A double-suffix rule is defined by a pair of suffixes: the target suffix and the source suffix. It matches any file whose name ends with the target suffix. The corresponding implicit dependency is made by replacing the target suffix with the source suffix in the file name.
A two-suffix rule (whose target and source suffixes are .o and .c) is equivalent to the pattern rule, %.o : %.c.
A single-suffix rule is defined by a single suffix, which is the source suffix. It matches any file name, and the corresponding implicit depen-dency name is made by appending the source suffix. A single-suffix rule whose source suffix is .c is equivalent to the pattern rule % : %.c.
Suffix rule definitions are
recognized by comparing each rules target against a defined list of known
suffixes. When make sees a rule whose target is a known suffix, this rule
is considered a single-suffix rule. When make
sees a rule whose target is two known suffixes concatenated, this rule
is taken as a double-suffix rule. For example, .c
and .o
are both on the default list of known suffixes. Therefore, if you define
a rule whose target is .c.o,
make takes it to be a double-suffix rule with source suffix, .c
and target suffix, .o.
The following is the old-fashioned way to define the rule for compiling
a C source file.
Suffix rules cannot have
any dependencies of their own. If they have any, they are treated as normal
files with funny names, not as suffix rules. Thus, use the following rule.
This rule tells how to make
the file, .c.o,
from the dependency file, foo.h,
and is not at all like the following pattern rule.
This rule tells how to make .o files from .c files, and makes all .o files using this pattern rule also depend on foo.h.
Suffix rules with no commands are also meaningless. They do not remove previous rules as do pattern rules with no commands (see Canceling implicit rules). They simply enter the suffix or pair of suffixes concatenated as a target in the data base.
The known suffixes are simply the names of the dependencies of the special target, .SUFFIXES. You can add your own suffixes by writing a rule for .SUFFIXES that adds more dependencies, as in: .SUFFIXES: .hack .win, which adds .hack and .win to the end of the list of suffixes.
If you wish to eliminate the default known suffixes instead of just adding to them, write a rule for .SUFFIXES with no dependencies. By special dispensation, this eliminates all existing dependencies of .SUFFIXES.
You can then write another
rule to add the suffixes you want. For example, use the following.
The -r or --no-builtin-rules flag causes the default list of suffixes to be empty. The variable, SUFFIXES, is defined to the default list of suffixes before make reads any makefiles. You can change the list of suffixes with a rule for the special target, .SUFFIXES, but that does not alter this variable.