NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | SEE ALSO | COLOPHON

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

NAME         top

       semctl - System V semaphore control operations

SYNOPSIS         top

       #include <sys/types.h>
       #include <sys/ipc.h>
       #include <sys/sem.h>

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

DESCRIPTION         top

       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         top

       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         top

       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.

       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         top

       POSIX.1-2001, POSIX.1-2001, 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         top

       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         top

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

COLOPHON         top

       This page is part of release 4.07 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                            2016-03-15                        SEMCTL(2)