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/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 SETALL 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
       structure 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 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.

       SEM_STAT_ANY (Linux-specific, since Linux 4.17)
              Return a semid_ds structure 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 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 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 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 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 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 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 success, semctl() 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.

       On failure, semctl() returns -1 and sets errno to indicate the
       error.

ERRORS         top

       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 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 5.13 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                          2021-03-22                      SEMCTL(2)

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