Miscellaneous Preprocessing Directives

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Miscellaneous preprocessing directives

There are three preprocessing directives that are not very useful, but are mentioned for completeness.

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.
The ANSI standard specifies that the #pragma directive has an arbitrary, implementation-defined effect. In the GNU C preprocessor, #pragma directives are not used, except for #pragma once (see Once-only include files). However, they are left in the preprocessor output, so they are available to the compilation pass.
The #ident directive is supported for compatibility with certain other systems. It is followed by a line of text. On some systems, the text is copied into a special place in the object file; on most systems, the text is ignored and this directive has no effect. Typically, #ident is only used in header files supplied with those systems where it is meaningful.