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<a name="Object-Size-Checking"></a>
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<hr>
<a name="Object-Size-Checking-Built_002din-Functions"></a>
<h3 class="section">6.58 Object Size Checking Built-in Functions</h3>
<a name="index-_005f_005fbuiltin_005fobject_005fsize"></a>
<a name="index-_005f_005fbuiltin_005f_005f_005fmemcpy_005fchk"></a>
<a name="index-_005f_005fbuiltin_005f_005f_005fmempcpy_005fchk"></a>
<a name="index-_005f_005fbuiltin_005f_005f_005fmemmove_005fchk"></a>
<a name="index-_005f_005fbuiltin_005f_005f_005fmemset_005fchk"></a>
<a name="index-_005f_005fbuiltin_005f_005f_005fstrcpy_005fchk"></a>
<a name="index-_005f_005fbuiltin_005f_005f_005fstpcpy_005fchk"></a>
<a name="index-_005f_005fbuiltin_005f_005f_005fstrncpy_005fchk"></a>
<a name="index-_005f_005fbuiltin_005f_005f_005fstrcat_005fchk"></a>
<a name="index-_005f_005fbuiltin_005f_005f_005fstrncat_005fchk"></a>
<a name="index-_005f_005fbuiltin_005f_005f_005fsprintf_005fchk"></a>
<a name="index-_005f_005fbuiltin_005f_005f_005fsnprintf_005fchk"></a>
<a name="index-_005f_005fbuiltin_005f_005f_005fvsprintf_005fchk"></a>
<a name="index-_005f_005fbuiltin_005f_005f_005fvsnprintf_005fchk"></a>
<a name="index-_005f_005fbuiltin_005f_005f_005fprintf_005fchk"></a>
<a name="index-_005f_005fbuiltin_005f_005f_005fvprintf_005fchk"></a>
<a name="index-_005f_005fbuiltin_005f_005f_005ffprintf_005fchk"></a>
<a name="index-_005f_005fbuiltin_005f_005f_005fvfprintf_005fchk"></a>

<p>GCC implements a limited buffer overflow protection mechanism that can
prevent some buffer overflow attacks by determining the sizes of objects
into which data is about to be written and preventing the writes when
the size isn&rsquo;t sufficient.  The built-in functions described below yield
the best results when used together and when optimization is enabled.
For example, to detect object sizes across function boundaries or to
follow pointer assignments through non-trivial control flow they rely
on various optimization passes enabled with <samp>-O2</samp>.  However, to
a limited extent, they can be used without optimization as well.
</p>
<dl>
<dt><a name="index-_005f_005fbuiltin_005fobject_005fsize-1"></a>Built-in Function: <em>size_t</em> <strong>__builtin_object_size</strong> <em>(const void * <var>ptr</var>, int <var>type</var>)</em></dt>
<dd><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).  To determine the sizes of dynamically allocated
objects the function relies on the allocation functions called to obtain
the storage to be declared with the <code>alloc_size</code> attribute (see <a href="Common-Function-Attributes.html#Common-Function-Attributes">Common Function Attributes</a>).  <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>
<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.
</p>
<div class="smallexample">
<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></div>
</dd></dl>

<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 the object the <var>dest</var>
argument points to or <code>(size_t) -1</code> if the size is not known.
</p>
<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 overflowed.  If the compiler can determine at compile time that the
object will always be overflowed, it issues a warning.
</p>
<p>The intended use can be e.g.
</p>
<div class="smallexample">
<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, &quot;abcde&quot;, n);
/* Destination is known and length too.  It is known at compile
   time there will be no overflow.  */
memcpy (&amp;buf[5], &quot;abcde&quot;, 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 run time.  */
memcpy (&amp;buf[5], &quot;abcde&quot;, 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 run time.  */
memcpy (&amp;buf[6], &quot;abcde&quot;, 5);
</pre></div>

<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>
<p>There are also checking built-in functions for formatted output functions.
</p><div class="smallexample">
<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></div>

<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>
<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>
<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 does, otherwise the checking function
is called and the <var>flag</var> argument passed to it.
</p>
<hr>
<div class="header">
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Next: <a href="Other-Builtins.html#Other-Builtins" accesskey="n" rel="next">Other Builtins</a>, Previous: <a href="x86-specific-memory-model-extensions-for-transactional-memory.html#x86-specific-memory-model-extensions-for-transactional-memory" accesskey="p" rel="prev">x86 specific memory model extensions for transactional memory</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> &nbsp; [<a href="index.html#SEC_Contents" title="Table of contents" rel="contents">Contents</a>][<a href="Option-Index.html#Option-Index" title="Index" rel="index">Index</a>]</p>
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