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.
EINVAL addr is not a valid pointer, or not a multiple of the system
page size.
ENOMEM Internal kernel structures could not be allocated.
ENOMEM 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. (Before kernel 2.4.19, the error EFAULT was
incorrectly produced for these cases.)
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 permissible 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
Program source
#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 3.24 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/.