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 contain 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 behaviour 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' 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. 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>
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) {
        perror("sigaction");
        exit(EXIT_FAILURE);
    }
    pagesize = sysconf(_SC_PAGE_SIZE);
    if (pagesize == -1) {
        perror("sysconf");
        exit(EXIT_FAILURE);
    }
    /* Allocate a buffer aligned on a page boundary;
       initial protection is PROT_READ | PROT_WRITE */
    buffer = memalign(pagesize, 4 * pagesize);
    if (buffer == NULL) {
        perror("memalign");
        exit(EXIT_FAILURE);
    }
    printf("Start of region:        0x%lx\n", (long) buffer);
    if (mprotect(buffer + pagesize * 2, pagesize,
                PROT_NONE) == -1) {
        perror("mprotect");
        exit(EXIT_FAILURE);
    }
    for (p = buffer ; ; )
        *(p++) = 'a';
    printf("Loop completed\n");     /* Should never happen */
    exit(EXIT_SUCCESS);
}

SEE ALSO
mmap(2), sysconf(3)