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<title>Using the GNU Compiler Collection (GCC): Integer Overflow Builtins</title>
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<a name="Integer-Overflow-Builtins"></a>
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<p>
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Next: <a href="x86-specific-memory-model-extensions-for-transactional-memory.html#x86-specific-memory-model-extensions-for-transactional-memory" accesskey="n" rel="next">x86 specific memory model extensions for transactional memory</a>, Previous: <a href="_005f_005fatomic-Builtins.html#g_t_005f_005fatomic-Builtins" accesskey="p" rel="prev">__atomic Builtins</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> [<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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</div>
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<hr>
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<a name="Built_002din-Functions-to-Perform-Arithmetic-with-Overflow-Checking"></a>
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<h3 class="section">6.54 Built-in Functions to Perform Arithmetic with Overflow Checking</h3>
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<p>The following built-in functions allow performing simple arithmetic operations
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together with checking whether the operations overflowed.
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</p>
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<dl>
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<dt><a name="index-_005f_005fbuiltin_005fadd_005foverflow"></a>Built-in Function: <em>bool</em> <strong>__builtin_add_overflow</strong> <em>(<var>type1</var> a, <var>type2</var> b, <var>type3</var> *res)</em></dt>
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<dt><a name="index-_005f_005fbuiltin_005fsadd_005foverflow"></a>Built-in Function: <em>bool</em> <strong>__builtin_sadd_overflow</strong> <em>(int a, int b, int *res)</em></dt>
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<dt><a name="index-_005f_005fbuiltin_005fsaddl_005foverflow"></a>Built-in Function: <em>bool</em> <strong>__builtin_saddl_overflow</strong> <em>(long int a, long int b, long int *res)</em></dt>
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<dt><a name="index-_005f_005fbuiltin_005fsaddll_005foverflow"></a>Built-in Function: <em>bool</em> <strong>__builtin_saddll_overflow</strong> <em>(long long int a, long long int b, long long int *res)</em></dt>
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<dt><a name="index-_005f_005fbuiltin_005fuadd_005foverflow"></a>Built-in Function: <em>bool</em> <strong>__builtin_uadd_overflow</strong> <em>(unsigned int a, unsigned int b, unsigned int *res)</em></dt>
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<dt><a name="index-_005f_005fbuiltin_005fuaddl_005foverflow"></a>Built-in Function: <em>bool</em> <strong>__builtin_uaddl_overflow</strong> <em>(unsigned long int a, unsigned long int b, unsigned long int *res)</em></dt>
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<dt><a name="index-_005f_005fbuiltin_005fuaddll_005foverflow"></a>Built-in Function: <em>bool</em> <strong>__builtin_uaddll_overflow</strong> <em>(unsigned long long int a, unsigned long long int b, unsigned long long int *res)</em></dt>
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<dd>
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<p>These built-in functions promote the first two operands into infinite precision signed
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type and perform addition on those promoted operands. The result is then
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cast to the type the third pointer argument points to and stored there.
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If the stored result is equal to the infinite precision result, the built-in
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functions return false, otherwise they return true. As the addition is
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performed in infinite signed precision, these built-in functions have fully defined
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behavior for all argument values.
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</p>
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<p>The first built-in function allows arbitrary integral types for operands and
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the result type must be pointer to some integral type other than enumerated or
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boolean type, the rest of the built-in functions have explicit integer types.
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</p>
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<p>The compiler will attempt to use hardware instructions to implement
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these built-in functions where possible, like conditional jump on overflow
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after addition, conditional jump on carry etc.
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</p>
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</dd></dl>
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<dl>
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<dt><a name="index-_005f_005fbuiltin_005fsub_005foverflow"></a>Built-in Function: <em>bool</em> <strong>__builtin_sub_overflow</strong> <em>(<var>type1</var> a, <var>type2</var> b, <var>type3</var> *res)</em></dt>
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<dt><a name="index-_005f_005fbuiltin_005fssub_005foverflow"></a>Built-in Function: <em>bool</em> <strong>__builtin_ssub_overflow</strong> <em>(int a, int b, int *res)</em></dt>
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<dt><a name="index-_005f_005fbuiltin_005fssubl_005foverflow"></a>Built-in Function: <em>bool</em> <strong>__builtin_ssubl_overflow</strong> <em>(long int a, long int b, long int *res)</em></dt>
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<dt><a name="index-_005f_005fbuiltin_005fssubll_005foverflow"></a>Built-in Function: <em>bool</em> <strong>__builtin_ssubll_overflow</strong> <em>(long long int a, long long int b, long long int *res)</em></dt>
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<dt><a name="index-_005f_005fbuiltin_005fusub_005foverflow"></a>Built-in Function: <em>bool</em> <strong>__builtin_usub_overflow</strong> <em>(unsigned int a, unsigned int b, unsigned int *res)</em></dt>
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<dt><a name="index-_005f_005fbuiltin_005fusubl_005foverflow"></a>Built-in Function: <em>bool</em> <strong>__builtin_usubl_overflow</strong> <em>(unsigned long int a, unsigned long int b, unsigned long int *res)</em></dt>
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<dt><a name="index-_005f_005fbuiltin_005fusubll_005foverflow"></a>Built-in Function: <em>bool</em> <strong>__builtin_usubll_overflow</strong> <em>(unsigned long long int a, unsigned long long int b, unsigned long long int *res)</em></dt>
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<dd>
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<p>These built-in functions are similar to the add overflow checking built-in
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functions above, except they perform subtraction, subtract the second argument
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from the first one, instead of addition.
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</p>
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</dd></dl>
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<dl>
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<dt><a name="index-_005f_005fbuiltin_005fmul_005foverflow"></a>Built-in Function: <em>bool</em> <strong>__builtin_mul_overflow</strong> <em>(<var>type1</var> a, <var>type2</var> b, <var>type3</var> *res)</em></dt>
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<dt><a name="index-_005f_005fbuiltin_005fsmul_005foverflow"></a>Built-in Function: <em>bool</em> <strong>__builtin_smul_overflow</strong> <em>(int a, int b, int *res)</em></dt>
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<dt><a name="index-_005f_005fbuiltin_005fsmull_005foverflow"></a>Built-in Function: <em>bool</em> <strong>__builtin_smull_overflow</strong> <em>(long int a, long int b, long int *res)</em></dt>
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<dt><a name="index-_005f_005fbuiltin_005fsmulll_005foverflow"></a>Built-in Function: <em>bool</em> <strong>__builtin_smulll_overflow</strong> <em>(long long int a, long long int b, long long int *res)</em></dt>
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<dt><a name="index-_005f_005fbuiltin_005fumul_005foverflow"></a>Built-in Function: <em>bool</em> <strong>__builtin_umul_overflow</strong> <em>(unsigned int a, unsigned int b, unsigned int *res)</em></dt>
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<dt><a name="index-_005f_005fbuiltin_005fumull_005foverflow"></a>Built-in Function: <em>bool</em> <strong>__builtin_umull_overflow</strong> <em>(unsigned long int a, unsigned long int b, unsigned long int *res)</em></dt>
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<dt><a name="index-_005f_005fbuiltin_005fumulll_005foverflow"></a>Built-in Function: <em>bool</em> <strong>__builtin_umulll_overflow</strong> <em>(unsigned long long int a, unsigned long long int b, unsigned long long int *res)</em></dt>
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<dd>
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<p>These built-in functions are similar to the add overflow checking built-in
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functions above, except they perform multiplication, instead of addition.
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</p>
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</dd></dl>
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<p>The following built-in functions allow checking if simple arithmetic operation
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would overflow.
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</p>
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<dl>
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<dt><a name="index-_005f_005fbuiltin_005fadd_005foverflow_005fp"></a>Built-in Function: <em>bool</em> <strong>__builtin_add_overflow_p</strong> <em>(<var>type1</var> a, <var>type2</var> b, <var>type3</var> c)</em></dt>
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<dt><a name="index-_005f_005fbuiltin_005fsub_005foverflow_005fp"></a>Built-in Function: <em>bool</em> <strong>__builtin_sub_overflow_p</strong> <em>(<var>type1</var> a, <var>type2</var> b, <var>type3</var> c)</em></dt>
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<dt><a name="index-_005f_005fbuiltin_005fmul_005foverflow_005fp"></a>Built-in Function: <em>bool</em> <strong>__builtin_mul_overflow_p</strong> <em>(<var>type1</var> a, <var>type2</var> b, <var>type3</var> c)</em></dt>
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<dd>
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<p>These built-in functions are similar to <code>__builtin_add_overflow</code>,
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<code>__builtin_sub_overflow</code>, or <code>__builtin_mul_overflow</code>, except that
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they don’t store the result of the arithmetic operation anywhere and the
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last argument is not a pointer, but some expression with integral type other
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than enumerated or boolean type.
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</p>
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<p>The built-in functions promote the first two operands into infinite precision signed type
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and perform addition on those promoted operands. The result is then
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cast to the type of the third argument. If the cast result is equal to the infinite
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precision result, the built-in functions return false, otherwise they return true.
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The value of the third argument is ignored, just the side-effects in the third argument
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are evaluated, and no integral argument promotions are performed on the last argument.
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If the third argument is a bit-field, the type used for the result cast has the
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precision and signedness of the given bit-field, rather than precision and signedness
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of the underlying type.
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</p>
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<p>For example, the following macro can be used to portably check, at
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compile-time, whether or not adding two constant integers will overflow,
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and perform the addition only when it is known to be safe and not to trigger
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a <samp>-Woverflow</samp> warning.
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</p>
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<div class="smallexample">
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<pre class="smallexample">#define INT_ADD_OVERFLOW_P(a, b) \
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__builtin_add_overflow_p (a, b, (__typeof__ ((a) + (b))) 0)
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enum {
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A = INT_MAX, B = 3,
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C = INT_ADD_OVERFLOW_P (A, B) ? 0 : A + B,
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D = __builtin_add_overflow_p (1, SCHAR_MAX, (signed char) 0)
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};
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</pre></div>
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<p>The compiler will attempt to use hardware instructions to implement
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these built-in functions where possible, like conditional jump on overflow
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after addition, conditional jump on carry etc.
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</p>
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</dd></dl>
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<hr>
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<div class="header">
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<p>
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Next: <a href="x86-specific-memory-model-extensions-for-transactional-memory.html#x86-specific-memory-model-extensions-for-transactional-memory" accesskey="n" rel="next">x86 specific memory model extensions for transactional memory</a>, Previous: <a href="_005f_005fatomic-Builtins.html#g_t_005f_005fatomic-Builtins" accesskey="p" rel="prev">__atomic Builtins</a>, Up: <a href="C-Extensions.html#C-Extensions" accesskey="u" rel="up">C Extensions</a> [<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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