MPROTECT(2)

NAME
mprotect - control allowable accesses to a region of memory

SYNOPSIS
#include <sys/mman.h>

int mprotect(caddr_t 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.

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>
int
main(void)
{
    char *p;
    char c;
    /* Allocate a buffer; it will have the default
       protection of PROT_READ|PROT_WRITE. */
    p = malloc(1024);
    if (!p) {
        perror("Couldn't malloc(1024)");
        exit(errno);
    }
    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);
}

SEE ALSO
mmap(2)