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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

       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


       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
       version-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)


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