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



NAME
       semctl - System V 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 Sys-
       tem V semaphore 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 long   sem_nsems; /* No. of semaphores in set */
	   };

       The  ipc_perm  structure	 is defined as follows (the highlighted fields
       are settable using IPC_SET):

	   struct ipc_perm {
	       key_t	      __key; /* Key supplied to semget(2) */
	       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 associated
		 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 significant 9 bits
		 of) sem_perm.mode.  The effective UID of the calling  process
		 must	 match	  the	 owner	 (sem_perm.uid)	  or   creator
		 (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 processes
		 blocked in semop(2) calls on the set (with  an	 error	return
		 and  errno set to EIDRM).  The effective user ID of the call-
		 ing process must match the creator or owner of the  semaphore
		 set,  or  the caller must be privileged.  The argument semnum
		 is ignored.

       IPC_INFO (Linux-specific)
		 Return 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;  /* Number of entries in semaphore
					 map; unused within kernel */
			 int semmni;  /* Maximum number of semaphore sets */
			 int semmns;  /* Maximum number of semaphores in all
					 semaphore sets */
			 int semmnu;  /* System-wide maximum number of undo
					 structures; unused within kernel */
			 int semmsl;  /* Maximum number of semaphores in a
					 set */
			 int semopm;  /* Maximum number of operations for
					 semop(2) */
			 int semume;  /* Maximum number of undo entries per
					 process; unused within kernel */
			 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)
		 Return 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	 sema-
		 phores: the semusz field returns the number of semaphore 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)
		 Return 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	infor-
		 mation 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 semaphore
		 set.

       GETNCNT	 Return 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 permission on the sema-
		 phore set.

       GETPID	 Return the value of sempid for the semnum-th semaphore of the
		 set.	This  is the PID of the process that last performed an
		 operation on that semaphore (but  see	NOTES).	  The  calling
		 process must have read permission on the semaphore set.

       GETVAL	 Return the value of semval for the semnum-th semaphore of the
		 set.  The calling process must have read  permission  on  the
		 semaphore set.

       GETZCNT	 Return	 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 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 structure
		 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(2)
		 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(2)
		 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,  GETVAL,
	      GETNCNT,	GETZCNT, IPC_STAT, SEM_STAT, SETALL, or SETVAL and the
	      calling process does not have the required  permissions  on  the
	      semaphore	 set and does not have the CAP_IPC_OWNER capability in
	      the user namespace that governs its IPC namespace.

       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.  Or: for a  SEM_STAT  operation,
	      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.

CONFORMING TO
       POSIX.1-2001, POSIX.1-2008, SVr4.

       POSIX.1 specifies the sem_nsems field of the semid_ds structure as hav-
       ing  the type unsigned short, and the field is so defined on most other
       systems.	 It was also so defined on Linux 2.2 and earlier,  but,	 since
       Linux 2.4, the field has the type unsigned long.

NOTES
       The  inclusion of <sys/types.h> and <sys/ipc.h> isn't required on Linux
       or by any version of POSIX.  However, some old implementations required
       the inclusion of these header files, and the SVID also documented their
       inclusion.  Applications intended to be portable to  such  old  systems
       may need to include these header files.

       The  IPC_INFO, SEM_STAT and SEM_INFO operations are used by the ipcs(1)
       program to provide information on allocated resources.  In  the	future
       these may modified or moved to a /proc filesystem interface.

       Various fields in a struct semid_ds were typed as short under Linux 2.2
       and have become long 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 an IPC_64 flag in cmd.)

       In some earlier versions of glibc,  the	semun  union  was  defined  in
       <sys/sem.h>,  but  POSIX.1  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.

   The sempid value
       POSIX.1 defines sempid as the "process ID of [the] last operation" on a
       semaphore,  and explicitly notes that this value is set by a successful
       semop(2) call, with the implication that no other interface affects the
       sempid value.

       While  some  implementations  conform  to  the  behavior	 specified  in
       POSIX.1, others do not.	(The fault here	 probably  lies	 with  POSIX.1
       inasmuch	 as  it	 likely	 failed	 to capture the full range of existing
       implementation behaviors.)  Various other implementations  also	update
       sempid  for  the other operations that update the value of a semaphore:
       the SETVAL and SETALL operations, as well as the semaphore  adjustments
       performed  on  process  termination  as a consequence of the use of the
       SEM_UNDO flag (see semop(2)).

       Linux also updates sempid for SETVAL operations and  semaphore  adjust-
       ments.	However,  somewhat  inconsistently,  up	 to and including 4.5,
       Linux did not update sempid for SETALL operations.  This was  rectified
       in Linux 4.6.

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

COLOPHON
       This  page  is  part of release 4.10 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
       https://www.kernel.org/doc/man-pages/.



Linux				  2016-10-08			     SEMCTL(2)