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Diffstat (limited to 'libstdc++-v3/doc/xml/faq.xml')
| -rw-r--r-- | libstdc++-v3/doc/xml/faq.xml | 33 |
1 files changed, 18 insertions, 15 deletions
diff --git a/libstdc++-v3/doc/xml/faq.xml b/libstdc++-v3/doc/xml/faq.xml index 92b81f2068c..1af41c2fbaf 100644 --- a/libstdc++-v3/doc/xml/faq.xml +++ b/libstdc++-v3/doc/xml/faq.xml @@ -551,31 +551,34 @@ <constant>_XOPEN_SOURCE</constant> and <constant>_GNU_SOURCE</constant> are always defined? </para> </question> + <answer xml:id="a-predefined"> - <para>On Solaris, <command>g++</command> (but not <command>gcc</command>) + <para>On GNU/Linux, <command>g++</command> (but not <command>gcc</command>) always defines the preprocessor macro - <constant>_XOPEN_SOURCE</constant>. On GNU/Linux, the same happens - with <constant>_GNU_SOURCE</constant>. (This is not an exhaustive list; + <constant>_GNU_SOURCE</constant>. On Solaris, the same happens + with <constant>_XOPEN_SOURCE</constant>. (This is not an exhaustive list; other macros and other platforms are also affected.) </para> <para>These macros are typically used in C library headers, guarding new - versions of functions from their older versions. The C++98 standard - library includes the C standard library, but it requires the C90 - version, which for backwards-compatibility reasons is often not the - default for many vendors. + versions of functions from their older versions. Historically, + libstdc++ needed these macros to ensure that the headers provided by + the C library declared all the functions that libstdc++ relies on. + For example, functions for working with <code>long long</code>, + such as <function>strtoll</function>, are needed by libstdc++ but + were not part of the C90 standard. </para> - <para>More to the point, the C++ standard requires behavior which is only - available on certain platforms after certain symbols are defined. + <para>Additionally the C++ standard requires behavior which is only + available on certain platforms after certain macros are defined. Usually the issue involves I/O-related typedefs. In order to - ensure correctness, the compiler simply predefines those symbols. + ensure correctness, the compiler simply predefines those macros. </para> - <para>Note that it's not enough to <literal>#define</literal> them only when the library is - being built (during installation). Since we don't have an 'export' - keyword, much of the library exists as headers, which means that - the symbols must also be defined as your programs are parsed and + <para>Note that it's not enough to <literal>#define</literal> them + only when the library is being built (during installation). + Much of the library exists as headers, which means that + the macros must also be defined as your programs are parsed and compiled. </para> - <para>To see which symbols are defined, look for + <para>To see which macros are defined, look for <varname>CPLUSPLUS_CPP_SPEC</varname> in the gcc config headers for your target (and try changing them to see what happens when building complicated code). You can also run |