This is the first paragraph of Common Attributes.
For the "virtual" and "comment" see Chapter 26.
Attributes can be added to a node with the ":attr" command and by using them in a dependency or rule. Note that an assignment does not directly associate the attribute with a node. This only happens when the variable is used in an ":attr" command or dependency. STICKY ATTRIBUTES When attributes are used in a rule or dependency, most of them are only used for that dependency. But some attributes are "sticky": Once used for an item they are used everywhere for that item. Sticky attributes are: virtual virtual target, not a file remember virtual target that is remembered directory item is a directory filetype type of file constant file contents never changes fetch list of locations where to fetch from (first one that works is used) commit list of locations for VCS publish list of locations to publish to (they are all used) force rebuild a target always depdir directory to put an automatically generated dependency file in; when omitted $BDIR is used signfile file used to store signatures for this target SIGNATURES: the check attribute The default check for a file that was changed is an md5 checksum. Each time a recipe is executed the checksums for the relevant items are computed and stored in the file "aap/sign". The next time the recipe is executed the current and the old checksums are compared. When they are different, the build commands are executed. This means that when you put back an old version of a file, rebuilding will take place even though the timestamp of the source might be older than the target. Another check can be specified with {check = name}. Example: foo.txt : foo.db {check = time} :sys db_extract $source >$target Other types of signatures supported: time Build the target when the timestamp of the source differs from the last time the target was build. newer Build the target if its timestamp is older than the timestamp of the source. This is what the good old "make" program uses. md5 Build the target if the md5 checksum of the source differs from the last time the target was build. This is the default. c_md5 Like "md5", but ignore changes in comments and amount of white space. Appropriate for C programs. Slows down computations considerably. none Don't check time or contents, only existence. Used for directories. When mixing "newer" with other methods, the build rules are executed if the target is older than the source with the "newer" check, or when one of the signatures for the other items differs. The "aap/sign" file is normally stored in the directory of the target. This means it will be found even when using several recipes that produce the same target. But for targets that get installed in system directories (use an absolute path), virtual targets and remote targets this is avoided. For these targets the "aap/sign" file is stored in the directory of the recipe that specifies how to build the target. To overrule the directory where "AAP/sign" is written, use the attribute {signdirectory = name} for the target. To overrule the file where the signatures are written, use the attribute {signfile = name} for the target. "name" cannot end in "sign". CIRCULAR DEPENDENCIES Two attributes can be used to handle circular dependencies: update Can be set to "no" to avoid updating a source that a target depends on. recursive Can be set to a number, which indicates the maximum recursive depth allowed. The use can best be illustrated with an example: -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- :attr {recursive = 3} index file.out index: file.out {update = no} # Get the current checksum for the index file. @sum = get_md5("index") # Generate the new index file from the output file. :system wc file.out >$target # Update the output file if the index file changed. @if sum != get_md5("index"): :update file.out file.out: file.in index {update = no} # Make sure index exists. @if not os.path.exists("index"): :print empty > index # Generate the output file. :cat $source >! $target # Need to generate the index file again. :update index all: file.out -=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=- The goal is to produce the file "file.out". It is created from "test.in" and "index". The "index" is created from "file.out", which includes the "index" file, thus a circular dependency exists. The idea is to repeat generating "file.out" until it no longer changes. The "recursive" attribute is set to 3 for "index" and "file.out". This allows rebuilding "file.out" three times before giving up. In the first dependency the "{update = no}" attribute is used to avoid updating "file.out". The build commands first update the "index" file before using ":update" to update "file.out". But this is only done when the index file has changed. That is where the circular dependency stops: When the generated index file no longer changes. In the second dependency a similar thing is done: The "index" file is not updated before executing the build commands but as part of the build commands. |