mprotect  - control allowable accesses to a region of mem­
       ory


SYNOPSIS

       #include <sys/mman.h>

       int mprotect(const void *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, or not a  multiple  of
              PAGESIZE.

       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 mpro­
              tect 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>

       #include <limits.h>    /* for PAGESIZE */
       #endif

       int
       main(void)
       {
           char *p;
           char c;

           /* Allocate a buffer; it will have the default
              protection of PROT_READ|PROT_WRITE. */
           p = malloc(1024+PAGESIZE-1);
           if (!p) {
               perror("Couldn't malloc(1024)");
               exit(errno);
           }

           /* Align to a multiple of PAGESIZE, assumed to be a power of two */
           p = (char *)(((int) p + PAGESIZE-1) & ~(PAGESIZE-1));

           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);
       }


CONFORMING TO

       SVr4,  POSIX.1b (formerly POSIX.4).  SVr4 defines an addi­
       tional error code EAGAIN. The SVr4 error conditions  don't
       map  neatly onto Linux's.  POSIX.1b says that mprotect can
       be used only on regions of memory obtained from mmap(2).


SEE ALSO

       mmap(2)