SEMCTL(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
       semctl()  performs  the  control  operation  specified  by  cmd  on the
       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 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 pro-
                   cess 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
                   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)
                   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
                   semaphore 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 pro-
                   cess 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
                   semaphore  of the set).  The calling process must have read
                   permission 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
                   semaphore 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  pro-
                   cesses.   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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