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)