SET or IPC_RMID but the calling process has
		  insufficient privileges to execute the command.

       ERANGE	  The argument cmd has value SETALL or SETVAL and the value to which  semval  has
		  to  be  set  (for some semaphore of the set) is less than 0 or greater than the
		  implementation value SEMVMX.

NOTES
       The IPC_INFO, SEM_STAT and SEM_INFO control calls are used by the ipcs(8) program to  pro-
       vide information on allocated resources.  In the future these can be modified as needed or
       moved to a proc file system interface.

       Various fields in a struct semid_ds were shorts under Linux  2.2  and  have  become  longs
       under  Linux  2.4.  To take advantage of this, a recompilation under glibc-2.1.91 or later
       should suffice.	(The kernel distinguishes old and new calls by a IPC_64 flag in cmd.)

       The following system limit on semaphore sets affects a semctl call:

       SEMVMX	  Maximum value for semval: implementation dependent (32767).

       For greater portability it is best to always call semctl with four arguments.

       Under Linux, the function semctl is not a system call, but is implemented via  the  system
       call ipc(2).

CONFORMING TO
       SVr4, SVID.  SVr4 documents more error conditions EINVAL and EOVERFLOW.

SEE ALSO
       ipc(2), shmget(2), shmat(2), shmdt(2), ipc(5)

Linux 2.4.1				    2001-12-21					SEMCTL(2)


All times are GMT -4. The time now is 10:45 AM.

Unix & Linux Forums Content Copyrightę1993-2018. All Rights Reserved.
×
UNIX.COM Login
Username:
Password:  
Show Password





Not a Forum Member?
Forgot Password?