SEMCTL(2)                 (2020-12-21)                  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 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 SETALL operation.

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          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 high-
          lighted 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:
          l l.  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 associ-
               ated with semid into the semid_ds structure pointed to
               by arg.buf. The argument semnum is ignored.  The call-
               ing process must have read permission on the semaphore
               set.

          IPC_SET
               Write the values of some members of the semid_ds struc-
               ture 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.

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          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 informa-
               tion 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

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               maintains information about all semaphore sets on the
               system.

          SEM_STAT_ANY (Linux-specific, since Linux 4.17)
               Return a seminfo structure containing the same informa-
               tion 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 sema-
               phores of the set into arg.array. The argument semnum
               is ignored.  The calling process must have read permis-
               sion 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 per-
               formed 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 sema-
               phores 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 pro-
               cess must have alter (write) permission on the sema-
               phore set.

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          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 sema-
               phore values would permit blocked semop(2) calls in
               other processes to proceed, then those processes are
               woken up.  The calling process must have alter permis-
               sion 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 sema-
               phore 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
          On failure, errno will be set to one of the following:

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          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 imple-
               mentation limit SEMVMX.

     CONFORMING TO
          POSIX.1-2001, POSIX.1-2008, SVr4.

          POSIX.1 specifies the sem_nsems field of the semid_ds struc-
          ture 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
          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.  Appli-
          cations 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

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          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 impli-
          cation 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 oper-
          ations that update the value of a semaphore: the SETVAL and
          SETALL operations, as well as the semaphore adjustments per-
          formed 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 sema-
          phore 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
          See shmop(2).

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

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     COLOPHON
          This page is part of release 5.10 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/.

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