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/.

Linux                                              2015-07-23                                        MPROTECT(2)