Unix/Linux Go Back    

Linux 2.6 - man page for mprotect (linux section 2)

Linux & Unix Commands - Search Man Pages
Man Page or Keyword Search:   man
Select Man Page Set:       apropos Keyword Search (sections above)

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

       mprotect - set protection on a region of memory

       #include <sys/mman.h>

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

       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

       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.

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

       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.)

       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).

       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 ver-
       sion-dependent.	  On   some  hardware  architectures  (e.g.,  i386),  PROT_WRITE  implies

       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.

       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

       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);

       main(int argc, char *argv[])
	   char *p;
	   int pagesize;
	   struct sigaction sa;

	   sa.sa_flags = SA_SIGINFO;
	   sa.sa_sigaction = handler;
	   if (sigaction(SIGSEGV, &sa, NULL) == -1)

	   pagesize = sysconf(_SC_PAGE_SIZE);
	   if (pagesize == -1)

	   /* Allocate a buffer aligned on a page boundary;
	      initial protection is PROT_READ | PROT_WRITE */

	   buffer = memalign(pagesize, 4 * pagesize);
	   if (buffer == NULL)

	   printf("Start of region:	   0x%lx\n", (long) buffer);

	   if (mprotect(buffer + pagesize * 2, pagesize,
		       PROT_READ) == -1)

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

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

       mmap(2), sysconf(3)

       This page is part of release 3.55 of the Linux man-pages project.  A  description  of  the
       project,     and    information	  about    reporting	bugs,	 can	be    found    at

Linux					    2012-08-14				      MPROTECT(2)
Unix & Linux Commands & Man Pages : ©2000 - 2018 Unix and Linux Forums

All times are GMT -4. The time now is 08:22 PM.