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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 System 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 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 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 calling 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  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)
                 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 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   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
                 semaphore 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  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 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 having 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  adjustments.   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

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