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SHMCTL(2)                   BSD System Calls Manual                  SHMCTL(2)

     shmctl -- shared memory control operations

     #include <sys/shm.h>

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

     The shmctl() system call performs some control operations on the shared
     memory area specified by shmid.  Each shared memory segment has a data
     structure associated with it, parts of which may be altered by shmctl()
     and parts of which determine the actions of shmctl().  This structure is
     defined as follows in <sys/shm.h>:

     struct shmid_ds {
         struct ipc_perm  shm_perm;     /* operation permissions */
         int              shm_segsz;    /* size of segment in bytes */
         pid_t            shm_lpid;     /* pid of last shm op */
         pid_t            shm_cpid;     /* pid of creator */
         short            shm_nattch;   /* # of current attaches */
         time_t           shm_atime;    /* last shmat() time*/
         time_t           shm_dtime;    /* last shmdt() time */
         time_t           shm_ctime;    /* last change by shmctl() */
         void            *shm_internal; /* sysv stupidity */

     The ipc_perm structure used inside the shmid_ds structure is defined in
     <sys/ipc.h> and looks like this:

     struct ipc_perm {
       uid_t           uid;   /* Owner's user ID */
       gid_t           gid;   /* Owner's group ID */
       uid_t           cuid;  /* Creator's user ID */
       gid_t           cgid;  /* Creator's group ID */
       mode_t          mode;  /* r/w permission (see chmod(2)) */
       unsigned short  _seq;  /* Reserved for internal use */
       key_t           _key;  /* Reserved for internal use */

     The operation to be performed by shmctl() is specified in cmd and is one

     IPC_STAT   Gather information about the shared memory segment and place
                it in the structure pointed to by buf.

     IPC_SET    Set the value of the shm_perm.uid, shm_perm.gid and
                shm_perm.mode fields in the structure associated with shmid.
                The values are taken from the corresponding fields in the
                structure pointed to by buf.  This operation can only be exe-cuted executed
                cuted by the super-user, or a process that has an effective
                user id equal to either shm_perm.cuid or shm_perm.uid in the
                data structure associated with the shared memory segment.

     IPC_RMID   Remove the shared memory segment specified by shmid and
                destroy the data associated with it. Only the super-user or a
                process with an effective uid equal to the shm_perm.cuid or
                shm_perm.uid values in the data structure associated with the
                queue can do this.

     The read and write permissions on a shared memory identifier are deter-mined determined
     mined by the shm_perm.mode field in the same way as is done with files
     (see chmod(2) ), but the effective uid can match either the shm_perm.cuid
     field or the shm_perm.uid field, and the effective gid can match either
     shm_perm.cgid or shm_perm.gid.

     Upon successful completion, a value of 0 is returned.  Otherwise, -1 is
     returned and the global variable errno is set to indicate the error.

     shmctl() will fail if:

     [EACCES]           The command is IPC_STAT and the caller has no read
                        permission for this shared memory segment.

     [EFAULT]           buf specifies an invalid address.

     [EINVAL]           shmid is not a valid shared memory segment identifier.
                        cmd is not a valid command.

     [EPERM]            cmd is equal to IPC_SET or IPC_RMID and the caller is
                        not the super-user,nor does the effective uid match
                        either the shm_perm.uid or shm_perm.cuid fields of the
                        data structure associated with the shared memory seg-ment. segment.
                        ment.  An attempt is made to increase the value of
                        shm_qbytes through IPC_SET but the caller is not the

     #include <sys/types.h>
     #include <sys/ipc.h>
     #include <sys/shm.h>

     All of these include files are necessary.

     The ipc_perm structure used inside the shmid_ds structure, as defined in
     <sys/ipc.h>, looks like this:

     struct ipc_perm {
         __uint16_t cuid;  /* Creator's user id */
         __uint16_t cgid;  /* Creator's group id */
         __uint16_t uid;   /* Owner's user id */
         __uint16_t gid;   /* Owner's group id */
         mode_t     mode;  /* r/w permission (see chmod(2)) */
         __uint16_t seq;   /* Reserved for internal use */
         key_t      key;   /* Reserved for internal use */

     This structure is maintained for binary backward compatibility with pre-vious previous
     vious versions of the interface.  New code should not use this interface,
     because ID values may be truncated.

     Specifically, LEGACY mode limits the allowable uid/gid ranges to 0-32767.
     If the user has a UID that is out of this range (e.g., "nobody"), soft-ware software
     ware using the LEGACY API will not behave as expected.

     shmat(2), shmdt(2), shmget(2), compat(5)

BSD                             August 17, 1995                            BSD