MPROTECT(2)		   Linux Programmers Manual		  MPROTECT(2)



NAME
       mprotect - set protection on a region of memory

SYNOPSIS
       #include 

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

DESCRIPTION
       mprotect()  changes protection for the calling processs 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 pro
       cess.

       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 hap
	      pen, 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 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 processs 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  architec
       ture-  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 follow
       ing:

	   $ ./a.out
	   Start of region:	   0x804c000
	   Got SIGSEGV at address: 0x804e000

       #include 
       #include 
       #include 
       #include 
       #include 
       #include 
       #include 

       #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 3.05 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				  2008-06-02			   MPROTECT(2)