MPROTECT(2)						     Linux Programmer's Manual						       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 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
",
		   (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
", (long) buffer);

	   if (mprotect(buffer + pagesize * 2, pagesize,
		       PROT_NONE) == -1)
	       handle_error("mprotect");

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

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

SEE ALSO

       mmap(2), sysconf(3)

COLOPHON

       This page is part of release 3.27 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								    2008-08-06							       MPROTECT(2)