#pragma GCC system_header用法

在看公司公共库的头文件中发现了：#pragma GCC system_header一行，以前没有见过这种用法，在网上查了一下，解释如下：

从#pragma GCC system_header直到文件结束之间的代码会被编译器视为系统头文件之中的代码。系统头文件中的代码往往不能完全遵循C标准, 所以头文件之中的警告信息往往不显示。(除非用 #warning显式指明)。

可以查看gcc参考手册：http://gcc.gnu.org/onlinedocs/cpp/System-Headers.html#System-Headers

The header files declaring interfaces to the operating system and runtime libraries often cannot be written in strictly conforming C. Therefore, GCC gives code found in system headersspecial treatment. All warnings, other than those generated by ‘#warning’ (see Diagnostics), are suppressed while GCC is processing a system header. Macros defined in a system header are immune to a few warnings wherever they are expanded. This immunity is granted on an ad-hoc basis, when we find that a warning generates lots of false positives because of code in macros defined in system headers.

Normally, only the headers found in specific directories are considered system headers. These directories are determined when GCC is compiled. There are, however, two ways to make normal headers into system headers:

• Header files found in directories added to the search path with the -isystem and -idirafter command-line options are treated as system headers for the purposes of diagnostics.
• There is also a directive, #pragma GCC system_header, which tells GCC to consider the rest of the current include file a system header, no matter where it was found. Code that comes before the ‘#pragma’ in the file is not affected. #pragma GCC system_header has no effect in the primary source file.