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 happen, for example, if you mmap(2) a file to which you have read-only access, then ask mprotect() to mark it PROT_WRITE.

EFAULT

The memory cannot be accessed.

EINVAL

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

ENOMEM

Internal kernel structures could not be allocated. Or: 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.

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 legal 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 architecture- 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 following:

$ ./a.out
Start of region:        0x804c000
Got SIGSEGV at address: 0x804e000
#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 2.79 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/.


  Copyright (C) 2007 Michael Kerrisk <mtk.manpagesgmail.com>
and Copyright (C) 1995 Michael Shields <shieldstembel.org>.

Permission is granted to make and distribute verbatim copies of this
manual provided the copyright notice and this permission notice are
preserved on all copies.

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permission notice identical to this one.

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manual page may be incorrect or out-of-date.  The author(s) assume no
responsibility for errors or omissions, or for damages resulting from
the use of the information contained herein.  The author(s) may not
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Formatted or processed versions of this manual, if unaccompanied by
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Modified 1996-10-22 by Eric S. Raymond <esrthyrsus.com>
Modified 1997-05-31 by Andries Brouwer <aebcwi.nl>
Modified 2003-08-24 by Andries Brouwer <aebcwi.nl>
Modified 2004-08-16 by Andi Kleen <akmuc.de>
2007-06-02, mtk: Fairly substantial rewrites and additions, and
a much improved example program.