Chapter 2. The Basics of PMake

As mentioned in the introduction, in any system, there are dependencies between the files that make up the system. For instance, in a program made up of several C source files and one header file, the C files will need to be re-compiled should the header file be changed. For a document of several chapters and one macro file, the chapters will need to be reprocessed if any of the macros changes. These are dependencies and are specified by means of dependency lines in the makefile.

On a dependency line, there are targets and sources, separated by a one- or two-character operator. The targets “depend” on the sources and are usually created from them. Any number of targets and sources may be specified on a dependency line. All the targets in the line are made to depend on all the sources. Targets and sources need not be actual files, but every source must be either an actual file or another target in the makefile. If you run out of room, use a backslash at the end of the line to continue onto the next one.

Any file may be a target and any file may be a source, but the relationship between the two (or however many) is determined by the “operator” that separates them. Three types of operators exist: one specifies that the datedness of a target is determined by the state of its sources, while another specifies other files (the sources) that need to be dealt with before the target can be re-created. The third operator is very similar to the first, with the additional condition that the target is out-of-date if it has no sources. These operations are represented by the colon, the exclamation point and the double-colon, respectively, and are mutually exclusive. Their exact semantics are as follows:

Enough words, now for an example. Take that C program I mentioned earlier. Say there are three C files (a.c, b.c and c.c) each of which includes the file defs.h. The dependencies between the files could then be expressed as follows:

program         : a.o b.o c.o

a.o b.o c.o     : defs.h

a.o             : a.c

b.o             : b.c

c.o             : c.c

You may be wondering at this point, where a.o, b.o and c.o came in and why they depend on defs.h and the C files do not. The reason is quite simple: program cannot be made by linking together .c files—it must be made from .o files. Likewise, if you change defs.h, it is not the .c files that need to be re-created, it is the .o files. If you think of dependencies in these terms—which files (targets) need to be created from which files (sources)—you should have no problems.

An important thing to notice about the above example, is that all the .o files appear as targets on more than one line. This is perfectly all right: the target is made to depend on all the sources mentioned on all the dependency lines. For example, a.o depends on both defs.h and a.c.

The order of the dependency lines in the makefile is important: the first target on the first dependency line in the makefile will be the one that gets made if you do not say otherwise. That is why program comes first in the example makefile, above.

Both targets and sources may contain the standard C-Shell wildcard characters ({, }, *, ?, [, and ]), but the non-curly-brace ones may only appear in the final component (the file portion) of the target or source. The characters mean the following things: