So far we've seen how SCons handles one-time builds. But one of the main functions of a build tool like SCons is to rebuild only the necessary things when source files change--or, put another way, SCons should not waste time rebuilding things that have already been built. You can see this at work simply by re-invoking SCons after building our simple hello example:
% scons -Q
cc -o hello.o -c hello.c
cc -o hello hello.o
% scons -Q
scons: `.' is up to date.
The second time it is executed, SCons realizes that the hello program is up-to-date with respect to the current hello.c source file, and avoids rebuilding it. You can see this more clearly by naming the hello program explicitly on the command line:
% scons -Q hello
cc -o hello.o -c hello.c
cc -o hello hello.o
% scons -Q hello
scons: `hello' is up to date.
Note that SCons reports "...is up to date" only for target files named explicitly on the command line, to avoid cluttering the output.
Decider FunctionAnother aspect of avoiding unnecessary rebuilds is the fundamental build tool behavior of rebuilding things when an input file changes, so that the built software is up to date. By default, SCons keeps track of this through an MD5 signature, or checksum, of the contents of each file, although you can easily configure SCons to use the modification times (or time stamps) instead. You can even specify your own Python function for deciding if an input file has changed.
By default, SCons keeps track of whether a file has changed based on an MD5 checksum of the file's contents, not the file's modification time. This means that you may be surprised by the default SCons behavior if you are used to the Make convention of forcing a rebuild by updating the file's modification time (using the touch command, for example):
% scons -Q hello
cc -o hello.o -c hello.c
cc -o hello hello.o
% touch hello.c
% scons -Q hello
scons: `hello' is up to date.
Even though the file's modification time has changed, SCons realizes that the contents of the hello.c file have not changed, and therefore that the hello program need not be rebuilt. This avoids unnecessary rebuilds when, for example, someone rewrites the contents of a file without making a change. But if the contents of the file really do change, then SCons detects the change and rebuilds the program as required:
% scons -Q hello
cc -o hello.o -c hello.c
cc -o hello hello.o
% edit hello.c
[CHANGE THE CONTENTS OF hello.c]
% scons -Q hello
cc -o hello.o -c hello.c
cc -o hello hello.o
Note that you can, if you wish,
specify this default behavior
(MD5 signatures) explicitly
using the Decider function as follows:
Program('hello.c')
Decider('MD5')
You can also use the string 'content'
as a synonym for 'MD5'
when calling the Decider function.
Using MD5 Signatures to decide if an input file has changed has one surprising benefit: if a source file has been changed in such a way that the contents of the rebuilt target file(s) will be exactly the same as the last time the file was built, then any "downstream" target files that depend on the rebuilt-but-not-changed target file actually need not be rebuilt.
So if, for example, a user were to only change a comment in a hello.c file, then the rebuilt hello.o file would be exactly the same as the one previously built (assuming the compiler doesn't put any build-specific information in the object file). SCons would then realize that it would not need to rebuild the hello program as follows:
% scons -Q hello
cc -o hello.o -c hello.c
cc -o hello hello.o
% edit hello.c
[CHANGE A COMMENT IN hello.c]
% scons -Q hello
cc -o hello.o -c hello.c
scons: `hello' is up to date.
In essence, SCons "short-circuits" any dependent builds when it realizes that a target file has been rebuilt to exactly the same file as the last build. This does take some extra processing time to read the contents of the target (hello.o) file, but often saves time when the rebuild that was avoided would have been time-consuming and expensive.
If you prefer, you can configure SCons to use the modification time of a file, not the file contents, when deciding if a target needs to be rebuilt. SCons gives you two ways to use time stamps to decide if an input file has changed since the last time a target has been built.
The most familiar way to use time stamps
is the way Make does:
that is, have SCons decide
and target must be rebuilt if
if a source file's modification time is
newer
than the target file.
To do this, call the Decider
function as follows:
Program('hello.c')
Decider('timestamp-newer')
This makes SCons act like Make when a file's modification time is updated (using the touch command, for example):
% scons -Q hello
cc -o hello.o -c hello.c
cc -o hello hello.o
% touch hello.c
% scons -Q hello
cc -o hello.o -c hello.c
cc -o hello hello.o
And, in fact, because this behavior is the same
as the behavior of Make,
you can also use the string 'make'
as a synonym for 'timestamp-newer'
when calling the Decider function:
Program('hello.c')
Decider('make')
One drawback to using times stamps exactly like Make is that if an input file's modification time suddenly becomes older than a target file, the target file will not be rebuilt. This can happen if an old copy of a source file is restored from a backup archive, for example. The contents of the restored file will likely be different than they were the last time a dependent target was built, but the target won't be rebuilt because the modification time of the source file is not newer than the target.
Because SCons actually stores information
about the source files' time stamps whenever a target is built,
it can handle this situation by checking for
an exact match of the source file time stamp,
instead of just whether or not the source file
is newer than the target file.
To do this, specify the argument
'timestamp-match'
when calling the Decider function:
Program('hello.c')
Decider('timestamp-match')
When configured this way, SCons will rebuild a target whenever a source file's modification time has changed. So if we use the touch -t option to change the modification time of hello.c to an old date (January 1, 1989), SCons will still rebuild the target file:
% scons -Q hello
cc -o hello.o -c hello.c
cc -o hello hello.o
% touch -t 198901010000 hello.c
% scons -Q hello
cc -o hello.o -c hello.c
scons: `hello' is up to date.
In general, the only reason to prefer timestamp-newer instead of timestamp-match, would be if you have some specific reason to require this Make-like behavior of not rebuilding a target when an otherwise-modified source file is older.
As a performance enhancement,
SCons provides a way to use
MD5 checksums of file contents
but to only read the contents
whenever the file's timestamp has changed.
To do this, call the Decider
function with 'MD5-timestamp'
argument as follows:
Program('hello.c')
Decider('MD5-timestamp')
So configured, SCons will still behave like it does when using Decider('MD5'):
% scons -Q hello
cc -o hello.o -c hello.c
cc -o hello hello.o
% touch hello.c
% scons -Q hello
scons: `hello' is up to date.
% edit hello.c
[CHANGE THE CONTENTS OF hello.c]
% scons -Q hello
scons: `hello' is up to date.
However, the second call to SCons in the above output, when the build is up-to-date, will have been performed by simply looking at the modification time of the hello.c file, not by opening it and performing an MD5 checksum calcuation on its contents. This can significantly speed up many up-to-date builds.
The only drawback to using Decider('MD5-timestamp') is that SCons will not rebuild a target file if a source file was modified within one second of the last time SCons built the file. While most developers are programming, this isn't a problem in practice, since it's unlikely that someone will have built and then thought quickly enought to make a substantive change to a source file within one second. Certain build scripts or continuous integration tools may, however, rely on the ability to applying changes to files automatically and then rebuild as quickly as possible, in which case use of Decider('MD5-timestamp') may not be appropriate.
Decider Function
The different string values that we've passed to
the Decider function are essentially used by SCons
to pick one of several specific internal functions
that implement various ways of deciding if a dependency
(usually a source file)
has changed since a target file has been built.
As it turns out,
you can also supply your own function
to decide if a dependency has changed.
For example, suppose we have an input file
that contains a lot of data,
in some specific regular format,
that is used to rebuild a lot of different target files,
but each target file really only depends on
one particular section of the input file.
We'd like to have each target file depend on
only its section of the input file.
However, since the input file may contain a lot of data,
we only want to open the input file if its timestamp has changed.
This could done with a custom
Decider function that might look something like this:
Program('hello.c')
def decide_if_changed(dependency, target, prev_ni):
if self.get_timestamp() != prev_ni.timestamp:
dep = str(dependency)
tgt = str(target)
if specific_part_of_file_has_changed(dep, tgt):
return True
return False
Decider(decide_if_changed)
Note that in the function definition,
the dependency
(input file) is the first argument,
and then the target.
Both of these are passed to the functions as
SCons Node objects,
which we convert to strings using the Python
str().
The third argument, prev_ni,
is an object that holds the
signature or timestamp information
that was recorded about the dependency
the last time the target was built.
Note that ignoring some of the arguments
in your custom Decider function
is a perfectly normal thing to do,
if they don't impact the way you want to
decide if the dependency file has changed.
The previous examples have all demonstrated calling
the global Decider function
to configure all dependency decisions that SCons makes.
Sometimes, however, you want to be able to configure
different decision-making for different targets.
When that's necessary, you can use the
env.Decider
method to affect only the configuration
decisions for targets built with a
specific construction environment.
For example, if we arbitrarily want to build one program using MD5 checkums and another use file modification times from the same source we might configure it this way:
env1 = Environment(CPPPATH = ['.'])
env2 = env1.Clone()
env2.Decider('timestamp-match')
env1.Program('prog-MD5', 'program1.c')
env2.Program('prog-timestamp', 'program2.c')
If both of the programs include the same inc.h file, then updating the modification time of inc.h (using the touch command) will cause only prog-timestamp to be rebuilt:
% scons -Q
cc -o program1.o -c -I. program1.c
cc -o prog-MD5 program1.o
cc -o program2.o -c -I. program2.c
cc -o prog-timestamp program2.o
% touch inc.h
% scons -Q
cc -o program2.o -c -I. program2.c
cc -o prog-timestamp program2.o