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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.

       SEM_STAT_ANY (Linux-specific, since Linux 4.17)
                 Return a seminfo structure containing the  same  information  as  for  SEM_STAT.
                 However, sem_perm.mode is not checked for read access for semid meaning that any
                 user can employ this operation (just as any user may read  /proc/sysvipc/sem  to
                 obtain the same information).

       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 or SEM_STAT_ANY 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.

       SEM_STAT_ANY
                 as for SEM_STAT.

       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, SEM_STAT_ANY, 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), sysvipc(7)

COLOPHON

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