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       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
              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.)


       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).


       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  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

       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.


       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

           $ ./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)


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