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<h3 class="section">6.54 Object Size Checking Built-in Functions</h3>
<p><a name="index-g_t_005f_005fbuiltin_005fobject_005fsize-3583"></a><a name="index-g_t_005f_005fbuiltin_005f_005f_005fmemcpy_005fchk-3584"></a><a name="index-g_t_005f_005fbuiltin_005f_005f_005fmempcpy_005fchk-3585"></a><a name="index-g_t_005f_005fbuiltin_005f_005f_005fmemmove_005fchk-3586"></a><a name="index-g_t_005f_005fbuiltin_005f_005f_005fmemset_005fchk-3587"></a><a name="index-g_t_005f_005fbuiltin_005f_005f_005fstrcpy_005fchk-3588"></a><a name="index-g_t_005f_005fbuiltin_005f_005f_005fstpcpy_005fchk-3589"></a><a name="index-g_t_005f_005fbuiltin_005f_005f_005fstrncpy_005fchk-3590"></a><a name="index-g_t_005f_005fbuiltin_005f_005f_005fstrcat_005fchk-3591"></a><a name="index-g_t_005f_005fbuiltin_005f_005f_005fstrncat_005fchk-3592"></a><a name="index-g_t_005f_005fbuiltin_005f_005f_005fsprintf_005fchk-3593"></a><a name="index-g_t_005f_005fbuiltin_005f_005f_005fsnprintf_005fchk-3594"></a><a name="index-g_t_005f_005fbuiltin_005f_005f_005fvsprintf_005fchk-3595"></a><a name="index-g_t_005f_005fbuiltin_005f_005f_005fvsnprintf_005fchk-3596"></a><a name="index-g_t_005f_005fbuiltin_005f_005f_005fprintf_005fchk-3597"></a><a name="index-g_t_005f_005fbuiltin_005f_005f_005fvprintf_005fchk-3598"></a><a name="index-g_t_005f_005fbuiltin_005f_005f_005ffprintf_005fchk-3599"></a><a name="index-g_t_005f_005fbuiltin_005f_005f_005fvfprintf_005fchk-3600"></a>
GCC implements a limited buffer overflow protection mechanism
that can prevent some buffer overflow attacks.
<div class="defun">
— 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-3601"></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> & 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 = &var.buf1[1], *q = &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 *) (&var + 1) - (char *) &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 (&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 run time. */
memcpy (&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 run time. */
memcpy (&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 does, otherwise the checking function
is called and the <var>flag</var> argument passed to it.
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