Each file is associated with a signature, which is a number or string that changes if the file has changed. Makepp compares signatures to see whether it needs to rebuild anything. The default signature for files is the file's modification time, unless you're executing a C/C++ compilation command, in which case the default signature is a cryptographic checksum on the file's contents, ignoring comments and whitespace. If you want, you can switch to a different method, or you can define your own signature functions.
In addition to the file's signature, it is also possible to
control how makepp compares these signature values. For
example, the exact_match
method requires that file
signatures be exactly the same as on the last build, whereas the
target_newer
method only requires that all dependencies
be older than the target.
If makepp is building a file, and you don't think it
should be, you might want to check the build log
(.makepp_log
). Makepp writes an explanation of
what it thought each file depended on, and why it chose to
rebuild.
At present, there are four signature checking methods included in
makepp. These methods may be selected on a rule-by-rule
basis by using the :signature
modifier, or on a makefile-by-makefile basis by using the signature
statement, or for all makefiles at once by using the -m
or
--signature-method
command line option. Usually,
makepp's default is fine.
This method uses the modification dates on the file as signatures. It rebuilds the targets unless all of the following conditions are true:
Makepp stores all the signature information and the build command from the last build, so that it can do these comparisons.
This is makepp's default algorithm unless it is trying to rebuild a makefile or compile C/C++ code. This is a highly reliable way of ensuring correct builds, and is almost always what you want. However, it does have a few side effects that may be surprising:
.makepp
, or don't copy it when you copy everything
else), then a rebuild is triggered.
lex
program won't compile on linux.
Rebuilds only if the target is newer than all of its dependencies. The dependencies may change their time stamp, but as long as they are older than the target, the target is not rebuilt. The target is also not rebuilt even if the command or the architecture has changed. (This is the signature method that the traditional make uses.)
This is makepp's default algorithm if it is trying to build the makefile before reading it in. (It loads the makefile and checks for a rule within the makefile to rebuild itself, and if such a rule is present and the makefile needs rebuilding, it is rebuild and then reread.) This is because it is common to modify a makefile using commands that are not under the control of makepp, e.g., running a configure procedure. Thus makepp doesn't insist that the last modification to the makefile be made by itself.
Using target_newer
compared to
exact_match
has the following disadvantages:
This is the same as exact_match
, except that instead
of using the file date as the signature, an MD5 checksum of the
files contents is used. This means that if you change the date on
the file but don't change its contents, makepp won't try to
rebuild anything that depends on it.
This is particularly useful if you have some file which is often regenerated during the build process that other files depend on, but which usually doesn't change. This will not work properly for libraries or other files which have compilation times contained in the file itself.
For C/C++ source code, you should use
c_compilation_md5
instead.
This is the same as md5
, except that signatures for
files which look like C or C++ source files are computed by an MD5
checksum of the file, ignoring comments and whitespace.
(Technically, comments are replaced by a single space, and multiple
whitespace is collapsed to a single space, before computing the MD5
checksum.) Ordinary file times are still used for signatures for
object files, and any other files that don't have an extension
typical of a C or C++ source file. (A file is considered to be
source code if it has an extension of c
,
h
, cc
, hh
, cxx
,
hxx
, hpp
, cpp
,
h++
, c++
, moc
, or upper case
versions of these.) If you use this, you can reindent your code or
add or change comments without triggering a rebuild.
This method is particularly useful for the following situations:
.cxx
and a
.h
file:
%.h %.cxx: %.qtdlg $(HLIB)/Qt/qt_dialog_generator $(HLIB)/Qt/qt_dialog_generator $(input)
However, most of the time when the input file changes, the
resulting .h
file contents are unchanged (except
for a comment about the build time written by the preprocessor),
although its date will change. This could trigger unnecessary
rebuilds of many modules without this kind of cryptographic
signature checking.
This is the default signature method for C or C++ compilation.
It overrides any default specified with the -m
or
--signature-method
command line option, but is
overridden by any signature method specified by the signature
statement or the :signature
rule modifier. Makepp determines that you are doing a
C/C++ compilation if it recognizes your command line as an
invocation of a C/C++ compiler (see on scanning).
You can, if you want, define your own methods for calculating
file signatures and comparing them. You will need to write a perl
module to do this. Have a look at the comments in
Signature.pm
in the distribution, and also at the
existing signature algorithms in Signature/*.pm
for
details.