Other Directives (The C Preprocessor)

## 9 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.