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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
       semctl()	 performs  the control operation specified by cmd on the sema‐
       phore set identified by semid, or on the semnum-th  semaphore  of  that
       set.  (The semaphores in a set are numbered starting at 0.)

       This  function  has  three  or  four arguments, depending on cmd.  When
       there are four, the fourth has the type union semun.  The calling  pro‐
       gram must define this union as follows:

	   union semun {
	       int		val;	/* Value for SETVAL */
	       struct semid_ds *buf;	/* Buffer for IPC_STAT, IPC_SET */
	       unsigned short  *array;	/* Array for GETALL, SETALL */
	       struct seminfo  *__buf;	/* Buffer for IPC_INFO
					   (Linux specific) */
	   };

       The semid_ds data structure is defined in <sys/sem.h> as follows:

	   struct semid_ds {
	       struct ipc_perm sem_perm;  /* Ownership and permissions
	       time_t	       sem_otime; /* Last semop time */
	       time_t	       sem_ctime; /* Last change time */
	       unsigned short  sem_nsems; /* No. of semaphores in set */
	   };

       The  ipc_perm structure is defined in <sys/ipc.h> as follows (the high‐
       lighted fields are settable using IPC_SET):

	   struct ipc_perm {
	       key_t key;	     /* Key supplied to semget() */
	       uid_t uid;	     /* Effective UID of owner */
	       gid_t gid;	     /* Effective GID of owner */
	       uid_t cuid;	     /* Effective UID of creator */
	       gid_t cgid;	     /* Effective GID of creator */
	       unsigned short mode;  /* Permissions */
	       unsigned short seq;   /* Sequence number */
	   };

       Valid values for cmd are:

       IPC_STAT	   Copy information from the kernel data structure  associated
		   with	 semid	into  the  semid_ds  structure	pointed	 to by
		   arg.buf.  The argument  semnum  is  ignored.	  The  calling
		   process must have read permission on the semaphore set.

       IPC_SET	   Write  the values of some members of the semid_ds structure
		   pointed to by arg.buf to the kernel data structure  associ‐
		   ated	 with  this semaphore set, updating also its sem_ctime
		   member.   The  following  members  of  the  structure   are
		   updated: sem_perm.uid, sem_perm.gid, and (the least signif‐
		   icant 9 bits of) sem_perm.mode.  The effective UID  of  the
		   calling process must match the owner (sem_perm.uid) or cre‐
		   ator (sem_perm.cuid) of the semaphore set,  or  the	caller
		   must be privileged.	The argument semnum is ignored.

       IPC_RMID	   Immediately	remove	the  semaphore set, awakening all pro‐
		   cesses blocked in semop() calls on the set (with  an	 error
		   return  and	errno set to EIDRM).  The effective user ID of
		   the calling process must match the creator or owner of  the
		   semaphore set, or the caller must be privileged.  The argu‐
		   ment semnum is ignored.

       IPC_INFO (Linux specific)
		   Returns information about system-wide semaphore limits  and
		   parameters  in the structure pointed to by arg.__buf.  This
		   structure is of type seminfo, defined in <sys/sem.h> if the
		   _GNU_SOURCE feature test macro is defined:

		     struct  seminfo {
			 int semmap;  /* # of entries in semaphore map;
					 unused */
			 int semmni;  /* Max. # of semaphore sets */
			 int semmns;  /* Max. # of semaphores in all
					 semaphore sets */
			 int semmnu;  /* System-wide max. # of undo
					 structures; unused */
			 int semmsl;  /* Max. # of semaphores in a set */
			 int semopm;  /* Max. # of operations for semop() */
			 int semume;  /* Max. # of undo entries per
					 process; unused */
			 int semusz;  /* size of struct sem_undo */
			 int semvmx;  /* Maximum semaphore value */
			 int semaem;  /* Max. value that can be recorded for
					 semaphore adjustment (SEM_UNDO) */
		     };

		   The	semmsl,	 semmns,  semopm,  and	semmni settings can be
		   changed via /proc/sys/kernel/sem; see proc(5) for details.

       SEM_INFO (Linux specific)
		   Returns a seminfo structure containing the same information
		   as  for  IPC_INFO,  except  that  the  following fields are
		   returned with information about system  resources  consumed
		   by semaphores: the semusz field returns the number of sema‐
		   phore sets that currently exist  on	the  system;  and  the
		   semaem  field returns the total number of semaphores in all
		   semaphore sets on the system.

       SEM_STAT (Linux specific)
		   Returns a semid_ds structure as for IPC_STAT.  However, the
		   semid  argument  is not a semaphore identifier, but instead
		   an index into the kernel's internal	array  that  maintains
		   information about all semaphore sets on the system.

       GETALL	   Return  semval (i.e., the current value) for all semaphores
		   of the set into arg.array.  The argument semnum is ignored.
		   The	calling process must have read permission on the sema‐
		   phore set.

       GETNCNT	   The system call returns the value  of  semncnt  (i.e.,  the
		   number of processes waiting for the value of this semaphore
		   to increase) 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 permission on the semaphore set.

       GETPID	   The	system	call  returns the value of sempid for the sem‐
		   num-th semaphore of the set (i.e. the PID  of  the  process
		   that executed the last semop() call for the semnum-th sema‐
		   phore of the set).  The calling process must have read per‐
		   mission on the semaphore set.

       GETVAL	   The	system	call  returns the value of semval for the sem‐
		   num-th semaphore of the set.	 The calling process must have
		   read permission on the semaphore set.

       GETZCNT	   The	system	call  returns  the value of semzcnt (i.e., the
		   number of processes waiting for the value of this semaphore
		   to  become  zero)  for  the	semnum-th semaphore of the set
		   (i.e. the number of processes waiting  for  semval  of  the
		   semnum-th  semaphore	 of the set to become 0).  The calling
		   process must have read permission 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 struc‐
		   ture associated with the set.  Undo entries (see  semop(2))
		   are	cleared	 for  altered semaphores in all processes.  If
		   the	changes	 to  semaphore	values	would  permit  blocked
		   semop()  calls  in  other  processes to proceed, then those
		   processes are woken up.  The argument  semnum  is  ignored.
		   The	calling	 process must have alter (write) permission on
		   the semaphore set.

       SETVAL	   Set the value of semval to arg.val for the semnum-th	 sema‐
		   phore of the set, updating also the sem_ctime member of the
		   semid_ds structure associated with the set.	 Undo  entries
		   are	cleared	 for  altered semaphores in all processes.  If
		   the	changes	 to  semaphore	values	would  permit  blocked
		   semop()  calls  in  other  processes to proceed, then those
		   processes are woken up.   The  calling  process  must  have
		   alter permission on the semaphore 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.

       IPC_INFO	  the index of the highest used entry in the kernel's internal
		  array recording information about all semaphore sets.	 (This
		  information can be used with repeated SEM_STAT operations to
		  obtain information about all semaphore sets on the system.)

       SEM_INFO	  As for IPC_INFO.

       SEM_STAT	  the identifier of the semaphore set whose index was given in
		  semid.

       All other cmd values return 0 on success.

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

       EACCES	  The argument cmd has one of the values GETALL, GETPID,  GET‐
		  VAL, GETNCNT, GETZCNT, IPC_STAT, SEM_STAT, SETALL, or SETVAL
		  and the calling process does not have the  required  permis‐
		  sions	  on   the   semaphore	set  and  does	not  have  the
		  CAP_IPC_OWNER capability.

       EFAULT	  The address pointed to by arg.buf or arg.array isn't	acces‐
		  sible.

       EIDRM	  The semaphore set was removed.

       EINVAL	  Invalid  value  for cmd or semid.  Or: for a SEM_STAT opera‐
		  tion, the index value specified  in  semid  referred	to  an
		  array slot that is currently unused.

       EPERM	  The  argument	 cmd has the value IPC_SET or IPC_RMID but the
		  effective user ID of the calling process is not the  creator
		  (as  found  in  sem_perm.cuid)  or  the  owner  (as found in
		  sem_perm.uid) of the semaphore set, and the process does not
		  have the CAP_SYS_ADMIN capability.

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

NOTES
       The IPC_INFO, SEM_STAT and SEM_INFO operations are used by the  ipcs(8)
       program	to  provide information on allocated resources.	 In the future
       these may modified 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 recom‐
       pilation under glibc-2.1.91 or later should suffice.  (The kernel  dis‐
       tinguishes old and new calls by an IPC_64 flag in cmd.)

       In  some	 earlier  versions  of	glibc,	the semun union was defined in
       <sys/sem.h>, but POSIX.1-2001 requires  that  the  caller  define  this
       union.  On versions of glibc where this union is not defined, the macro
       _SEM_SEMUN_UNDEFINED is defined in <sys/sem.h>.

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

CONFORMING TO
       SVr4, POSIX.1-2001.

SEE ALSO
       ipc(2), semget(2), semop(2), capabilities(7), sem_overview(7), svipc(7)

Linux 2.6.9			  2004-11-10			     SEMCTL(2)
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