NAME

       mprotect - set protection on a region of memory

SYNOPSIS

       #include <sys/mman.h>

       int mprotect(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-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

       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 allow write access only 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 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)

       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

       mmap(2), sysconf(3)

COLOPHON

       This page is part of release 3.54 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/.

Linux                                              2012-08-14                                        MPROTECT(2)