Description

   The mprotect function modifies the access protection of a mapped
   file or shared memory region.

   The addr and len arguments specify the address and length, in
   bytes, of the region that you want to modify. The len argument
   must be a multiple of the page size as returned by sysconf(_SC_
   PAGE_SIZE). If len is not a multiple of the page size as returned
   by sysconf(_SC_PAGE_SIZE), the length of the region is rounded up
   to the next multiple of the page size.

   The prot argument specifies access permissions for the mapped
   region. Specify one of the following:

   PROT_NONE      No access
   PROT_READ      Read-only
   PROT_WRITE     Read/Write access

   The mprotect function does not modify the access permission of
   any region that lies outside of the specified region, except that
   the effect on addresses between the end of the region, and the
   end of the page containing the end of the region, is unspecified.

   If the mprotect function fails under a condition other than that
   specified by EINVAL, the access protection of some of the pages
   in the range [addr, addr + len] can change. Suppose the error
   occurs on some page at an addr2; mprotect can modify protections
   of all whole pages in the range [addr, addr2].

   See also sysconf.