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60
61<h4 class="subsection">3.7.2 Common Predefined Macros</h4>
62
63<p><a name="index-common-predefined-macros-61"></a>
64The common predefined macros are GNU C extensions.  They are available
65with the same meanings regardless of the machine or operating system on
66which you are using GNU C or GNU Fortran.  Their names all start with
67double underscores.
68
69     <dl>
70<dt><code>__COUNTER__</code><dd>This macro expands to sequential integral values starting from 0.  In
71conjunction with the <code>##</code> operator, this provides a convenient means to
72generate unique identifiers.  Care must be taken to ensure that
73<code>__COUNTER__</code> is not expanded prior to inclusion of precompiled headers
74which use it.  Otherwise, the precompiled headers will not be used.
75
76     <br><dt><code>__GFORTRAN__</code><dd>The GNU Fortran compiler defines this.
77
78     <br><dt><code>__GNUC__</code><dt><code>__GNUC_MINOR__</code><dt><code>__GNUC_PATCHLEVEL__</code><dd>These macros are defined by all GNU compilers that use the C
79preprocessor: C, C++, Objective-C and Fortran.  Their values are the major
80version, minor version, and patch level of the compiler, as integer
81constants.  For example, GCC 3.2.1 will define <code>__GNUC__</code> to 3,
82<code>__GNUC_MINOR__</code> to 2, and <code>__GNUC_PATCHLEVEL__</code> to 1.  These
83macros are also defined if you invoke the preprocessor directly.
84
85     <p><code>__GNUC_PATCHLEVEL__</code> is new to GCC 3.0; it is also present in the
86widely-used development snapshots leading up to 3.0 (which identify
87themselves as GCC 2.96 or 2.97, depending on which snapshot you have).
88
89     <p>If all you need to know is whether or not your program is being compiled
90by GCC, or a non-GCC compiler that claims to accept the GNU C dialects,
91you can simply test <code>__GNUC__</code>.  If you need to write code
92which depends on a specific version, you must be more careful.  Each
93time the minor version is increased, the patch level is reset to zero;
94each time the major version is increased (which happens rarely), the
95minor version and patch level are reset.  If you wish to use the
96predefined macros directly in the conditional, you will need to write it
97like this:
98
99     <pre class="smallexample">          /* <span class="roman">Test for GCC &gt; 3.2.0</span> */
100          #if __GNUC__ &gt; 3 || \
101              (__GNUC__ == 3 &amp;&amp; (__GNUC_MINOR__ &gt; 2 || \
102                                 (__GNUC_MINOR__ == 2 &amp;&amp; \
103                                  __GNUC_PATCHLEVEL__ &gt; 0))
104</pre>
105     <p class="noindent">Another approach is to use the predefined macros to
106calculate a single number, then compare that against a threshold:
107
108     <pre class="smallexample">          #define GCC_VERSION (__GNUC__ * 10000 \
109                               + __GNUC_MINOR__ * 100 \
110                               + __GNUC_PATCHLEVEL__)
111          ...
112          /* <span class="roman">Test for GCC &gt; 3.2.0</span> */
113          #if GCC_VERSION &gt; 30200
114</pre>
115     <p class="noindent">Many people find this form easier to understand.
116
117     <br><dt><code>__GNUG__</code><dd>The GNU C++ compiler defines this.  Testing it is equivalent to
118testing <code>(__GNUC__&nbsp;&amp;&amp;&nbsp;__cplusplus)<!-- /@w --></code>.
119
120     <br><dt><code>__STRICT_ANSI__</code><dd>GCC defines this macro if and only if the <samp><span class="option">-ansi</span></samp> switch, or a
121<samp><span class="option">-std</span></samp> switch specifying strict conformance to some version of ISO C,
122was specified when GCC was invoked.  It is defined to &lsquo;<samp><span class="samp">1</span></samp>&rsquo;. 
123This macro exists primarily to direct GNU libc's header files to
124restrict their definitions to the minimal set found in the 1989 C
125standard.
126
127     <br><dt><code>__BASE_FILE__</code><dd>This macro expands to the name of the main input file, in the form
128of a C string constant.  This is the source file that was specified
129on the command line of the preprocessor or C compiler.
130
131     <br><dt><code>__INCLUDE_LEVEL__</code><dd>This macro expands to a decimal integer constant that represents the
132depth of nesting in include files.  The value of this macro is
133incremented on every &lsquo;<samp><span class="samp">#include</span></samp>&rsquo; directive and decremented at the
134end of every included file.  It starts out at 0, its value within the
135base file specified on the command line.
136
137     <br><dt><code>__ELF__</code><dd>This macro is defined if the target uses the ELF object format.
138
139     <br><dt><code>__VERSION__</code><dd>This macro expands to a string constant which describes the version of
140the compiler in use.  You should not rely on its contents having any
141particular form, but it can be counted on to contain at least the
142release number.
143
144     <br><dt><code>__OPTIMIZE__</code><dt><code>__OPTIMIZE_SIZE__</code><dt><code>__NO_INLINE__</code><dd>These macros describe the compilation mode.  <code>__OPTIMIZE__</code> is
145defined in all optimizing compilations.  <code>__OPTIMIZE_SIZE__</code> is
146defined if the compiler is optimizing for size, not speed. 
147<code>__NO_INLINE__</code> is defined if no functions will be inlined into
148their callers (when not optimizing, or when inlining has been
149specifically disabled by <samp><span class="option">-fno-inline</span></samp>).
150
151     <p>These macros cause certain GNU header files to provide optimized
152definitions, using macros or inline functions, of system library
153functions.  You should not use these macros in any way unless you make
154sure that programs will execute with the same effect whether or not they
155are defined.  If they are defined, their value is 1.
156
157     <br><dt><code>__GNUC_GNU_INLINE__</code><dd>GCC defines this macro if functions declared <code>inline</code> will be
158handled in GCC's traditional gnu90 mode.  Object files will contain
159externally visible definitions of all functions declared <code>inline</code>
160without <code>extern</code> or <code>static</code>.  They will not contain any
161definitions of any functions declared <code>extern inline</code>.
162
163     <br><dt><code>__GNUC_STDC_INLINE__</code><dd>GCC defines this macro if functions declared <code>inline</code> will be
164handled according to the ISO C99 standard.  Object files will contain
165externally visible definitions of all functions declared <code>extern
166inline</code>.  They will not contain definitions of any functions declared
167<code>inline</code> without <code>extern</code>.
168
169     <p>If this macro is defined, GCC supports the <code>gnu_inline</code> function
170attribute as a way to always get the gnu90 behavior.  Support for
171this and <code>__GNUC_GNU_INLINE__</code> was added in GCC 4.1.3.  If
172neither macro is defined, an older version of GCC is being used:
173<code>inline</code> functions will be compiled in gnu90 mode, and the
174<code>gnu_inline</code> function attribute will not be recognized.
175
176     <br><dt><code>__CHAR_UNSIGNED__</code><dd>GCC defines this macro if and only if the data type <code>char</code> is
177unsigned on the target machine.  It exists to cause the standard header
178file <samp><span class="file">limits.h</span></samp> to work correctly.  You should not use this macro
179yourself; instead, refer to the standard macros defined in <samp><span class="file">limits.h</span></samp>.
180
181     <br><dt><code>__WCHAR_UNSIGNED__</code><dd>Like <code>__CHAR_UNSIGNED__</code>, this macro is defined if and only if the
182data type <code>wchar_t</code> is unsigned and the front-end is in C++ mode.
183
184     <br><dt><code>__REGISTER_PREFIX__</code><dd>This macro expands to a single token (not a string constant) which is
185the prefix applied to CPU register names in assembly language for this
186target.  You can use it to write assembly that is usable in multiple
187environments.  For example, in the <code>m68k-aout</code> environment it
188expands to nothing, but in the <code>m68k-coff</code> environment it expands
189to a single &lsquo;<samp><span class="samp">%</span></samp>&rsquo;.
190
191     <br><dt><code>__USER_LABEL_PREFIX__</code><dd>This macro expands to a single token which is the prefix applied to
192user labels (symbols visible to C code) in assembly.  For example, in
193the <code>m68k-aout</code> environment it expands to an &lsquo;<samp><span class="samp">_</span></samp>&rsquo;, but in the
194<code>m68k-coff</code> environment it expands to nothing.
195
196     <p>This macro will have the correct definition even if
197<samp><span class="option">-f(no-)underscores</span></samp> is in use, but it will not be correct if
198target-specific options that adjust this prefix are used (e.g. the
199OSF/rose <samp><span class="option">-mno-underscores</span></samp> option).
200
201     <br><dt><code>__SIZE_TYPE__</code><dt><code>__PTRDIFF_TYPE__</code><dt><code>__WCHAR_TYPE__</code><dt><code>__WINT_TYPE__</code><dt><code>__INTMAX_TYPE__</code><dt><code>__UINTMAX_TYPE__</code><dt><code>__SIG_ATOMIC_TYPE__</code><dt><code>__INT8_TYPE__</code><dt><code>__INT16_TYPE__</code><dt><code>__INT32_TYPE__</code><dt><code>__INT64_TYPE__</code><dt><code>__UINT8_TYPE__</code><dt><code>__UINT16_TYPE__</code><dt><code>__UINT32_TYPE__</code><dt><code>__UINT64_TYPE__</code><dt><code>__INT_LEAST8_TYPE__</code><dt><code>__INT_LEAST16_TYPE__</code><dt><code>__INT_LEAST32_TYPE__</code><dt><code>__INT_LEAST64_TYPE__</code><dt><code>__UINT_LEAST8_TYPE__</code><dt><code>__UINT_LEAST16_TYPE__</code><dt><code>__UINT_LEAST32_TYPE__</code><dt><code>__UINT_LEAST64_TYPE__</code><dt><code>__INT_FAST8_TYPE__</code><dt><code>__INT_FAST16_TYPE__</code><dt><code>__INT_FAST32_TYPE__</code><dt><code>__INT_FAST64_TYPE__</code><dt><code>__UINT_FAST8_TYPE__</code><dt><code>__UINT_FAST16_TYPE__</code><dt><code>__UINT_FAST32_TYPE__</code><dt><code>__UINT_FAST64_TYPE__</code><dt><code>__INTPTR_TYPE__</code><dt><code>__UINTPTR_TYPE__</code><dd>These macros are defined to the correct underlying types for the
202<code>size_t</code>, <code>ptrdiff_t</code>, <code>wchar_t</code>, <code>wint_t</code>,
203<code>intmax_t</code>, <code>uintmax_t</code>, <code>sig_atomic_t</code>, <code>int8_t</code>,
204<code>int16_t</code>, <code>int32_t</code>, <code>int64_t</code>, <code>uint8_t</code>,
205<code>uint16_t</code>, <code>uint32_t</code>, <code>uint64_t</code>,
206<code>int_least8_t</code>, <code>int_least16_t</code>, <code>int_least32_t</code>,
207<code>int_least64_t</code>, <code>uint_least8_t</code>, <code>uint_least16_t</code>,
208<code>uint_least32_t</code>, <code>uint_least64_t</code>, <code>int_fast8_t</code>,
209<code>int_fast16_t</code>, <code>int_fast32_t</code>, <code>int_fast64_t</code>,
210<code>uint_fast8_t</code>, <code>uint_fast16_t</code>, <code>uint_fast32_t</code>,
211<code>uint_fast64_t</code>, <code>intptr_t</code>, and <code>uintptr_t</code> typedefs,
212respectively.  They exist to make the standard header files
213<samp><span class="file">stddef.h</span></samp>, <samp><span class="file">stdint.h</span></samp>, and <samp><span class="file">wchar.h</span></samp> work correctly. 
214You should not use these macros directly; instead, include the
215appropriate headers and use the typedefs.  Some of these macros may
216not be defined on particular systems if GCC does not provide a
217<samp><span class="file">stdint.h</span></samp> header on those systems.
218
219     <br><dt><code>__CHAR_BIT__</code><dd>Defined to the number of bits used in the representation of the
220<code>char</code> data type.  It exists to make the standard header given
221numerical limits work correctly.  You should not use
222this macro directly; instead, include the appropriate headers.
223
224     <br><dt><code>__SCHAR_MAX__</code><dt><code>__WCHAR_MAX__</code><dt><code>__SHRT_MAX__</code><dt><code>__INT_MAX__</code><dt><code>__LONG_MAX__</code><dt><code>__LONG_LONG_MAX__</code><dt><code>__WINT_MAX__</code><dt><code>__SIZE_MAX__</code><dt><code>__PTRDIFF_MAX__</code><dt><code>__INTMAX_MAX__</code><dt><code>__UINTMAX_MAX__</code><dt><code>__SIG_ATOMIC_MAX__</code><dt><code>__INT8_MAX__</code><dt><code>__INT16_MAX__</code><dt><code>__INT32_MAX__</code><dt><code>__INT64_MAX__</code><dt><code>__UINT8_MAX__</code><dt><code>__UINT16_MAX__</code><dt><code>__UINT32_MAX__</code><dt><code>__UINT64_MAX__</code><dt><code>__INT_LEAST8_MAX__</code><dt><code>__INT_LEAST16_MAX__</code><dt><code>__INT_LEAST32_MAX__</code><dt><code>__INT_LEAST64_MAX__</code><dt><code>__UINT_LEAST8_MAX__</code><dt><code>__UINT_LEAST16_MAX__</code><dt><code>__UINT_LEAST32_MAX__</code><dt><code>__UINT_LEAST64_MAX__</code><dt><code>__INT_FAST8_MAX__</code><dt><code>__INT_FAST16_MAX__</code><dt><code>__INT_FAST32_MAX__</code><dt><code>__INT_FAST64_MAX__</code><dt><code>__UINT_FAST8_MAX__</code><dt><code>__UINT_FAST16_MAX__</code><dt><code>__UINT_FAST32_MAX__</code><dt><code>__UINT_FAST64_MAX__</code><dt><code>__INTPTR_MAX__</code><dt><code>__UINTPTR_MAX__</code><dt><code>__WCHAR_MIN__</code><dt><code>__WINT_MIN__</code><dt><code>__SIG_ATOMIC_MIN__</code><dd>Defined to the maximum value of the <code>signed char</code>, <code>wchar_t</code>,
225<code>signed short</code>,
226<code>signed int</code>, <code>signed long</code>, <code>signed long long</code>,
227<code>wint_t</code>, <code>size_t</code>, <code>ptrdiff_t</code>,
228<code>intmax_t</code>, <code>uintmax_t</code>, <code>sig_atomic_t</code>, <code>int8_t</code>,
229<code>int16_t</code>, <code>int32_t</code>, <code>int64_t</code>, <code>uint8_t</code>,
230<code>uint16_t</code>, <code>uint32_t</code>, <code>uint64_t</code>,
231<code>int_least8_t</code>, <code>int_least16_t</code>, <code>int_least32_t</code>,
232<code>int_least64_t</code>, <code>uint_least8_t</code>, <code>uint_least16_t</code>,
233<code>uint_least32_t</code>, <code>uint_least64_t</code>, <code>int_fast8_t</code>,
234<code>int_fast16_t</code>, <code>int_fast32_t</code>, <code>int_fast64_t</code>,
235<code>uint_fast8_t</code>, <code>uint_fast16_t</code>, <code>uint_fast32_t</code>,
236<code>uint_fast64_t</code>, <code>intptr_t</code>, and <code>uintptr_t</code> types and
237to the minimum value of the <code>wchar_t</code>, <code>wint_t</code>, and
238<code>sig_atomic_t</code> types respectively.  They exist to make the
239standard header given numerical limits work correctly.  You should not
240use these macros directly; instead, include the appropriate headers. 
241Some of these macros may not be defined on particular systems if GCC
242does not provide a <samp><span class="file">stdint.h</span></samp> header on those systems.
243
244     <br><dt><code>__INT8_C</code><dt><code>__INT16_C</code><dt><code>__INT32_C</code><dt><code>__INT64_C</code><dt><code>__UINT8_C</code><dt><code>__UINT16_C</code><dt><code>__UINT32_C</code><dt><code>__UINT64_C</code><dt><code>__INTMAX_C</code><dt><code>__UINTMAX_C</code><dd>Defined to implementations of the standard <samp><span class="file">stdint.h</span></samp> macros with
245the same names without the leading <code>__</code>.  They exist the make the
246implementation of that header work correctly.  You should not use
247these macros directly; instead, include the appropriate headers.  Some
248of these macros may not be defined on particular systems if GCC does
249not provide a <samp><span class="file">stdint.h</span></samp> header on those systems.
250
251     <br><dt><code>__SIZEOF_INT__</code><dt><code>__SIZEOF_LONG__</code><dt><code>__SIZEOF_LONG_LONG__</code><dt><code>__SIZEOF_SHORT__</code><dt><code>__SIZEOF_POINTER__</code><dt><code>__SIZEOF_FLOAT__</code><dt><code>__SIZEOF_DOUBLE__</code><dt><code>__SIZEOF_LONG_DOUBLE__</code><dt><code>__SIZEOF_SIZE_T__</code><dt><code>__SIZEOF_WCHAR_T__</code><dt><code>__SIZEOF_WINT_T__</code><dt><code>__SIZEOF_PTRDIFF_T__</code><dd>Defined to the number of bytes of the C standard data types: <code>int</code>,
252<code>long</code>, <code>long long</code>, <code>short</code>, <code>void *</code>, <code>float</code>,
253<code>double</code>, <code>long double</code>, <code>size_t</code>, <code>wchar_t</code>, <code>wint_t</code>
254and <code>ptrdiff_t</code>.
255
256     <br><dt><code>__BYTE_ORDER__</code><dt><code>__ORDER_LITTLE_ENDIAN__</code><dt><code>__ORDER_BIG_ENDIAN__</code><dt><code>__ORDER_PDP_ENDIAN__</code><dd><code>__BYTE_ORDER__</code> is defined to one of the values
257<code>__ORDER_LITTLE_ENDIAN__</code>, <code>__ORDER_BIG_ENDIAN__</code>, or
258<code>__ORDER_PDP_ENDIAN__</code> to reflect the layout of multi-byte and
259multi-word quantities in memory.  If <code>__BYTE_ORDER__</code> is equal to
260<code>__ORDER_LITTLE_ENDIAN__</code> or <code>__ORDER_BIG_ENDIAN__</code>, then
261multi-byte and multi-word quantities are laid out identically: the
262byte (word) at the lowest address is the least significant or most
263significant byte (word) of the quantity, respectively.  If
264<code>__BYTE_ORDER__</code> is equal to <code>__ORDER_PDP_ENDIAN__</code>, then
265bytes in 16-bit words are laid out in a little-endian fashion, whereas
266the 16-bit subwords of a 32-bit quantity are laid out in big-endian
267fashion.
268
269     <p>You should use these macros for testing like this:
270
271     <pre class="smallexample">          /* <span class="roman">Test for a little-endian machine</span> */
272          #if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
273</pre>
274     <br><dt><code>__FLOAT_WORD_ORDER__</code><dd><code>__FLOAT_WORD_ORDER__</code> is defined to one of the values
275<code>__ORDER_LITTLE_ENDIAN__</code> or <code>__ORDER_BIG_ENDIAN__</code> to reflect
276the layout of the words of multi-word floating-point quantities.
277
278     <br><dt><code>__DEPRECATED</code><dd>This macro is defined, with value 1, when compiling a C++ source file
279with warnings about deprecated constructs enabled.  These warnings are
280enabled by default, but can be disabled with <samp><span class="option">-Wno-deprecated</span></samp>.
281
282     <br><dt><code>__EXCEPTIONS</code><dd>This macro is defined, with value 1, when compiling a C++ source file
283with exceptions enabled.  If <samp><span class="option">-fno-exceptions</span></samp> is used when
284compiling the file, then this macro is not defined.
285
286     <br><dt><code>__GXX_RTTI</code><dd>This macro is defined, with value 1, when compiling a C++ source file
287with runtime type identification enabled.  If <samp><span class="option">-fno-rtti</span></samp> is
288used when compiling the file, then this macro is not defined.
289
290     <br><dt><code>__USING_SJLJ_EXCEPTIONS__</code><dd>This macro is defined, with value 1, if the compiler uses the old
291mechanism based on <code>setjmp</code> and <code>longjmp</code> for exception
292handling.
293
294     <br><dt><code>__GXX_EXPERIMENTAL_CXX0X__</code><dd>This macro is defined when compiling a C++ source file with the option
295<samp><span class="option">-std=c++0x</span></samp> or <samp><span class="option">-std=gnu++0x</span></samp>. It indicates that some
296features likely to be included in C++0x are available. Note that these
297features are experimental, and may change or be removed in future
298versions of GCC.
299
300     <br><dt><code>__GXX_WEAK__</code><dd>This macro is defined when compiling a C++ source file.  It has the
301value 1 if the compiler will use weak symbols, COMDAT sections, or
302other similar techniques to collapse symbols with &ldquo;vague linkage&rdquo;
303that are defined in multiple translation units.  If the compiler will
304not collapse such symbols, this macro is defined with value 0.  In
305general, user code should not need to make use of this macro; the
306purpose of this macro is to ease implementation of the C++ runtime
307library provided with G++.
308
309     <br><dt><code>__NEXT_RUNTIME__</code><dd>This macro is defined, with value 1, if (and only if) the NeXT runtime
310(as in <samp><span class="option">-fnext-runtime</span></samp>) is in use for Objective-C.  If the GNU
311runtime is used, this macro is not defined, so that you can use this
312macro to determine which runtime (NeXT or GNU) is being used.
313
314     <br><dt><code>__LP64__</code><dt><code>_LP64</code><dd>These macros are defined, with value 1, if (and only if) the compilation
315is for a target where <code>long int</code> and pointer both use 64-bits and
316<code>int</code> uses 32-bit.
317
318     <br><dt><code>__SSP__</code><dd>This macro is defined, with value 1, when <samp><span class="option">-fstack-protector</span></samp> is in
319use.
320
321     <br><dt><code>__SSP_ALL__</code><dd>This macro is defined, with value 2, when <samp><span class="option">-fstack-protector-all</span></samp> is
322in use.
323
324     <br><dt><code>__TIMESTAMP__</code><dd>This macro expands to a string constant that describes the date and time
325of the last modification of the current source file. The string constant
326contains abbreviated day of the week, month, day of the month, time in
327hh:mm:ss form, year and looks like <code>"Sun&nbsp;Sep&nbsp;16&nbsp;01:03:52&nbsp;1973"<!-- /@w --></code>. 
328If the day of the month is less than 10, it is padded with a space on the left.
329
330     <p>If GCC cannot determine the current date, it will emit a warning message
331(once per compilation) and <code>__TIMESTAMP__</code> will expand to
332<code>"???&nbsp;???&nbsp;??&nbsp;??:??:??&nbsp;????"<!-- /@w --></code>.
333
334     <br><dt><code>__GCC_HAVE_SYNC_COMPARE_AND_SWAP_1</code><dt><code>__GCC_HAVE_SYNC_COMPARE_AND_SWAP_2</code><dt><code>__GCC_HAVE_SYNC_COMPARE_AND_SWAP_4</code><dt><code>__GCC_HAVE_SYNC_COMPARE_AND_SWAP_8</code><dt><code>__GCC_HAVE_SYNC_COMPARE_AND_SWAP_16</code><dd>These macros are defined when the target processor supports atomic compare
335and swap operations on operands 1, 2, 4, 8 or 16 bytes in length, respectively.
336
337     <br><dt><code>__GCC_HAVE_DWARF2_CFI_ASM</code><dd>This macro is defined when the compiler is emitting Dwarf2 CFI directives
338to the assembler.  When this is defined, it is possible to emit those same
339directives in inline assembly.
340
341     <br><dt><code>__FP_FAST_FMA</code><dt><code>__FP_FAST_FMAF</code><dt><code>__FP_FAST_FMAL</code><dd>These macros are defined with value 1 if the backend supports the
342<code>fma</code>, <code>fmaf</code>, and <code>fmal</code> builtin functions, so that
343the include file <samp><span class="file">math.h</span></samp> can define the macros
344<code>FP_FAST_FMA</code>, <code>FP_FAST_FMAF</code>, and <code>FP_FAST_FMAL</code>
345for compatibility with the 1999 C standard. 
346</dl>
347
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