sdenter(S)                     6 January 1993                     sdenter(S)


 Name

    sdenter, sdleave - synchronizes access to a shared data segment

 Syntax


    cc  . . .  -lx


    #include  <sys/sd.h>

    int sdenter(addr,flags)
    char *addr;
    int flags;

    int sdleave(addr)
    char *addr;


 Description

    sdenter is used to indicate that the current process is about to access
    the contents of a shared data segment.  addr is the valid return code
    from a previous sdget call.  The actions performed depend on the value of
    flags.  flags values are formed by OR-ing together entries from the fol-
    lowing list:


    SDNOWAIT      If another process has called sdenter but not sdleave for
                   the indicated segment, and the segment was not created
                   with the SDUNLOCK flag set, return an ENAVAIL error
                   instead of waiting for the segment to become free.

    SDWRITE       Indicates that the process wants to write data to the
                   shared data segment.  A process that has attached to a
                   shared data segment with the SDRDONLY flag set is not
                   allowed to enter with the SDWRITE flag set.

    sdleave is used to indicate that the current process is done modifying
    the contents of a shared data segment.

    Only changes made between invocations of sdenter and sdleave are
    guaranteed to be reflected in other processes.  sdenter and sdleave are
    very fast; consequently, it is recommended that they be called frequently
    rather than leave sdenter in effect for any period of time.  In particu-
    lar, system calls should be avoided between sdenter and sdleave calls.

    The fork system call is forbidden between calls to sdenter and sdleave if
    the segment was created without the SDUNLOCK flag.

 Return value

    Successful calls return 0. Unsuccessful calls return -1, and errno is set
    to indicate the error.  errno is set to EINVAL if a process does an
    sdenter with the SDWRITE flag set and the segment is already attached
    with the SDRDONLY flag set.  errno is set to ENAVAIL if the SDNOWAIT
    flag is set for sdenter call and the shared data segment is not free.

 Notes

    This feature is a XENIX specific enhancement  and may not be present on
    all UNIX implementations.

 See also

    sdget(S), sdgetv(S)

 Standards conformance

    sdenter and sdleave are not part of any currently supported standard;
    they are an extension of AT&T System V provided by the Santa Cruz Opera-
    tion.