File: make.info,  Node: Suffix Rules,  Next: Implicit Rule Search,  Prev: Last Resort,  Up: Implicit Rules

10.7 Old-Fashioned Suffix Rules
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"Suffix rules" are the old-fashioned way of defining implicit rules for
'make'.  Suffix rules are obsolete because pattern rules are more
general and clearer.  They are supported in GNU 'make' for compatibility
with old makefiles.  They come in two kinds: "double-suffix" and
"single-suffix".

   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 prerequisite is made by
replacing the target suffix with the source suffix in the file name.  A
two-suffix rule '.c.o' (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
prerequisite 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 rule's
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'.  Here is the old-fashioned way to define the rule
for compiling a C source file:

     .c.o:
             $(CC) -c $(CFLAGS) $(CPPFLAGS) -o $@ $<

   Suffix rules cannot have any prerequisites of their own.  If they
have any, they are treated as normal files with funny names, not as
suffix rules.  Thus, the rule:

     .c.o: foo.h
             $(CC) -c $(CFLAGS) $(CPPFLAGS) -o $@ $<

tells how to make the file '.c.o' from the prerequisite file 'foo.h',
and is not at all like the pattern rule:

     %.o: %.c foo.h
             $(CC) -c $(CFLAGS) $(CPPFLAGS) -o $@ $<

which 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 recipe are also meaningless.  They do not remove
previous rules as do pattern rules with no recipe (*note Canceling
Implicit Rules: Canceling 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 prerequisites of the
special target '.SUFFIXES'.  You can add your own suffixes by writing a
rule for '.SUFFIXES' that adds more prerequisites, 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 prerequisites.  By
special dispensation, this eliminates all existing prerequisites of
'.SUFFIXES'.  You can then write another rule to add the suffixes you
want.  For example,

     .SUFFIXES:            # Delete the default suffixes
     .SUFFIXES: .c .o .h   # Define our suffix list

   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.