View Source: mprotect(2) - Waikato Linux Users Group

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!!NAME
mprotect - control allowable accesses to a region of memory

!!SYNOPSIS
 __#include <sys/mman.h>__
 __int mprotect(const void *__''addr''__, size_t__ ''len''__, int__ ''prot''__);__
!!DESCRIPTION
__mprotect__ controls how a section of memory may be accessed. If an access is disallowed by the protection given it, the program receives a __SIGSEGV__.

''prot'' is a bitwise-or of the following values:
;__PROT_NONE__: The memory cannot be accessed at all.
;__PROT_READ__: The memory can be read.
;__PROT_WRITE__: The memory can be written to.
;__PROT_EXEC__: The memory can contain executing code.

The new protection replaces any existing protection. For example, if the memory had previously been marked __PROT_READ__, and __mprotect__ is then called with
''prot'' __PROT_WRITE__, it will no longer be readable.

!!RETURN VALUE
On success, __mprotect__ returns zero. On error, -1 is returned, and ''errno'' is set appropriately.
!!ERRORS
;[EINVAL]: ''addr'' is not a valid pointer, or not a multiple of PAGESIZE.
;[EFAULT]: The memory cannot be accessed.
;[EACCES]: The memory cannot be given the specified access. This can happen, for example, if you mmap(2) a file to which you have read-only access, then ask __mprotect__ to mark it __PROT_WRITE__.
;[ENOMEM]: Internal kernel structures could not be allocated.

!!EXAMPLE
       #include <stdio.h>
       #include <stdlib.h>
       #include <errno.h>
       #include <sys/mman.h>

       #include <limits.h>    /* for PAGESIZE */
       #ifndef PAGESIZE
       #define PAGESIZE 4096
       #endif

       int
       main(void)
       {
           char *p;
           char c;

           /* Allocate a buffer; it will have the default
              protection of PROT_READ|PROT_WRITE. */
           p = malloc(1024+PAGESIZE&#8208;1);
           if (!p) {
               perror("Couldn�t malloc(1024)");
               exit(errno);
           }

           /* Align to a multiple of PAGESIZE, assumed to be a power of two */
           p = (char *)(((int) p + PAGESIZE&#8208;1) & ~(PAGESIZE&#8208;1));

           c = p[666];         /* Read; ok */
           p[666] = 42;        /* Write; ok */

           /* Mark the buffer read&#8208;only. */
           if (mprotect(p, 1024, PROT_READ)) {
               perror("Couldn't mprotect");
               exit(errno);
           }

           c = p[[666];         /* Read; ok */
           p[[666] = 42;        /* Write; program dies on SIGSEGV */

           exit(0);
       }

!!CONFORMING TO
SVr4, [POSIX].1b (formerly POSIX.4). SVr4 defines an additional error code [EAGAIN]. The SVr4 error conditions don't map neatly onto [Linux]'s. POSIX.1b says that
__mprotect__ can be used only on regions of memory obtained from mmap(2).

!!SEE ALSO
mmap(2)

3 pages link to mprotect(2):

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Last edited on Wednesday, September 10, 2003 9:14:19 pm by JohnMcPherson

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