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mprotect
========

Syntax
------

     #include <sys/types.h>
     #include <sys/mman.h>
     
     int mprotect(void *addr, size_t len, int prot);

Description
-----------

This function modifies the access protection of a memory region.
Protection occurs in 4Kbyte regions (pages) aligned on 4Kbyte
boundaries.  All pages in the region will be changed, so ADDR and LEN
should be multiples of 4096.

The protection PROT for each page is specified with the values:
PROT_NONE  Region can not be touched (if or'ed is ignored).
PROT_READ  Region can be read (can be or'ed with PROT_WRITE).
PROT_WRITE Region can be written (implies read access).

This function is only supported on DPMI hosts which provide some V1.0
extensions on V0.9 memory blocks.

Return Value
------------

The function returns 0 if successful and the value -1 if all the pages
could not be set.

Example
-------

     mprotect(readonly_buffer,8192,PROT_READ);
     mprotect(guard_area,4096,PROT_NONE);
     mprotect(NULL,4096,PROT_WRITE);       /* Let NULL pointers not generate exceptions */