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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 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 program 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 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 permis-
                 sion 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 mem-
                 bers 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 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 processes 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 argument sem-
                 num 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;  /* 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 follow-
                 ing 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 identi-
                 fier,  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 process must
                 have read permission 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(2) call for the semnum-th semaphore of the set).  The call-
                 ing process must have read permission on the semaphore set.

       GETVAL    The system call returns the value of semval for the semnum-th semaphore of the set.  The calling pro-
                 cess 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  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  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 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 informa-
                   tion 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.

       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
       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  allo-
       cated 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  distin-
       guishes 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_UNDE-
       FINED 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.

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

COLOPHON
       This  page  is part of release 3.22 of the Linux man-pages project.  A description of the project, and informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-08-06                         SEMCTL(2)