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

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 PAGESIZE.

ENOMEM

Internal kernel structures could not be allocated. Or: addreses 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.

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 says that mprotect can be used only on regions of memory obtained from mmap(2).

NOTES

On Linux it is always legal to call mprotect on any address in a process' 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.

SEE ALSO

mmap(2)