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.)

       ENOMEM Changing the protection of a memory region would result in  the  total  number  of  mappings  with
              distinct attributes (e.g., read versus read/write protection) exceeding the allowed maximum.  (For
              example, making the protection of a range PROT_READ in the middle of a region currently  protected
              as  PROT_READ|PROT_WRITE would result in three mappings: two read/write mappings at each end and a
              read-only mapping in the middle.)

CONFORMING TO

       POSIX.1-2001, POSIX.1-2008, SVr4.  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 depends on processor architecture, kernel
       version, and process state.  If  READ_IMPLIES_EXEC  is  set  in  the  process's  personality  flags  (see
       personality(2)), specifying PROT_READ will implicitly add PROT_EXEC.

       On some hardware architectures (e.g., i386), PROT_WRITE implies PROT_READ.

       POSIX.1 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)

       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);
           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 4.04 of  the  Linux  man-pages  project.   A  description  of  the  project,
       information   about   reporting   bugs,   and   the  latest  version  of  this  page,  can  be  found  at
       http://www.kernel.org/doc/man-pages/.