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<h3 class="section">6.50 Object Size Checking Builtins</h3>

<p><a name="index-g_t_005f_005fbuiltin_005fobject_005fsize-2630"></a><a name="index-g_t_005f_005fbuiltin_005f_005f_005fmemcpy_005fchk-2631"></a><a name="index-g_t_005f_005fbuiltin_005f_005f_005fmempcpy_005fchk-2632"></a><a name="index-g_t_005f_005fbuiltin_005f_005f_005fmemmove_005fchk-2633"></a><a name="index-g_t_005f_005fbuiltin_005f_005f_005fmemset_005fchk-2634"></a><a name="index-g_t_005f_005fbuiltin_005f_005f_005fstrcpy_005fchk-2635"></a><a name="index-g_t_005f_005fbuiltin_005f_005f_005fstpcpy_005fchk-2636"></a><a name="index-g_t_005f_005fbuiltin_005f_005f_005fstrncpy_005fchk-2637"></a><a name="index-g_t_005f_005fbuiltin_005f_005f_005fstrcat_005fchk-2638"></a><a name="index-g_t_005f_005fbuiltin_005f_005f_005fstrncat_005fchk-2639"></a><a name="index-g_t_005f_005fbuiltin_005f_005f_005fsprintf_005fchk-2640"></a><a name="index-g_t_005f_005fbuiltin_005f_005f_005fsnprintf_005fchk-2641"></a><a name="index-g_t_005f_005fbuiltin_005f_005f_005fvsprintf_005fchk-2642"></a><a name="index-g_t_005f_005fbuiltin_005f_005f_005fvsnprintf_005fchk-2643"></a><a name="index-g_t_005f_005fbuiltin_005f_005f_005fprintf_005fchk-2644"></a><a name="index-g_t_005f_005fbuiltin_005f_005f_005fvprintf_005fchk-2645"></a><a name="index-g_t_005f_005fbuiltin_005f_005f_005ffprintf_005fchk-2646"></a><a name="index-g_t_005f_005fbuiltin_005f_005f_005fvfprintf_005fchk-2647"></a>
GCC implements a limited buffer overflow protection mechanism
that can prevent some buffer overflow attacks.

<div class="defun">
&mdash; Built-in Function: size_t <b>__builtin_object_size</b> (<var>void * ptr, int type</var>)<var><a name="index-g_t_005f_005fbuiltin_005fobject_005fsize-2648"></a></var><br>
<blockquote><p>is a built-in construct that returns a constant number of bytes from
<var>ptr</var> to the end of the object <var>ptr</var> pointer points to
(if known at compile time).  <code>__builtin_object_size</code> never evaluates
its arguments for side-effects.  If there are any side-effects in them, it
returns <code>(size_t) -1</code> for <var>type</var> 0 or 1 and <code>(size_t) 0</code>
for <var>type</var> 2 or 3.  If there are multiple objects <var>ptr</var> can
point to and all of them are known at compile time, the returned number
is the maximum of remaining byte counts in those objects if <var>type</var> &amp; 2 is
0 and minimum if nonzero.  If it is not possible to determine which objects
<var>ptr</var> points to at compile time, <code>__builtin_object_size</code> should
return <code>(size_t) -1</code> for <var>type</var> 0 or 1 and <code>(size_t) 0</code>
for <var>type</var> 2 or 3.

      <p><var>type</var> is an integer constant from 0 to 3.  If the least significant
bit is clear, objects are whole variables, if it is set, a closest
surrounding subobject is considered the object a pointer points to. 
The second bit determines if maximum or minimum of remaining bytes
is computed.

     <pre class="smallexample">          struct V { char buf1[10]; int b; char buf2[10]; } var;
          char *p = &amp;var.buf1[1], *q = &amp;var.b;
          
          /* Here the object p points to is var.  */
          assert (__builtin_object_size (p, 0) == sizeof (var) - 1);
          /* The subobject p points to is var.buf1.  */
          assert (__builtin_object_size (p, 1) == sizeof (var.buf1) - 1);
          /* The object q points to is var.  */
          assert (__builtin_object_size (q, 0)
                  == (char *) (&amp;var + 1) - (char *) &amp;var.b);
          /* The subobject q points to is var.b.  */
          assert (__builtin_object_size (q, 1) == sizeof (var.b));
</pre>
      </blockquote></div>

 <p>There are built-in functions added for many common string operation
functions, e.g., for <code>memcpy</code> <code>__builtin___memcpy_chk</code>
built-in is provided.  This built-in has an additional last argument,
which is the number of bytes remaining in object the <var>dest</var>
argument points to or <code>(size_t) -1</code> if the size is not known.

 <p>The built-in functions are optimized into the normal string functions
like <code>memcpy</code> if the last argument is <code>(size_t) -1</code> or if
it is known at compile time that the destination object will not
be overflown.  If the compiler can determine at compile time the
object will be always overflown, it issues a warning.

 <p>The intended use can be e.g.

<pre class="smallexample">     #undef memcpy
     #define bos0(dest) __builtin_object_size (dest, 0)
     #define memcpy(dest, src, n) \
       __builtin___memcpy_chk (dest, src, n, bos0 (dest))
     
     char *volatile p;
     char buf[10];
     /* It is unknown what object p points to, so this is optimized
        into plain memcpy - no checking is possible.  */
     memcpy (p, "abcde", n);
     /* Destination is known and length too.  It is known at compile
        time there will be no overflow.  */
     memcpy (&amp;buf[5], "abcde", 5);
     /* Destination is known, but the length is not known at compile time.
        This will result in __memcpy_chk call that can check for overflow
        at runtime.  */
     memcpy (&amp;buf[5], "abcde", n);
     /* Destination is known and it is known at compile time there will
        be overflow.  There will be a warning and __memcpy_chk call that
        will abort the program at runtime.  */
     memcpy (&amp;buf[6], "abcde", 5);
</pre>
 <p>Such built-in functions are provided for <code>memcpy</code>, <code>mempcpy</code>,
<code>memmove</code>, <code>memset</code>, <code>strcpy</code>, <code>stpcpy</code>, <code>strncpy</code>,
<code>strcat</code> and <code>strncat</code>.

 <p>There are also checking built-in functions for formatted output functions.
<pre class="smallexample">     int __builtin___sprintf_chk (char *s, int flag, size_t os, const char *fmt, ...);
     int __builtin___snprintf_chk (char *s, size_t maxlen, int flag, size_t os,
                                   const char *fmt, ...);
     int __builtin___vsprintf_chk (char *s, int flag, size_t os, const char *fmt,
                                   va_list ap);
     int __builtin___vsnprintf_chk (char *s, size_t maxlen, int flag, size_t os,
                                    const char *fmt, va_list ap);
</pre>
 <p>The added <var>flag</var> argument is passed unchanged to <code>__sprintf_chk</code>
etc. functions and can contain implementation specific flags on what
additional security measures the checking function might take, such as
handling <code>%n</code> differently.

 <p>The <var>os</var> argument is the object size <var>s</var> points to, like in the
other built-in functions.  There is a small difference in the behavior
though, if <var>os</var> is <code>(size_t) -1</code>, the built-in functions are
optimized into the non-checking functions only if <var>flag</var> is 0, otherwise
the checking function is called with <var>os</var> argument set to
<code>(size_t) -1</code>.

 <p>In addition to this, there are checking built-in functions
<code>__builtin___printf_chk</code>, <code>__builtin___vprintf_chk</code>,
<code>__builtin___fprintf_chk</code> and <code>__builtin___vfprintf_chk</code>. 
These have just one additional argument, <var>flag</var>, right before
format string <var>fmt</var>.  If the compiler is able to optimize them to
<code>fputc</code> etc. functions, it will, otherwise the checking function
should be called and the <var>flag</var> argument passed to it.

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