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

NAME
       mprotect - set protection on a region of memory

SYNOPSIS
       #include <sys/mman.h>

       int mprotect(const void *addr, size_t len, int prot);

DESCRIPTION
       mprotect()  changes protection for the calling process's memory page(s)
       containing  any	part  of   the	 address   range   in	the   interval
       [addr, addr+len-1].  addr must be aligned to a page boundary.

       If the calling process tries to access memory in a manner that violates
       the protection, then the kernel generates  a  SIGSEGV  signal  for  the
       process.

       prot  is	 either	 PROT_NONE  or a bitwise-or of the other values in the
       following list:

       PROT_NONE  The memory cannot be accessed at all.

       PROT_READ  The memory can be read.

       PROT_WRITE The memory can be modified.

       PROT_EXEC  The memory can be executed.

RETURN VALUE
       On success, mprotect() returns zero.  On error,	-1  is	returned,  and
       errno is set appropriately.

ERRORS
       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.

       EINVAL addr  is	not  a	valid pointer, or not a multiple of the system
	      page size.

       ENOMEM Internal kernel structures could not be allocated.

       ENOMEM Addresses in the range [addr,  addr+len]	are  invalid  for  the
	      address  space of the process, or specify one or more pages that
	      are not mapped.  (Before kernel 2.4.19,  the  error  EFAULT  was
	      incorrectly produced for these cases.)

CONFORMING TO
       SVr4,  POSIX.1-2001.   POSIX  says  that	 the behavior of mprotect() is
       unspecified if it is applied  to	 a  region  of	memory	that  was  not
       obtained via mmap(2).

NOTES
       On  Linux it is always permissible to call mprotect() on any address in
       a process's address space (except for the kernel	 vsyscall  area).   In
       particular  it  can  be	used  to  change  existing code mappings to be
       writable.

       Whether PROT_EXEC has any effect different from PROT_READ is  architec‐
       ture-  and  kernel  version-dependent.	On some hardware architectures
       (e.g., i386), PROT_WRITE implies PROT_READ.

       POSIX.1-2001 says that an implementation may permit access  other  than
       that specified in prot, but at a minimum can only allow write access if
       PROT_WRITE has been set, and must not allow any access if PROT_NONE has
       been set.

EXAMPLE
       The  program  below  allocates four pages of memory, makes the third of
       these pages read-only, and then executes	 a  loop  that	walks  upwards
       through the allocated region modifying bytes.

       An example of what we might see when running the program is the follow‐
       ing:

	   $ ./a.out
	   Start of region:	   0x804c000
	   Got SIGSEGV at address: 0x804e000

   Program source

       #include <unistd.h>
       #include <signal.h>
       #include <stdio.h>
       #include <malloc.h>
       #include <stdlib.h>
       #include <errno.h>
       #include <sys/mman.h>

       #define handle_error(msg) \
	   do { perror(msg); exit(EXIT_FAILURE); } while (0)

       char *buffer;

       static void
       handler(int sig, siginfo_t *si, void *unused)
       {
	   printf("Got SIGSEGV at address: 0x%lx\n",
		   (long) si->si_addr);
	   exit(EXIT_FAILURE);
       }

       int
       main(int argc, char *argv[])
       {
	   char *p;
	   int pagesize;
	   struct sigaction sa;

	   sa.sa_flags = SA_SIGINFO;
	   sigemptyset(&sa.sa_mask);
	   sa.sa_sigaction = handler;
	   if (sigaction(SIGSEGV, &sa, NULL) == -1)
	       handle_error("sigaction");

	   pagesize = sysconf(_SC_PAGE_SIZE);
	   if (pagesize == -1)
	       handle_error("sysconf");

	   /* Allocate a buffer aligned on a page boundary;
	      initial protection is PROT_READ | PROT_WRITE */

	   buffer = memalign(pagesize, 4 * pagesize);
	   if (buffer == NULL)
	       handle_error("memalign");

	   printf("Start of region:	   0x%lx\n", (long) buffer);

	   if (mprotect(buffer + pagesize * 2, pagesize,
		       PROT_NONE) == -1)
	       handle_error("mprotect");

	   for (p = buffer ; ; )
	       *(p++) = 'a';

	   printf("Loop completed\n");	   /* Should never happen */
	   exit(EXIT_SUCCESS);
       }

SEE ALSO
       mmap(2), sysconf(3)

COLOPHON
       This page is part of release 3.27 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at http://www.kernel.org/doc/man-pages/.

Linux				  2008-08-06			   MPROTECT(2)
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