mprotect

MPROTECT(2)		  Linux Programmer’s Manual		  MPROTECT(2)



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
       The  function mprotect specifies the desired protection for the memory
       page(s) containing part or all of the interval [addr,addr+len-1].   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 hap-
	      pen, 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-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-1) & ~(PAGESIZE-1));

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

	   /* Mark the buffer read-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)



Linux 2.4			  2003-08-24			  MPROTECT(2)