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man:semctl

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 4.16 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 2017-09-15 SEMCTL(2)

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