mprotect man page on Archlinux

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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 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

       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 appropriately.

       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.

       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

       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 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 bytes.

       An example of what we might see when running the program is the follow‐

	   $ ./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);

       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 3.65 of the Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found at

Linux				  2014-01-05			   MPROTECT(2)

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