| A special target is a built-in phony target used to change make 's default behavior. For instance, .PHONY , a special target, which we've already seen, declares that its prerequisite does not refer to an actual file and should always be considered out of date. The .PHONY target is the most common special target you will see, but there are others as well. These special targets follow the syntax of normal targets, that is target : prerequisite , but the target is not a file or even a normal phony. They are really more like directives for modifying make 's internal algorithms. There are twelve special targets. They fall into three categories: as we've just said many are used to alter the behavior of make when updating a target, another set act simply as global flags to make and ignore their targets, finally the .SUFFIXES special target is used when specifying old-fashioned suffix rules (discussed in the Section 2.4.3 earlier in this chapter). The most useful target modifiers (aside from .PHONY ) are: -
- .INTERMEDIATE
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Prerequisites of this special target are treated as intermediate files. If make creates the file while updating another target, the file will be deleted automatically when make exits. If the file already exists when make considers updating the file, the file will not be deleted. -
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This can be very useful when building custom rule chains. For instance, most Java tools accept Windows-like file lists. Creating rules to build the file lists and marking their output files as intermediate allows make to clean up many temporary files. -
- .SECONDARY
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Prerequisites of this special target are treated as intermediate files but are never automatically deleted. The most common use of .SECONDARY is to mark object files stored in libraries. Normally these object files will be deleted as soon as they are added to an archive. Sometimes it is more convenient during development to keep these object files, but still use the make support for updating archives. -
- .PRECIOUS
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When make is interrupted during execution, it may delete the target file it is updating if the file was modified since make started. This is so make doesn't leave a partially constructed (possibly corrupt) file laying around in the build tree. There are times when you don't want this behavior, particularly if the file is large and computationally expensive to create. If you mark the file as precious, make will never delete the file if interrupted. -
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Use of .PRECIOUS is relatively rare, but when it is needed it is often a life saver. Note that make will not perform an automatic delete if the commands of a rule generate an error. It does so only when interrupted by a signal. -
- .DELETE_ON_ERROR
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This is sort of the opposite of .PRECIOUS . Marking a target as .DELETE_ON_ERROR says that make should delete the target if any of the commands associated with the rule generates an error. make normally only deletes the target if it is interrupted by a signal. The other special targets will be covered later when their use is more relevant. We'll discuss .EXPORT_ALL_VARIABLES in Chapter 3 and the targets relating to parallel execution in Chapter 10. |