NAME | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | CONFORMING TO | NOTES | EXAMPLE | SEE ALSO | COLOPHON

MPROTECT(2)               Linux Programmer's Manual              MPROTECT(2)

NAME         top

       mprotect - set protection on a region of memory

SYNOPSIS         top

       #include <sys/mman.h>

       int mprotect(void *addr, size_t len, int prot);

DESCRIPTION         top

       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         top

       On success, mprotect() returns zero.  On error, -1 is returned, and
       errno is set appropriately.

ERRORS         top

       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-1] 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         top

       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         top

       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 allow write access only
       if PROT_WRITE has been set, and must not allow any access if
       PROT_NONE has been set.

EXAMPLE         top

       The program below allocates four pages of memory, makes the third of
       these pages read-only, and then executes a loop that walks upward
       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)

       static 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_READ) == -1)
               handle_error("mprotect");

           for (p = buffer ; ; )
               *(p++) = 'a';

           printf("Loop completed\n");     /* Should never happen */
           exit(EXIT_SUCCESS);
       }

SEE ALSO         top

       mmap(2), sysconf(3)

COLOPHON         top

       This page is part of release 3.72 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at
       http://www.kernel.org/doc/man-pages/.

Linux                            2014-01-05                      MPROTECT(2)