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UNIFDEF(1) General Commands Manual UNIFDEF(1)

unifdefremove preprocessor conditionals from code

unifdef [-BbcdehKkmnSstV] [-[i]Dsym[=val]] [-[i]Usym] [-f defile] [-M backext] [-o outfile] [-x 0 | 1 | 2] file ...

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, using macros specified in -D and -U command line options or in -f definition files. A directive is processed if the macro specifications are sufficient to provide a definite value for its control expression. If the result is false, the directive and the following lines under its control are removed. If the result is true, only the directive is removed. An #ifdef or #ifndef directive is passed through unchanged if its controlling macro is not specified. Any #if or #elif control expression that has an unknown value or that unifdef cannot parse is passed through unchanged. By default, unifdef ignores #if and #elif lines with constant expressions; it can be told to process them by specifying the -k flag on the command line.

It understands a commonly-used subset of the expression syntax for #if and #elif lines: integer constants, integer values of macros defined on the command line, the () operator, the operators !, <, >, <=, >=, ==, !=, &&, ||, and parenthesized expressions. A kind of “short circuit” evaluation is used for the && operator: if either operand is definitely false then the result is false, even if the value of the other operand is unknown. Similarly, if either operand of || is definitely true then the result is true.

When evaluating an expression, unifdef does not expand macros first. The value of a macro must be a simple number, not an expression. A limited form of indirection is allowed, where one macro's value is the name of another.

In most cases, unifdef does not distinguish between object-like macros (without arguments) and function-like macros (with arguments). A function-like macro invocation can appear in #if and #elif control expressions. If the macro is not explicitly defined, or is defined with the -D flag on the command-line, or with #define in a -f definition file, its arguments are ignored. If a macro is explicitly undefined on the command line with the -U flag, or with #undef in a -f definition file, it may not have any arguments since this leads to a syntax error.

The unifdef utility 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 for it to handle.

The options are as follows:

Compress blank lines around a deleted section. Mutually exclusive with the -b option.

Replace removed lines with blank lines instead of deleting them. Mutually exclusive with the -B option.

Complement. That is, lines that would have been removed or blanked are retained and vice versa.

Specify that a macro is defined (-D) to a given value, to 1 if no value is given, or undefined (-U).

If the same macro appears in more than one argument, the last occurrence dominates.

Turn on printing of debugging messages.

By default, unifdef will report an error if it needs to remove a preprocessor directive that spans more than one line, for example, if it has a multi-line comment hanging off its right hand end. The -e flag makes it ignore the line instead.

The file defile contains #define and #undef preprocessor directives, which have the same effect as the corresponding -D and -U command line arguments. Multiple -f arguments can be given and mixed with -D and -U arguments; later options override earlier ones.

Each directive must be on a single line. Object-like macro definitions (without arguments) are set to the given value. Function-like macro definitions (with arguments) are treated as if they are set to 1.

Ignore #ifdefs. If C code uses #ifdefs to delimit non-C lines, such as comments or code which is under construction, this tells unifdef which symbols are used for that purpose so that it will not try to parse comments and line continuations inside those #ifdefs.

If the same macro appears in more than one argument, the last occurrence dominates.

Print help.

Always treat the result of && and || operators as unknown if either operand is unknown, instead of short-circuiting when unknown operands can't affect the result. This option is for compatibility with older versions of unifdef.

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.

Modify input files in place, and keep backups of the original files by appending the backext to the input filenames. If a zero length backext is given, no backup will be saved.

Modify one or more input files in place.

Add #line directives to the output following any deleted lines, so that errors produced when compiling the output file correspond to line numbers in the input file.

Write output to the file outfile instead of the standard output when processing a single file.

Like the -s option, but the nesting depth of each macro is also printed. This is useful for working out the number of possible combinations of interdependent defined/undefined macros.

Instead of processing an input file as usual, this option causes unifdef to produce a list of macros that are used in preprocessor directive controlling expressions.

Disables parsing for C strings, comments, and line continuations, which is useful for plain text. This is a blanket version of the -iD and -iU flags.

Print version details.

0 | |
Set exit status mode to zero, one, or two. See the EXIT STATUS section below for details.

The unifdef utility takes its input from if there are no file arguments. The -m or -M options must be used if there are multiple input files. A dash (-) specifies input from stdin or output to stdout.

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

In normal usage the unifdef utility's exit status depends on the mode set using the -x option.

If the exit mode is zero (the default) then unifdef exits with status 0 if the output is an exact copy of the input, or with status 1 if the output differs.

If the exit mode is one, unifdef exits with status 1 if the output is unmodified or 0 if it differs.

If the exit mode is two, unifdef exits with status zero in both cases.

In all exit modes, unifdef exits with status 2 if there is an error.

The exit status is 0 if the -h or -V command line options are given.

cpp(1), diff(1)

The unifdef home page is

The unifdef command appeared in 4.1cBSD. ANSI C support was added in FreeBSD 4.7.

The original implementation was written by Dave Yost <>.
Tony Finch <> rewrote it to support ANSI C.

Expression evaluation is very limited.

Handling one line at a time means preprocessor directives 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 macros with different definitions at different points in the source file.

The text-mode and ignore functionality does not correspond to modern cpp(1) behaviour.

August 13, 2023 OpenBSD-current