mprotect

NAME

       mprotect  - control allowable accesses to a region of mem-
       ory

SYNOPSIS

       #include <sys/mman.h>

       int mprotect(caddr_t addr, size_t *len, int prot);

DESCRIPTION

       mprotect controls how a section of memory may be accessed.
       If an access is disallowed by the protection given it, the
       program receives a SIGSEGV.

       prot is a bitwise-or of the following values:

       PROT_NONE
               The memory cannot be accessed at all.

       PROT_READ
               The memory can be read.

       PROT_WRITE
               The memory can be written to.

       PROT_EXEC
               The memory can contain executing code.

       The new protection replaces any existing protection.   For
       example,   if   the  memory  had  previously  been  marked
       PROT_READ,  and  mprotect  is  then   called   with   prot
       PROT_WRITE, it will no longer be readable.

RETURN VALUE

       On  success,  mprotect  returns  zero.   On  error,  -1 is
       returned, and errno is set appropriately.

ERRORS

       EINVAL  addr is not a valid pointer.

       EFAULT  The memory cannot be accessed.

       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.

       ENOMEM  Internal kernel structures could not be allocated.

EXAMPLE

       #include <stdio.h>
       #include <stdlib.h>
       #include <errno.h>
       #include <sys/mman.h>
       int
       main(void)
       {
           char *p;
           char c;

           /* Allocate a buffer; it will have the default
              protection of PROT_READ|PROT_WRITE. */
           p = malloc(1024);
           if (!p) {
               perror("Couldn't malloc(1024)");
               exit(errno);
           }

           c = p[666];         /* Read; ok */
           p[666] = 42;        /* Write; ok */

           /* Mark the buffer read-only. */
           if (mprotect(p, 1024, PROT_READ)) {
               perror("Couldn't mprotect");
               exit(errno);
           }

           c = p[666];         /* Read; ok */
           p[666] = 42;        /* Write; program dies on SIGSEGV */

           exit(0);
       }

SEE ALSO

       mmap(2)