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shmctl(2) - shmctl, shmctl - shared memory control - man 2 shmctl

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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 seg-
       ment, set(7,n,1 builtins) the owner, group, and permissions of a shared memory segment,
       or  destroy  a segment. The information about the segment identified by
       shmid is returned in(1,8) 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(1,2,n) */
               time_t shm_dtime;          /* last detach time(1,2,n) */
               time_t shm_ctime;          /* last change time(1,2,n) */
               unsigned short shm_cpid;   /* pid of creator */
               unsigned short shm_lpid;   /* pid of last operator */
               short shm_nattch;          /* no. of current attaches */
               ...
           };

       The highlighted fields in(1,8) 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(2,5) 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(2,n,1 builtins) access(2,5)
                   to the shared memory 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 low-
                   est 9 bits of mode are used.  The shm_ctime member is  also
                   updated.   The  user  must  be  the owner or creator, or be
                   privileged.

       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 or creator, or be privi-
                   leged.

       The user must ensure that a segment is eventually destroyed;  otherwise
       its pages that were faulted in(1,8) will remain in(1,8) memory or swap.

   Linux additions
       A privileged user can prevent or allow swapping of a shared memory seg-
       ment with the following cmds:

       SHM_LOCK    prevents swapping of a shared memory segment. The user must
                   fault  in(1,8)  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.

RETURN VALUE
       On  success,  zero is returned.  On error(8,n), -1 is returned, and errno is
       set(7,n,1 builtins) appropriately.

ERRORS
       EACCES     IPC_STAT or SHM_STAT is requested  and  shm_perm.modes  does
                  not  allow  read(2,n,1 builtins)  access(2,5)  for shmid, and the calling process
                  does not have the CAP_IPC_OWNER capability.

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

       EIDRM      shmid points to a removed identifier.

       EINVAL     shmid  is not a valid identifier, or cmd is not a valid com-
                  mand.

       EOVERFLOW  IPC_STAT is attempted, and the gid or uid value is too large
                  to be stored in(1,8) the structure pointed to by buf.

       EPERM      IPC_SET  or IPC_RMID is attempted, and the effective user ID
                  of the calling process is not that of the creator (found  in(1,8)
                  shm_perm.cuid),  or  the  owner (found in(1,8) shm_perm.uid), and
                  the process was not privileged  (Linux:  did  not  have  the
                  CAP_SYS_ADMIN capability).

                  Or,  SHM_LOCK  or  SHM_UNLOCK was specified, but the process
                  was not privileged (Linux: did  not  have  the  CAP_IPC_LOCK
                  capability).

NOTE
       Various  fields  in(1,8)  a  struct shmid_ds were shorts under Linux 2.2 and
       have become longs under Linux 2.4. To take advantage of this, a  recom-
       pilation  under glibc-2.1.91 or later should suffice.  (The kernel dis-
       tinguishes old and new calls by an IPC_64 flag in(1,8) cmd.)

CONFORMING TO
       SVr4, SVID.  SVr4 documents additional error(8,n) conditions EINVAL, ENOENT,
       ENOSPC, ENOMEM, EEXIST.  Neither SVr4 nor SVID documents an EIDRM error(8,n)
       condition.

SEE ALSO
       shmget(2), shmop(2), capabilities(7)



Linux 2.6.7                       2004-06-17                         SHMCTL(2)

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