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NAME

       semctl - System V semaphore control operations

LIBRARY

       Standard C library (libc, -lc)

SYNOPSIS

       #include <sys/sem.h>

       int semctl(int semid, int semnum, int op, ...);

DESCRIPTION

       semctl()  performs  the  control  operation  specified by op 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 op.  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; /* Creation time/time of last
                                             modification via semctl() */
               unsigned long   sem_nsems; /* No. of semaphores in set */
           };

       The fields of the semid_ds structure are as follows:

       sem_perm   This is an ipc_perm structure (see below) that specifies the access permissions
                  on the semaphore set.

       sem_otime  Time of last semop(2) system call.

       sem_ctime  Time  of  creation of semaphore set or time of last semctl() IPCSET, SETVAL, or
                  SETALL operation.

       sem_nsems  Number of semaphores in the set.  Each semaphore of the set is referenced by  a
                  nonnegative integer ranging from 0 to sem_nsems-1.

       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 */
           };

       The least significant 9 bits of the mode field of the ipc_perm structure define the access
       permissions for the shared memory segment.  The permission bits are as follows:

       0400   Read by user
       0200   Write by user
       0040   Read by group
       0020   Write by group
       0004   Read by others
       0002   Write by others

       In  effect,  "write"  means  "alter"  for a semaphore set.  Bits 0100, 0010, and 0001 (the
       execute bits) are unused by the system.

       Valid values for op 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 semid_ds structure 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  semncnt value for the semnum-th semaphore of the set (i.e., the number
              of processes waiting for the semaphore's value to increase).  The  calling  process
              must have read permission on the semaphore set.

       GETPID Return the sempid value 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  semval  (i.e., the semaphore value) for the semnum-th semaphore of the set.
              The calling process must have read permission on the semaphore set.

       GETZCNT
              Return the semzcnt value for the semnum-th semaphore of the set (i.e.,  the  number
              of  processes  waiting  for  the semaphore value to become 0).  The calling process
              must have read permission on the semaphore set.

       SETALL Set the semval values 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 semaphore value (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 success, semctl() returns a nonnegative value depending on op 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 op values return 0 on success.

       On failure, semctl() returns -1 and sets errno to indicate the error.

ERRORS

       EACCES The  argument  op  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 op 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 op 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 op 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.

VERSIONS

       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.

   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 Linux 4.5, the kernel did not  update  sempid
       for SETALL operations.  This was rectified in Linux 4.6.

STANDARDS

       POSIX.1-2008.

HISTORY

       POSIX.1-2001, SVr4.

       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
       op.)

       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>.

NOTES

       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.

       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.

EXAMPLES

       See shmop(2).

SEE ALSO

       ipc(2), semget(2), semop(2), capabilities(7), sem_overview(7), sysvipc(7)