NAME

       deheader - report which includes in C or C++ compiles can be removed

SYNOPSIS

       deheader [-h] [-m command] [-b builddir] [-i pattern] [-q] [-r] [-v] [-x pattern] [-V]
                [file-or-dir]

DESCRIPTION

       This tool takes a list of C or C++ sourcefiles and generates a report on which #includes
       can be omitted from them; also, what standard inclusions may be required for portability.
       The test, for each foo.c or foo.cc or foo.cpp, is simply whether "rm foo.o; make foo.o"
       returns a zero status (but the build command may be overridden).

       Exception: Under cmake, foo.o is a phony target. Therefore, when a "CMakeList.txt" is
       detected, "make clean" is done rather than "rm foo.o".

       Optionally, with the -r switch, the unneeded headers are removed from the sourcefiles.
       Don't use this option unless you have your sourcefiles safely under version control and
       can revert!

       If a sourcefile argument is a directory, the report is generated on all source files
       beneath it. Subdirectories beginning with a dot are assumed to be repository directories
       for version-control systems and ignored. If no arguments are given, the program runs as if
       the name of the current directory had been passed to it.

       Inclusions within the scope of #if/#ifdef/#else/#endif directives are left alone, because
       trying to reason about potential combinations of -D and U options would be too complicated
       and prone to weird errors. One exception: headers protected only by S_SPLINT_S, the
       conditional for blocking scanning by the static analysis tool splint(1), are scanned
       normally.

       The tool will also emit warnings about duplicate inclusions, and inclusions required for
       portability but not present.

       It is recommended that you arrange to compile with options that will stop the compiler on
       warnings when using this tool; otherwise it will report headers that only declare
       prototypes and return types (and thus throw only warnings) as being not required. Under
       gcc the compiler options to accomplish this are -Werror -Wfatal-errors. If your makefile
       follows normal conventions, running with -m "make CFLAGS='-Werror -Wfatal-errors' %s" may
       do the right thing; you can check this by running with -v -v -v to see what compilation
       commands are actually emitted.

       On each test compile, the original sourcefile is moved to a name with an .orig suffix and
       restored on interrupt or after processing with its original timestamp, unless the -r
       option was given and headers removed.

       If the -b option is given, it tells the program that generated .o files live in a file
       tree parallel to the source tree but rooted at the specified argument. If the argument is
       a relative path, it is interpreted relative to the directory in which deheader is run.

       If the first test compilation from the top-level directory fails, deheader descends into
       the subdirectory of the source file and retries compiling inside there.

       At verbosity level 0, only messages indicating removable headers are issued. At verbosity
       1, test compilations are timed and progress indicated with a twirling-baton prompt. At
       verbosity level 2, you get verbose progress messages on the analysis. At verbosity level
       3, you see the output from the make and compilation commands.

       If the -q (--quiet) option flag was not set, the last line of the output will be a
       statistical summary.

       Running deheader will leave a lot of binaries in your directory that were compiled in ways
       possibly not invoked by your normal build process. Running "make clean" afterwards (or the
       equivalent under whatever build system you are using) is strongly recommended.

OPTIONS

       -h
           Display some help and exit.

       -m
           Set the build command used for test compiles. Defaults to 'make %s'. A %s in the build
           command is replaced with the make target. Targetless builders such as meson can be
           supported with, e.g. "-m 'ninja -C build'"

       -b
           Set the build directory for object files.

       -i
           Set a pattern for includes to be ignored. Takes a Python regular expression.

       -q
           Suppress statistical summary.

       -r
           Remove header inclusions from sourcefiles where they are not required.

       -v
           Set verbosity.

       -x
           Exclude files with names matching the specified Python regexp.

       -V
           Show version of program and exit.

RETURN VALUES

       Returns 1 if unneeded includes were found, 0 otherwise. Thus, you can use it for
       pre-release sanity checking in Makefile.

BUGS

       Very rarely, test-compiling after running with -r may show that this tool removed some
       headers that are actually required for your build. This can happen because deheader
       doesn't know about all the strange things your build system gets up to, and the problem of
       analyzing your build to understand them would be Turing-complete. Simply revert the
       altered files and continue.

       Due to minor variations in system headers, it is possible your program may not port
       correctly to other Unix variants after being deheadered. This is normally not a problem
       with the portion of the API specified by POSIX and ANSI C, but may be for headers that are
       not standardized or only weakly standardized. The sockets API (sys/select.h,
       sys/sockets.h, and friends such as sys/types.h and sys.stat.h) is perhaps the most serious
       trouble spot.  deheader has an internal table of rules that heads off the most common
       problems by suppressing deletion of headers that are required for portability, but your
       mileage may vary.

       The dependency scanner does not ignore the text of comments. This, e.g, a reference to
       "log10" in a comment will produce a spurious warning that <math.h> is required for
       portability.

       Sufficiently perverse C++ can silently invalidate the brute-force algorithm this tool
       uses. Example: if an overloaded function has different overloads from two different files,
       removing one may expose the other, changing runtime semantics without a compile-time
       warning. Similarly, removing a later file containing a template specialization may lead to
       undefined behavior from a template defined in an earlier file. Use this with caution near
       such features, and test carefully.

AUTHOR

       Eric S. Raymond <esr@snark.thyrsus.com>; (home page at http://www.catb.org/~esr/).