GCC扩展符（#，##）

先来说说这两个扩展符的各自用处吧，“#”是将宏字符串化（Stringification），“##”是将##左右两边的标签组合在一起（token pasting or token concatenation），下面从两个简单例子着手：

    Macro arguments are completely macro-expanded before they are substituted into a macro body, unless they are stringified or pasted with other tokens. After substitution, the en-tire macrobody, including the substituted arguments, is scanned again for macros to be expanded.

【参考文献】
1. The C Preprocessor （For gcc version 4.7.2）