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/GCC 7 CPP

8 Other Directives

The ‘#ident’ directive takes one argument, a string constant. On some systems, that string constant is copied into a special segment of the object file. On other systems, the directive is ignored. The ‘#sccs’ directive is a synonym for ‘#ident’.

These directives are not part of the C standard, but they are not official GNU extensions either. What historical information we have been able to find, suggests they originated with System V.

The null directive consists of a ‘#’ followed by a newline, with only whitespace (including comments) in between. A null directive is understood as a preprocessing directive but has no effect on the preprocessor output. The primary significance of the existence of the null directive is that an input line consisting of just a ‘#’ will produce no output, rather than a line of output containing just a ‘#’. Supposedly some old C programs contain such lines.

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https://gcc.gnu.org/onlinedocs/gcc-7.1.0/cpp/Other-Directives.html