How make Processes a Makefile

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How make Processes a Makefile

By default, make starts with the first rule (not counting rules whose target names start with ‘.’). This is called the default goal.(Goals are the targets that make strives ultimately to update. See Arguments to Specify the Goals.)

In the example shown in A Simple Makefile, the default goal is to update the executable program ‘edit’; therefore, we put that rule first.

When you give the command, make, make reads the makefile in the current directory and begins by processing the first rule. In the example, this rule is for relinking edit; but before make can fully process this rule, it must process the rules for the files that edit depends on; in this case, they are the object files. Each of these files is processed according to its own rule. These rules say to update each ‘.o’ file by compiling its source file. The recompilation must be done if the source file, or any of the header files named as dependencies, is more recent than the object file, or if the object file does not exist.

The other rules are processed because their targets appear as dependencies of the goal. If some other rule is not depended on by the goal (or anything it depends on, etc.), that rule is not processed, unless you tell make to do so (with a command such as make clean).

Before recompiling an object file, make considers updating its dependencies, the source file and header files. This makefile does not specify anything to be done for them—the ‘.c’ and ‘.h’ files are not the targets of any rules—so make does nothing for these files. But make would update automatically generated C programs, such as those made by Bison or Yacc, by their own rules at this time. 7d6

After recompiling whichever object files need it, make decides whether to relink ‘edit’. This must be done if the file ‘edit’ does not exist, or if any of the object files are newer than it. If an object file was just recompiled, it is now newer than ‘edit’, so ‘edit’ is relinked.

Thus, if we change the file ‘insert.c’ and run make, make will compile that file to update ‘insert.o’, and then link ‘edit’. If we change the file ‘command.h’ and run make, make will recompile the object files ‘kbd.o’ along with ‘command.o’ and ‘files.o’ and then link the file ‘edit’.