The
unifdef utility selectively processes conditional
cpp(1) 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
unifdef 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
unifdef 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(1) directives from a file. It uses
unifdef -s and
cpp -dM to get lists of all the controlling symbols and their definitions (or lack thereof), then invokes
unifdef with appropriate arguments to process the file.
Available options:
-Dsym[=val]
Specify that a symbol is defined, and optionally specify what value to give it for the purpose of handling #if and #elif directives.
-Usym
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 unifdef is complemented, i.e., the lines that would have been removed or blanked are retained and vice versa.
-e
Because unifdef 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 unifdef 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 “#if 0” 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.
-o output
The argument given is the name of an output file to be used instead of the standard output. This file can be the same as the input file.
-s
Instead of processing the input file as usual, this option causes
unifdef to produce a list of symbols that appear in expressions that
unifdef understands. It is useful in conjunction with the
-dM option of
cpp(1) for creating
unifdef command lines.
-t
Disables parsing for C comments and line continuations, which is useful for plain text.
-iUsym
Ignore #ifdefs. If your C code uses #ifdefs to delimit non-C lines, such as comments or code which is under construction, then you must tell unifdef which symbols are used for that purpose so that it will not try to parse comments and line continuations inside those #ifdefs. One specifies ignored symbols with -iDsym[=val] and -iUsym similar to -Dsym[=val] and -Usym above.
-Ipath
Specifies to
unifdefall an additional place to look for
#include files. This option is ignored by
unifdef for compatibility with
cpp(1) and to simplify the implementation of
unifdefall.
The
unifdef utility copies its output to
stdout and will take its input from
stdin if no
file argument is given.
The
unifdef utility works nicely with the
-Dsym option of
diff(1).