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
%     [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.

% scons -Q hello
cc -o hello.o -c hello.c
cc -o hello hello.o
%   [CHANGE A COMMENT IN hello.c]
% scons -Q hello
cc -o hello.o -c hello.c
scons: `hello' is up to date.

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 that a target must be rebuilt if a source file's modification time is newer than the target file. To do this, call the Decider function as follows:

Object('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.o
cc -o hello.o -c hello.c
% touch hello.c
% scons -Q hello.o
cc -o hello.o -c hello.c

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:

Object('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:

Object('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.o
cc -o hello.o -c hello.c
% touch -t 198901010000 hello.c
% scons -Q hello.o
cc -o hello.o -c hello.c

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 read those contents only when 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
cc -o hello.o -c hello.c
cc -o hello hello.o
      

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 enough 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 apply changes to files automatically and then rebuild as quickly as possible, in which case use of Decider('MD5-timestamp') may not be appropriate.

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 want to open the input file only if its timestamp has changed. This could be done with a custom Decider function that might look something like this:

Program('hello.c')
def decide_if_changed(dependency, target, prev_ni, repo_node=None):
    if dependency.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. A prev_ni object can hold different information, depending on the type of thing that the dependency argument represents. For normal files, the prev_ni object has the following attributes:

These attributes may not be present at the time of the first run. Without any prior build, no targets have been created and no .sconsign DB file exists yet. So you should always check whether the prev_ni attribute in question is available (use the Python hasattr method or a try-except block).

The fourth argument repo_node is the Node to use if it is not None when comparing BuildInfo. This is typically only set when the target node only exists in a Repository

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.

We finally present a small example for a csig-based decider function. Note how the signature information for the dependency file has to get initialized via get_csig during each function call (this is mandatory!).

env = Environment()


def config_file_decider(dependency, target, prev_ni, repo_node=None):
    import os.path

    # We always have to init the .csig value...
    dep_csig = dependency.get_csig()
    # .csig may not exist, because no target was built yet...
    if not prev_ni.hasattr("csig"):
        return True
    # Target file may not exist yet
    if not os.path.exists(str(target.abspath)):
        return True
    if dep_csig != prev_ni.csig:
        # Some change on source file => update installed one
        return True
    return False


def update_file():
    with open("test.txt", "a") as f:
        f.write("some line\n")


update_file()

# Activate our own decider function
env.Decider(config_file_decider)

env.Install("install", "test.txt")
      

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 using 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