redhat man page for 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
       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-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 2.0							    1997-05-31							       MPROTECT(2)