From K&R
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Use
#as replacement text
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And Use
##to concatenate
The preprocessor operator##provides a way to concatenate actual arguments during macro expansion.If a parameter in the replacement text is adjacent to athe parameter is replaced by the actual argument, the##and surrounding white space are removed, and the result is re-scanned.For example, the macro paste concatenates its two arguments:
#define paste(front, back) front ## back
Nonetheless, macros are valuable.One practical example comes from <stdio.h>, in which getchar and putchar are often defined as macros to avoid the e run-time overhead of a function call per character processed.The functions in <ctype.h> are also usually implemented as macros.