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 what is necessary when source files change--or, put another way, &SCons; should not waste time rebuilding things that don't need to be rebuilt. You can see this at work simply by re-invoking &SCons; after building our simple &hello; example: Program('hello.c') int main() { printf("Hello, world!\n"); } scons -Q scons -Q 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 scons -Q hello Note that &SCons; reports "...is up to date" only for target files named explicitly on the command line, to avoid cluttering the output.

Another 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 touch hello.c scons -Q hello 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 edit hello.c scons -Q hello 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 edit hello.c scons -Q hello 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 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: Program('hello.c') Decider('timestamp-newer') int main() { printf("Hello, world!\n"); } This makes &SCons; act like &Make; when a file's modification time is updated (using the &touch; command, for example): scons -Q hello touch hello.c scons -Q hello 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') int main() { printf("Hello, world!\n"); } 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 touch -t 198901010000 hello.c scons -Q hello 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') int main() { printf("Hello, world!\n"); } 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 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) int main() { printf("Hello, world!\n"); } Note that in the function definition, the dependency (input file) is the first argument, and then the ⌖. 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: .csig The content signature, or MD5 checksum, of the contents of the dependency file the list time the ⌖ was built. .size The size in bytes of the dependency file the list time the target was built. .timestamp The modification time of the dependency file the list time the ⌖ 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 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') #include "inc.h" int main() { printf("Hello, world!\n"); } #include "inc.h" int main() { printf("Hello, world!\n"); } #define INC 1 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 touch inc.h scons -Q

&SCons; still supports two functions that used to be the primary methods for configuring the decision about whether or not an input file has changed. Although they're not officially deprecated yet, their use is discouraged, mainly because they rely on a somewhat confusing distinction between how source files and target files are handled. These functions are documented here mainly in case you encounter them in existing &SConscript; files.

The &SourceSignatures; function is fairly straightforward, and supports two different argument values to configure whether source file changes should be decided using MD5 signatures: Program('hello.c') SourceSignatures('MD5') Or using time stamps: Program('hello.c') SourceSignatures('timestamp') These are roughly equivalent to specifying Decider('MD5') or Decider('timestamp-match'), respectively, although it only affects how SCons makes decisions about dependencies on source files--that is, files that are not built from any other files.

The &TargetSignatures; function specifies how &SCons; decides when a target file has changed when it is used as a dependency of (input to) another target--that is, the &TargetSignatures; function configures how the signatures of "intermediate" target files are used when deciding if a "downstream" target file must be rebuilt. This easily-overlooked distinction between how &SCons; decides if the target itself must be rebuilt and how the target is then used to decide if a different target must be rebuilt is one of the confusing things that has led to the &TargetSignatures; and &SourceSignatures; functions being replaced by the simpler &Decider; function. The &TargetSignatures; function supports the same 'MD5' and 'timestamp' argument values that are supported by the &SourceSignatures;, with the same meanings, but applied to target files. That is, in the example: Program('hello.c') TargetSignatures('MD5') The MD5 checksum of the &hello_o; target file will be used to decide if it has changed since the last time the "downstream" &hello; target file was built. And in the example: Program('hello.c') TargetSignatures('timestamp') The modification time of the &hello_o; target file will be used to decide if it has changed since the last time the "downstream" &hello; target file was built. The &TargetSignatures; function supports two additional argument values: 'source' and 'build'. The 'source' argument specifies that decisions involving whether target files have changed since a previous build should use the same behavior for the decisions configured for source files (using the &SourceSignatures; function). So in the example: Program('hello.c') TargetSignatures('source') SourceSignatures('timestamp') All files, both targets and sources, will use modification times when deciding if an input file has changed since the last time a target was built. Lastly, the 'build' argument specifies that &SCons; should examine the build status of a target file and always rebuild a "downstream" target if the target file was itself rebuilt, without re-examining the contents or timestamp of the newly-built target file. If the target file was not rebuilt during this &scons; invocation, then the target file will be examined the same way as configured by the &SourceSignature; call to decide if it has changed. This mimics the behavior of build signatures in earlier versions of &SCons;. A &buildsignature; re-combined signatures of all the input files that went into making the target file, so that the target file itself did not need to have its contents read to compute an MD5 signature. This can improve performance for some configurations, but is generally not as effective as using Decider('MD5-timestamp').

Now suppose that our "Hello, World!" program actually has an #include line to include the &hello_h; file in the compilation: Program('hello.c', CPPPATH = '.') #include <hello.h> int main() { printf("Hello, %s!\n", string); } #define string "world" And, for completeness, the &hello_h; file looks like this: In this case, we want &SCons; to recognize that, if the contents of the &hello_h; file change, the &hello; program must be recompiled. To do this, we need to modify the &SConstruct; file like so: The &cv-link-CPPPATH; value tells &SCons; to look in the current directory ('.') for any files included by C source files (.c or .h files). With this assignment in the &SConstruct; file: scons -Q hello scons -Q hello edit hello.h scons -Q hello First, notice that &SCons; added the -I. argument from the &cv-CPPPATH; variable so that the compilation would find the &hello_h; file in the local directory. Second, realize that &SCons; knows that the &hello; program must be rebuilt because it scans the contents of the &hello_c; file for the #include lines that indicate another file is being included in the compilation. &SCons; records these as implicit dependencies of the target file, Consequently, when the &hello_h; file changes, &SCons; realizes that the &hello_c; file includes it, and rebuilds the resulting &hello; program that depends on both the &hello_c; and &hello_h; files. Like the &cv-link-LIBPATH; variable, the &cv-CPPPATH; variable may be a list of directories, or a string separated by the system-specific path separation character (':' on POSIX/Linux, ';' on Windows). Either way, &SCons; creates the right command-line options so that the following example: Program('hello.c', CPPPATH = ['include', '/home/project/inc']) int main() { printf("Hello, world!\n"); } Will look like this on POSIX or Linux: scons -Q hello And like this on Windows: scons -Q hello.exe

Scanning each file for #include lines does take some extra processing time. When you're doing a full build of a large system, the scanning time is usually a very small percentage of the overall time spent on the build. You're most likely to notice the scanning time, however, when you rebuild all or part of a large system: &SCons; will likely take some extra time to "think about" what must be built before it issues the first build command (or decides that everything is up to date and nothing must be rebuilt). In practice, having &SCons; scan files saves time relative to the amount of potential time lost to tracking down subtle problems introduced by incorrect dependencies. Nevertheless, the "waiting time" while &SCons; scans files can annoy individual developers waiting for their builds to finish. Consequently, &SCons; lets you cache the implicit dependencies that its scanners find, for use by later builds. You can do this by specifying the &implicit-cache; option on the command line: scons -Q --implicit-cache hello scons -Q hello If you don't want to specify &implicit-cache; on the command line each time, you can make it the default behavior for your build by setting the &implicit_cache; option in an &SConscript; file: SetOption('implicit_cache', 1) &SCons; does not cache implicit dependencies like this by default because the &implicit-cache; causes &SCons; to simply use the implicit dependencies stored during the last run, without any checking for whether or not those dependencies are still correct. Specifically, this means &implicit-cache; instructs &SCons; to not rebuild "correctly" in the following cases: When &implicit-cache; is used, &SCons; will ignore any changes that may have been made to search paths (like &cv-CPPPATH; or &cv-LIBPATH;,). This can lead to &SCons; not rebuilding a file if a change to &cv-CPPPATH; would normally cause a different, same-named file from a different directory to be used. When &implicit-cache; is used, &SCons; will not detect if a same-named file has been added to a directory that is earlier in the search path than the directory in which the file was found last time.

When using cached implicit dependencies, sometimes you want to "start fresh" and have &SCons; re-scan the files for which it previously cached the dependencies. For example, if you have recently installed a new version of external code that you use for compilation, the external header files will have changed and the previously-cached implicit dependencies will be out of date. You can update them by running &SCons; with the &implicit-deps-changed; option: scons -Q --implicit-deps-changed hello scons -Q hello In this case, &SCons; will re-scan all of the implicit dependencies and cache updated copies of the information.

By default when caching dependencies, &SCons; notices when a file has been modified and re-scans the file for any updated implicit dependency information. Sometimes, however, you may want to force &SCons; to use the cached implicit dependencies, even if the source files changed. This can speed up a build for example, when you have changed your source files but know that you haven't changed any #include lines. In this case, you can use the &implicit-deps-unchanged; option: scons -Q --implicit-deps-unchanged hello scons -Q hello In this case, &SCons; will assume that the cached implicit dependencies are correct and will not bother to re-scan changed files. For typical builds after small, incremental changes to source files, the savings may not be very big, but sometimes every bit of improved performance counts.

Sometimes a file depends on another file that is not detected by an &SCons; scanner. For this situation, &SCons; allows you to specific explicitly that one file depends on another file, and must be rebuilt whenever that file changes. This is specified using the &Depends; method: hello = Program('hello.c') Depends(hello, 'other_file') % scons -Q hello cc -c hello.c -o hello.o cc -o hello hello.o % scons -Q hello scons: `hello' is up to date. % edit other_file [CHANGE THE CONTENTS OF other_file] % scons -Q hello cc -c hello.c -o hello.o cc -o hello hello.o Note that the dependency (the second argument to &Depends;) may also be a list of Node objects (for example, as returned by a call to a Builder): hello = Program('hello.c') goodbye = Program('goodbye.c') Depends(hello, goodbye) in which case the dependency or dependencies will be built before the target(s): % scons -Q hello cc -c goodbye.c -o goodbye.o cc -o goodbye goodbye.o cc -c hello.c -o hello.o cc -o hello hello.o

&SCons; has built-in scanners for a number of languages. Sometimes these scanners fail to extract certain implicit dependencies due to limitations of the scanner implementation. The following example illustrates a case where the built-in C scanner is unable to extract the implicit dependency on a header file. #define FOO_HEADER <foo.h> #include FOO_HEADER int main() { return FOO; } Program('hello', 'hello.c', CPPPATH='.') #define FOO 42 scons -Q edit foo.h scons -Q Apparently, the scanner does not know about the header dependency. Being not a full-fledged C preprocessor, the scanner does not expand the macro. In these cases, you may also use the compiler to extract the implicit dependencies. &ParseDepends; can parse the contents of the compiler output in the style of &Make;, and explicitly establish all of the listed dependencies. The following example uses &ParseDepends; to process a compiler generated dependency file which is generated as a side effect during compilation of the object file: #define FOO_HEADER <foo.h> #include FOO_HEADER int main() { return FOO; } obj = Object('hello.c', CCFLAGS='-MD -MF hello.d', CPPPATH='.') SideEffect('hello.d', obj) ParseDepends('hello.d') Program('hello', obj) #define FOO 42 hello.o: hello.c foo.h scons -Q edit foo.h scons -Q Parsing dependencies from a compiler-generated .d file has a chicken-and-egg problem, that causes unnecessary rebuilds: #define FOO_HEADER <foo.h> #include FOO_HEADER int main() { return FOO; } obj = Object('hello.c', CCFLAGS='-MD -MF hello.d', CPPPATH='.') SideEffect('hello.d', obj) ParseDepends('hello.d') Program('hello', obj) #define FOO 42 % scons -Q cc -o hello.o -c -MD -MF hello.d -I. hello.c cc -o hello hello.o % scons -Q --debug=explain scons: rebuilding `hello.o' because `foo.h' is a new dependency cc -o hello.o -c -MD -MF hello.d -I. hello.c % scons -Q scons: `.' is up to date. In the first pass, the dependency file is generated while the object file is compiled. At that time, &SCons; does not know about the dependency on foo.h. In the second pass, the object file is regenerated because foo.h is detected as a new dependency. &ParseDepends; immediately reads the specified file at invocation time and just returns if the file does not exist. A dependency file generated during the build process is not automatically parsed again. Hence, the compiler-extracted dependencies are not stored in the signature database during the same build pass. This limitation of &ParseDepends; leads to unnecessary recompilations. Therefore, &ParseDepends; should only be used if scanners are not available for the employed language or not powerful enough for the specific task.

Sometimes it makes sense to not rebuild a program, even if a dependency file changes. In this case, you would tell &SCons; specifically to ignore a dependency as follows: hello = Program('hello.c') Ignore(hello, 'hello.h') #include "hello.h" int main() { printf("Hello, %s!\n", string); } #define string "world" % scons -Q hello cc -c -o hello.o hello.c cc -o hello hello.o % scons -Q hello scons: `hello' is up to date. % edit hello.h [CHANGE THE CONTENTS OF hello.h] % scons -Q hello scons: `hello' is up to date. Now, the above example is a little contrived, because it's hard to imagine a real-world situation where you wouldn't want to rebuild &hello; if the &hello_h; file changed. A more realistic example might be if the &hello; program is being built in a directory that is shared between multiple systems that have different copies of the &stdio_h; include file. In that case, &SCons; would notice the differences between the different systems' copies of &stdio_h; and would rebuild &hello; each time you change systems. You could avoid these rebuilds as follows: hello = Program('hello.c', CPPPATH=['/usr/include']) Ignore(hello, '/usr/include/stdio.h') &Ignore; can also be used to prevent a generated file from being built by default. This is due to the fact that directories depend on their contents. So to ignore a generated file from the default build, you specify that the directory should ignore the generated file. Note that the file will still be built if the user specifically requests the target on scons command line, or if the file is a dependency of another file which is requested and/or is built by default. hello_obj=Object('hello.c') hello = Program(hello_obj) Ignore('.',[hello,hello_obj]) #include "stdio.h" int main() { printf("Hello!\n"); } scons -Q scons -Q hello scons -Q hello

Occasionally, it may be useful to specify that a certain file or directory must, if necessary, be built or created before some other target is built, but that changes to that file or directory do not require that the target itself be rebuilt. Such a relationship is called an order-only dependency because it only affects the order in which things must be built--the dependency before the target--but it is not a strict dependency relationship because the target should not change in response to changes in the dependent file. For example, suppose that you want to create a file every time you run a build that identifies the time the build was performed, the version number, etc., and which is included in every program that you build. The version file's contents will change every build. If you specify a normal dependency relationship, then every program that depends on that file would be rebuilt every time you ran &SCons;. For example, we could use some Python code in a &SConstruct; file to create a new version.c file with a string containing the current date every time we run &SCons;, and then link a program with the resulting object file by listing version.c in the sources: import time version_c_text = """ char *date = "%s"; """ % time.ctime(time.time()) open('version.c', 'w').write(version_c_text) hello = Program(['hello.c', 'version.c']) extern char *date; int main() { printf("Hello, %s! I was built: %s\n", date); } If we list version.c as an actual source file, though, then version.o will get rebuilt every time we run &SCons; (because the &SConstruct; file itself changes the contents of version.c) and the hello executable will get re-linked every time (because the version.o file changes): % scons -Q gcc -o hello.o -c hello.c gcc -o version.o -c version.c gcc -o hello hello.o version.o % scons -Q gcc -o version.o -c version.c gcc -o hello hello.o version.o % scons -Q gcc -o version.o -c version.c gcc -o hello hello.o version.o One solution is to use the &Requires; function to specify that the version.o must be rebuilt before it is used by the link step, but that changes to version.o should not actually cause the hello executable to be re-linked: import time version_c_text = """ char *date = "%s"; """ % time.ctime(time.time()) open('version.c', 'w').write(version_c_text) version_obj = Object('version.c') hello = Program('hello.c', LINKFLAGS = str(version_obj[0])) Requires(hello, version_obj) extern char *date; int main() { printf("Hello, %s! I was built: %s\n", date); } Notice that because we can no longer list version.c as one of the sources for the hello program, we have to find some other way to get it into the link command line. For this example, we're cheating a bit and stuffing the object file name (extracted from version_obj list returned by the &b-Object; call) into the &cv-link-LINKFLAGS; variable, because &cv-LINKFLAGS; is already included in the &cv-link-LINKCOM; command line. With these changes, we get the desired behavior of re-building the version.o file, and therefore re-linking the hello executable, only when the hello.c has changed: scons -Q scons -Q edit hello.c scons -Q scons -Q

How &SCons; handles dependencies can also be affected by the &AlwaysBuild; method. When a file is passed to the &AlwaysBuild; method, like so: hello = Program('hello.c') AlwaysBuild(hello) int main() { printf("Hello, %s!\n", string); } Then the specified target file (&hello; in our example) will always be considered out-of-date and rebuilt whenever that target file is evaluated while walking the dependency graph: scons -Q scons -Q The &AlwaysBuild; function has a somewhat misleading name, because it does not actually mean the target file will be rebuilt every single time &SCons; is invoked. Instead, it means that the target will, in fact, be rebuilt whenever the target file is encountered while evaluating the targets specified on the command line (and their dependencies). So specifying some other target on the command line, a target that does not itself depend on the &AlwaysBuild; target, will still be rebuilt only if it's out-of-date with respect to its dependencies: scons -Q scons -Q hello.o