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

NAME
       shmctl - shared memory control

SYNOPSIS
       #include <sys/ipc.h>

       #include <sys/shm.h>

       int shmctl(int shmid, int cmd, struct shmid_ds *buf);

DESCRIPTION
       shmctl()  allows  the  user  to  receive  information on a
       shared memory segment, set the owner, group,  and  permis-
       sions  of  a  shared memory segment, or destroy a segment.
       The information about the segment identified by  shmid  is
       returned in a shmid_ds structure:

            struct shmid_ds {
                 struct    ipc_perm shm_perm;  /* operation perms */
                 int  shm_segsz;          /* size of segment (bytes) */
                 time_t    shm_atime;          /* last attach time */
                 time_t    shm_dtime;          /* last detach time */
                 time_t    shm_ctime;          /* last change time */
                 unsigned short shm_cpid; /* pid of creator */
                 unsigned short shm_lpid; /* pid of last operator */
                 short     shm_nattch;         /* no. of current attaches */
                 /* the following are private */
                 unsigned short   shm_npages;  /* size of segment (pages) */
                 unsigned long   *shm_pages;
                 struct shm_desc *attaches;    /* descriptors for attaches */
            };

       The fields in the member shm_perm can be set:

            struct ipc_perm
            {
              key_t  key;
              ushort uid;   /* owner euid and egid */
              ushort gid;
              ushort cuid;  /* creator euid and egid */
              ushort cgid;
              ushort mode;  /* lower 9 bits of access modes */
              ushort seq;   /* sequence number */
            };

       The following cmds are available:

       IPC_STAT    is  used  to  copy  the  information about the
                   shared memory segment into the buffer buf. The
                   user  must have read access to the shared mem-
                   ory segment.

       IPC_SET     is used to apply the changes the user has made
                   to  the  uid,  gid,  or  mode  members  of the
                   shm_perms field.  Only the lowest  9  bits  of
                   mode  are  used.  The shm_ctime member is also
                   updated.  The user must be the owner, creator,
                   or the super-user.

       IPC_RMID    is  used  to mark the segment as destroyed. It
                   will actually  be  destroyed  after  the  last
                   detach.   (I.e., when the shm_nattch member of
                   the associated structure  shmid_ds  is  zero.)
                   The  user  must  be the owner, creator, or the
                   super-user.

       The  user  must  ensure  that  a  segment  is   eventually
       destroyed;  otherwise  its pages that were faulted in will
       remain in memory or swap.

       In addition, the super-user can prevent or allow  swapping
       of a shared memory segment with the following cmds: (Linux
       only)

       SHM_LOCK    prevents swapping of a shared memory  segment.
                   The  user  must  fault  in  any pages that are
                   required  to  be  present  after  locking   is
                   enabled.

       SHM_UNLOCK  allows the shared memory segment to be swapped
                   out.

       The IPC_INFO, SHM_STAT and SHM_INFO control calls are used
       by the ipcs(8) program to provide information on allocated
       resources.  In the future, these man be modified as needed
       or moved to a proc file system interface.

SYSTEM CALLS
       fork() After  a  fork()  the  child  inherits the attached
              shared memory segments.

       exec() After an exec() all attached shared memory segments
              are detached (not destroyed).

       exit() Upon exit() all attached shared memory segments are
              detached (not destroyed).

RETURN VALUE
       0 is returned on success, -1 on error.

ERRORS
       On error, errno will be set to one of the following:

       EACCES      is  returned  if  IPC_STAT  is  requested  and
                   shm_perm.modes  does not allow read access for
                   msqid.

       EFAULT      The argument cmd has value IPC_SET or IPC_STAT
                   but the address pointed to by buf isn't acces-
                   sible.

       EINVAL      is returned if shmid is not  a  valid  identi-
                   fier, or cmd is not a valid command.

       EIDRM       is returned if shmid points to a removed iden-
                   tifier.

       EPERM       is  returned  if  IPC_SET   or   IPC_RMID   is
                   attempted,  and  the  user is not the creator,
                   the owner, or the  super-user,  and  the  user
                   does  not  have  permission  granted  to their
                   group or to the world.

CONFORMING TO
       SVr4, SVID.  SVr4 documents  additional  error  conditions
       EINVAL,  ENOENT, ENOSPC, ENOMEM, EEXIST.  Neither SVr4 nor
       SVID documents an EIDRM error condition.

SEE ALSO
       shmget(2), shmop(2)

Linux 0.99.11           November 28, 1993                       1