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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

     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

     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 of:

     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 executed 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 determined 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

     Upon successful completion, a value of 0 is returned.  Otherwise, -1 is returned and the global vari-able variable
     able 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

     [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 segment.  An attempt is made to increase
                        the value of shm_qbytes through IPC_SET but the caller is not the super-user.

     #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 previous 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"), software using the LEGACY API will not behave as expected.

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

BSD                             August 17, 1995                            BSD

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