SEMCTL(2)



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

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 Sys-
       tem 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  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 <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 call-
                 ing 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  sema-
                 phores: 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  infor-
                 mation 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 sema-
                 phore 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  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 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 hav-
       ing  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  adjust-
       ments.   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
       This  page  is  part of release 4.08 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at
       https://www.kernel.org/doc/man-pages/.

Linux                             2016-10-08                         SEMCTL(2)

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