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README

cppp is a partial preprocessor for C/C++ code. It implements enough of
the C preprocessor to identify the preprocessor statements #if,
#ifdef, et al., and applies their effects for the identifiers that are
specified on the command-line. Preprocessor statements using
identifiers not specified by the user are passed through without
modification. And no other part of the input is altered.

Thus, if cppp is invoked with the options -DFOO -UBAR, then FOO will
be defined, BAR will be undefined, and all other identifiers will be
unaffected. A section guarded by "#ifdef FOO" will be included in the
output without the guard statements, and a section guarded by "#ifdef
BAR" will be entirely dropped from the output.

In addition to removing #ifdef statements, cppp understands complex
expressions in #if statements that involve the defined operator, and
will apply the effects even partially. Thus for example the
preprocessor statement "#if (defined(BAR) || defined(BAZ))" would be
edited to read "#if defined(BAZ)" in the output.

cppp can also handle simple numeric tests. For example, giving cppp
the option -D_LIB_VERSION=5 would cause the preprocessor statement
"#if _LIB_VERSION >= 4" to succeed. Note, however, that cppp is
limited to dealing with integer constants. The preprocessor statement
"#if _LIB_VERSION >= _HEADER_FILE_VERSION" would simply get rewritten
to "#if 5 >= _HEADER_FILE_VERSION", even if a definition for
_HEADER_FILE_VERSION was provided in the same file.

cppp assumes that its input files are valid C code. It will only do as
much error-checking on the input as it needs to in order to identify
the preprocessor directives. When cppp does find an error in an input
file, it will output a message and do its best to skip over the
invalid construct and continue parsing. Note that it is possible
(though rare) for compiler extensions to be reported as errors -- e.g.
some compilers reportedly allow #elif directives in a sequence
beginning with #ifdef.

The general purpose of this program is to deal with legacy code bases
where #ifdefs have crept in and infiltrated the source. This sort of
thing usually happens while code is transitioning from one platform to
another, in the misguided belief that this will ease the process
rather than make it more confusing. Then, if the time finally comes
that the ifdefs can be safely removed, it often doesn't get done
because it's too tedious and error-prone to do it manually.

I originally wrote cppp as an interesting exercise, but I later wound
up using it to save myself from just such a tedious task. Sadly,
because the code was not originally intended to be seen by anyone, it
is not well-written and in some places it is downright opaque. I have
attempted to ameliorate this by cleaning up some of the most egregious
inconsistencies and adding a handful of comments, but the central
functions are quite large and not easily explicated (particularly with
all the intervening years since the time of its original creation). I
offer it solely as a tool that might save you from a mind-numbing
chore, but not as something to learn from.

That said, cppp is free software, so you can redistribute it and/or
modify it under the terms of the GNU General Public License as
published by the Free Software Foundation; either version 2 of the
License, or (at your option) any later version. This program is
distributed in the hope that it will be useful, but WITHOUT ANY
WARRANTY; without even the implied warranty of MERCHANTABILITY or
FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License,
included in this distribution in the file COPYING, for more details.

cppp website: <http://www.muppetlabs.com/~breadbox/software/cppp.html>.

Brian Raiter
breadbox@muppetlabs.com
June, 2011

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A program for automatically resolving #ifdef statements in C code.

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