semctl(2) — Linux manual page

NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | EXAMPLES | 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; /* 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 SETVALL 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 struc‐
       ture 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 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 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 informa‐
              tion 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 opera‐
              tion (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 per‐
              mission 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 per‐
              mission 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 per‐
              mission 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 pro‐
              cesses are woken up.  The argument semnum is ignored.  The
              calling process must have alter (write) permission on the sem‐
              aphore 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         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 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         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, 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         top

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

EXAMPLES         top

       See shmop(2).

SEE ALSO         top

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

COLOPHON         top

       This page is part of release 5.07 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                            2020-04-11                        SEMCTL(2)

Pages that refer to this page: ipcrm(1)ipcs(1)ipc(2)semget(2)semop(2)semtimedop(2)syscalls(2)svipc(7)sysvipc(7)