PMake -- A Tutorial
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2.3. Variables

PMake, like Make before it, has the ability to save text in variables to be recalled later at your convenience. Variables in PMake are used much like variables in the shell and, by tradition, consist of all upper-case letters (you do not have to use all upper-case letters. In fact there is nothing to stop you from calling a variable @^&$%$. Just tradition). Variables are assigned-to using lines of the form:

VARIABLE = value

appended-to by:

VARIABLE += value

conditionally assigned-to (if the variable is not already defined) by:

VARIABLE ?= value

and assigned-to with expansion (i.e. the value is expanded (see below) before being assigned to the variable--useful for placing a value at the beginning of a variable, or other things) by:

VARIABLE := value

Any whitespace before value is stripped off. When appending, a space is placed between the old value and the stuff being appended.

The final way a variable may be assigned to is using:

VARIABLE != shell-command

In this case, shell-command has all its variables expanded (see below) and is passed off to a shell to execute. The output of the shell is then placed in the variable. Any newlines (other than the final one) are replaced by spaces before the assignment is made. This is typically used to find the current directory via a line like:

CWD             != pwd

Note: This is intended to be used to execute commands that produce small amounts of output (e.g. pwd). The implementation is less than intelligent and will likely freeze if you execute something that produces thousands of bytes of output (8 Kb is the limit on many UNIX® systems). The value of a variable may be retrieved by enclosing the variable name in parentheses or curly braces and preceding the whole thing with a dollar sign.

For example, to set the variable CFLAGS to the string -I/sprite/src/lib/libc -O, you would place a line:

CFLAGS = -I/sprite/src/lib/libc -O

in the makefile and use the word $(CFLAGS) wherever you would like the string -I/sprite/src/lib/libc -O to appear. This is called variable expansion.

Note: Unlike Make, PMake will not expand a variable unless it knows the variable exists. E.g. if you have a ${i} in a shell command and you have not assigned a value to the variable i (the empty string is considered a value, by the way), where Make would have substituted the empty string, PMake will leave the ${i} alone. To keep PMake from substituting for a variable it knows, precede the dollar sign with another dollar sign (e.g. to pass ${HOME} to the shell, use $${HOME}). This causes PMake, in effect, to expand the $ macro, which expands to a single $.

For compatibility, Make's style of variable expansion will be used if you invoke PMake with any of the compatibility flags (-V, -B or -M. The -V flag alters just the variable expansion). There are two different times at which variable expansion occurs: when parsing a dependency line, the expansion occurs immediately upon reading the line. If any variable used on a dependency line is undefined, PMake will print a message and exit. Variables in shell commands are expanded when the command is executed. Variables used inside another variable are expanded whenever the outer variable is expanded (the expansion of an inner variable has no effect on the outer variable. For example, if the outer variable is used on a dependency line and in a shell command, and the inner variable changes value between when the dependency line is read and the shell command is executed, two different values will be substituted for the outer variable).

Variables come in four flavors, though they are all expanded the same and all look about the same. They are (in order of expanding scope):

Local variables.
Command-line variables.
Global variables.
Environment variables.

The classification of variables does not matter much, except that the classes are searched from the top (local) to the bottom (environment) when looking up a variable. The first one found wins.

2.3.1. Local Variables

Each target can have as many as seven local variables. These are variables that are only “visible” within that target's shell script and contain such things as the target's name, all of its sources (from all its dependency lines), those sources that were out-of-date, etc. Four local variables are defined for all targets. They are:

.TARGET: The name of the target.
.OODATE: The list of the sources for the target that were considered out-of-date. The order in the list is not guaranteed to be the same as the order in which the dependencies were given.
.ALLSRC: The list of all sources for this target in the order in which they were given.
.PREFIX: The target without its suffix and without any leading path. E.g. for the target ../../lib/compat/fsRead.c, this variable would contain fsRead.

Three other local variables are set only for certain targets under special circumstances. These are the .IMPSRC, .ARCHIVE, and .MEMBER variables. When they are set and how they are used is described later.

Four of these variables may be used in sources as well as in shell scripts. These are .TARGET, .PREFIX, .ARCHIVE and .MEMBER. The variables in the sources are expanded once for each target on the dependency line, providing what is known as a “dynamic source,” allowing you to specify several dependency lines at once. For example:

$(OBJS)         : $(.PREFIX).c

will create a dependency between each object file and its corresponding C source file.

2.3.2. Command-line Variables

Command-line variables are set when PMake is first invoked by giving a variable assignment as one of the arguments. For example:

pmake "CFLAGS = -I/sprite/src/lib/libc -O"

would make CFLAGS be a command-line variable with the given value. Any assignments to CFLAGS in the makefile will have no effect, because once it is set, there is (almost) nothing you can do to change a command-line variable (the search order, you see). Command-line variables may be set using any of the four assignment operators, though only = and ?= behave as you would expect them to, mostly because assignments to command-line variables are performed before the makefile is read, thus the values set in the makefile are unavailable at the time. += is the same as =, because the old value of the variable is sought only in the scope in which the assignment is taking place (for reasons of efficiency that I won't get into here). := and ?= will work if the only variables used are in the environment. != is sort of pointless to use from the command line, since the same effect can no doubt be accomplished using the shell's own command substitution mechanisms (backquotes and all that).

2.3.3. Global Variables

Global variables are those set or appended-to in the makefile. There are two classes of global variables: those you set and those PMake sets. As I said before, the ones you set can have any name you want them to have, except they may not contain a colon or an exclamation point. The variables PMake sets (almost) always begin with a period and always contain upper-case letters, only. The variables are as follows:

.PMAKE: The name by which PMake was invoked is stored in this variable. For compatibility, the name is also stored in the MAKE variable.
.MAKEFLAGS: All the relevant flags with which PMake was invoked. This does not include such things as -f or variable assignments. Again for compatibility, this value is stored in the MFLAGS variable as well.

Two other variables, .INCLUDES and .LIBS, are covered in the section on special targets in Chapter 3.

Global variables may be deleted using lines of the form:

#undef variable

The # must be the first character on the line. Note that this may only be done on global variables.

2.3.4. Environment Variables

Environment variables are passed by the shell that invoked PMake and are given by PMake to each shell it invokes. They are expanded like any other variable, but they cannot be altered in any way.

One special environment variable, PMAKE, is examined by PMake for command-line flags, variable assignments, etc., it should always use. This variable is examined before the actual arguments to PMake are. In addition, all flags given to PMake, either through the PMAKE variable or on the command line, are placed in this environment variable and exported to each shell PMake executes. Thus recursive invocations of PMake automatically receive the same flags as the top-most one.

Using all these variables, you can compress the sample makefile even more:

OBJS            = a.o b.o c.o

program         : $(OBJS)
        cc $(.ALLSRC) -o $(.TARGET)

$(OBJS)         : defs.h

a.o             : a.c
        cc -c a.c

b.o             : b.c
        cc -c b.c

c.o             : c.c
        cc -c c.c

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