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



NAME
       semctl - semaphore control operations

SYNOPSIS
       #include <&lt;sys/types.h>&gt;
       #include <&lt;sys/ipc.h>&gt;
       #include <&lt;sys/sem.h>&gt;

       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 &lt;sys/sem.h&gt; 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 &lt;sys/ipc.h&gt; as follows (the high-
       lighted 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  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(2) 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 &lt;sys/sem.h&gt; 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)
                   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(2) call for the semnum-th sem-
                   aphore 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(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 non-negative 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.

CONFORMING TO
       SVr4, POSIX.1-2001.

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 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
       &lt;sys/sem.h&gt;, 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 &lt;sys/sem.h&gt;.

       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.

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

COLOPHON
       This  page  is  part of release 3.05 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-05-20                         SEMCTL(2)