The utility selectively processes conditional cpp() directives. It removes from a file both the directives and any additional text that they specify should be removed, while otherwise leaving the file alone.

The utility acts on #if , #ifdef , #ifndef , #elif , #else , and #endif lines, and it understands only the commonly-used subset of the expression syntax for #if and #elif lines. It handles integer values of symbols defined on the command line, the defined operator applied to symbols defined or undefined on the command line, the operators ! , < , > , <= , >= , == , != , && , || , and parenthesized expressions. Anything that it does not understand is passed through unharmed. It only processes #ifdef and #ifndef directives if the symbol is specified on the command line, otherwise they are also passed through unchanged. By default, it ignores #if and #elif lines with constant expressions, or they may be processed by specifying the -k flag on the command line.

The utility also understands just enough about C to know when one of the directives is inactive because it is inside a comment, or affected by a backslash-continued line. It spots unusually-formatted preprocessor directives and knows when the layout is too odd to handle.

A script called unifdefall can be used to remove all conditional cpp() directives from a file. It uses -s and cpp -dM to get lists of all the controlling symbols and their definitions (or lack thereof), then invokes with appropriate arguments to process the file.

Available options:

-D Ns sym Ns [= Ns val]

Specify that a symbol is defined, and optionally specify what value to give it for the purpose of handling #if and #elif directives.

-U Ns sym

Specify that a symbol is undefined. If the same symbol appears in more than one argument, the last occurrence dominates.

-c

If the -c flag is specified, then the operation of is complemented, i.e., the lines that would have been removed or blanked are retained and vice versa.

-e

Because processes its input one line at a time, it cannot remove preprocessor directives that span more than one line. The most common example of this is a directive with a multi-line comment hanging off its right hand end. By default, if has to process such a directive, it will complain that the line is too obfuscated. The -e option changes the behaviour so that, where possible, such lines are left unprocessed instead of reporting an error.

-k

Process #if and #elif lines with constant expressions. By default, sections controlled by such lines are passed through unchanged because they typically start and are used as a kind of comment to sketch out future or past development. It would be rude to strip them out, just as it would be for normal comments.

-l

Replace removed lines with blank lines instead of deleting them.

-s

Instead of processing the input file as usual, this option causes to produce a list of symbols that appear in expressions that understands. It is useful in conjunction with the -dM option of cpp() for creating command lines.

-t

Disables parsing for C comments and line continuations, which is useful for plain text.

-iD Ns sym Ns [= Ns val]

-iU Ns sym

Ignore #ifdef Ns s . If your C code uses #ifdef Ns s to delimit non-C lines, such as comments or code which is under construction, then you must tell which symbols are used for that purpose so that it will not try to parse comments and line continuations inside those #ifdef Ns s . One specifies ignored symbols with -iD Ns sym Ns Oo = Ns val Oc and -iU Ns sym similar to -D Ns sym Ns [= Ns val] and -U Ns sym above.

-I Ns path

Specifies to unifdefall an additional place to look for #include files. This option is ignored by for compatibility with cpp() and to simplify the implementation of unifdefall .

The utility copies its output to and will take its input from if no file argument is given.

The utility works nicely with the -D Ns sym option of diff(1) .

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Too many levels of nesting.

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Inappropriate #elif , #else or #endif .

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Obfuscated preprocessor control line.

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Premature EOF (with the line number of the most recent unterminated #if ) .

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EOF in comment.

The utility exits 0 if the output is an exact copy of the input, 1 if not, and 2 if in trouble.

Expression evaluation is very limited.

Preprocessor control lines split across more than one physical line (because of comments or backslash-newline) cannot be handled in every situation.

Trigraphs are not recognized.

There is no support for symbols with different definitions at different points in the source file.

The text-mode and ignore functionality does not correspond to modern cpp() behaviour.

The command appeared in ANSI~C support was added in