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## Catalogue of Implicit Rules

Here is a catalogue of predefined implicit rules which are always available unless the makefile explicitly overrides or cancels them. See section Canceling Implicit Rules, for information on canceling or overriding an implicit rule. The ``-r'` or ``--no-builtin-rules'` option cancels all predefined rules.

Not all of these rules will always be defined, even when the ``-r'` option is not given. Many of the predefined implicit rules are implemented in `make` as suffix rules, so which ones will be defined depends on the suffix list (the list of dependencies of the special target `.SUFFIXES`). The default suffix list is: `.out`, `.a`, `.ln`, `.o`, `.c`, `.cc`, `.C`, `.p`, `.f`, `.F`, `.r`, `.y`, `.l`, `.s`, `.S`, `.mod`, `.sym`, `.def`, `.h`, `.info`, `.dvi`, `.tex`, `.texinfo`, `.texi`, `.txinfo`, `.w`, `.ch` `.web`, `.sh`, `.elc`, `.el`. All of the implicit rules described below whose dependencies have one of these suffixes are actually suffix rules. If you modify the suffix list, the only predefined suffix rules in effect will be those named by one or two of the suffixes that are on the list you specify; rules whose suffixes fail to be on the list are disabled. See section Old-Fashioned Suffix Rules, for full details on suffix rules.

Compiling C programs
  ``n.o'` is made automatically from ``n.c'` with a command of the form ``$(CC) -c $(CPPFLAGS) $(CFLAGS)'`.
Compiling C++ programs
  ``n.o'` is made automatically from ``n.cc'` or ``n.C'` with a command of the form ``$(CXX) -c $(CPPFLAGS) $(CXXFLAGS)'`. We encourage you to use the suffix ``.cc'` for C++ source files instead of ``.C'`.
Compiling Pascal programs
  ``n.o'` is made automatically from ``n.p'` with the command ``$(PC) -c $(PFLAGS)'`.
Compiling Fortran and Ratfor programs
  ``n.o'` is made automatically from ``n.r'`, ``n.F'` or ``n.f'` by running the Fortran compiler. The precise command used is as follows:
  ``.f'`
    ``$(FC) -c $(FFLAGS)'`.
  ``.F'`
    ``$(FC) -c $(FFLAGS) $(CPPFLAGS)'`.
  ``.r'`
    ``$(FC) -c $(FFLAGS) $(RFLAGS)'`.
Preprocessing Fortran and Ratfor programs
  ``n.f'` is made automatically from ``n.r'` or ``n.F'`. This rule runs just the preprocessor to convert a Ratfor or preprocessable Fortran program into a strict Fortran program. The precise command used is as follows:
  ``.F'`
    ``$(FC) -F $(CPPFLAGS) $(FFLAGS)'`.
  ``.r'`
    ``$(FC) -F $(FFLAGS) $(RFLAGS)'`.
Compiling Modula-2 programs
  ``n.sym'` is made from ``n.def'` with a command of the form ``$(M2C) $(M2FLAGS) $(DEFFLAGS)'`. ``n.o'` is made from ``n.mod'`; the form is: ``$(M2C) $(M2FLAGS) $(MODFLAGS)'`.
Assembling and preprocessing assembler programs
  ``n.o'` is made automatically from ``n.s'` by running the assembler, `as`. The precise command is ``$(AS) $(ASFLAGS)'`. ``n.s'` is made automatically from ``n.S'` by running the C preprocessor, `cpp`. The precise command is ``$(CPP) $(CPPFLAGS)'`.
Linking a single object file
  ``n'` is made automatically from ``n.o'` by running the linker (usually called `ld`) via the C compiler. The precise command used is ``$(CC) $(LDFLAGS) n.o $(LOADLIBES)'`. This rule does the right thing for a simple program with only one source file. It will also do the right thing if there are multiple object files (presumably coming from various other source files), one of which has a name matching that of the executable file. Thus,

      x: y.o z.o

  when ``x.c'`, ``y.c'` and ``z.c'` all exist will execute:

      cc -c x.c -o x.o
      cc -c y.c -o y.o
      cc -c z.c -o z.o
      cc x.o y.o z.o -o x
      rm -f x.o
      rm -f y.o
      rm -f z.o

  In more complicated cases, such as when there is no object file whose name derives from the executable file name, you must write an explicit command for linking. Each kind of file automatically made into ``.o'` object files will be automatically linked by using the compiler (``$(CC)'`, ``$(FC)'` or ``$(PC)'`; the C compiler ``$(CC)'` is used to assemble ``.s'` files) without the ``-c'` option. This could be done by using the ``.o'` object files as intermediates, but it is faster to do the compiling and linking in one step, so that's how it's done.
Yacc for C programs
  ``n.c'` is made automatically from ``n.y'` by running Yacc with the command ``$(YACC) $(YFLAGS)'`.
Lex for C programs
  ``n.c'` is made automatically from ``n.l'` by by running Lex. The actual command is ``$(LEX) $(LFLAGS)'`.
Lex for Ratfor programs
  ``n.r'` is made automatically from ``n.l'` by by running Lex. The actual command is ``$(LEX) $(LFLAGS)'`. The convention of using the same suffix ``.l'` for all Lex files regardless of whether they produce C code or Ratfor code makes it impossible for `make` to determine automatically which of the two languages you are using in any particular case. If `make` is called upon to remake an object file from a ``.l'` file, it must guess which compiler to use. It will guess the C compiler, because that is more common. If you are using Ratfor, make sure `make` knows this by mentioning ``n.r'` in the makefile. Or, if you are using Ratfor exclusively, with no C files, remove ``.c'` from the list of implicit rule suffixes with:

      .SUFFIXES:
      .SUFFIXES: .o .r .f .l ...

Making Lint Libraries from C, Yacc, or Lex programs
  ``n.ln'` is made from ``n.c'` by running `lint`. The precise command is ``$(LINT) $(LINTFLAGS) $(CPPFLAGS) -i'`. The same command is used on the C code produced from ``n.y'` or ``n.l'`.
TeX and Web
  ``n.dvi'` is made from ``n.tex'` with the command ``$(TEX)'`. ``n.tex'` is made from ``n.web'` with ``$(WEAVE)'`, or from ``n.w'` (and from ``n.ch'` if it exists or can be made) with ``$(CWEAVE)'`. ``n.p'` is made from ``n.web'` with ``$(TANGLE)'` and ``n.c'` is made from ``n.w'` (and from ``n.ch'` if it exists or can be made) with ``$(CTANGLE)'`.
Texinfo and Info
  ``n.dvi'` is made from ``n.texinfo'`, ``n.texi'`, or ``n.txinfo'`, with the command ``$(TEXI2DVI) $(TEXI2DVI_FLAGS)'`. ``n.info'` is made from ``n.texinfo'`, ``n.texi'`, or ``n.txinfo'`, with the command ``$(MAKEINFO) $(MAKEINFO_FLAGS)'`.
RCS
  Any file ``n'` is extracted if necessary from an RCS file named either ``n,v'` or ``RCS/n,v'`. The precise command used is ``$(CO) $(COFLAGS)'`. ``n'` will not be extracted from RCS if it already exists, even if the RCS file is newer. The rules for RCS are terminal (see section Match-Anything Pattern Rules), so RCS files cannot be generated from another source; they must actually exist.
SCCS
  Any file ``n'` is extracted if necessary from an SCCS file named either ``s.n'` or ``SCCS/s.n'`. The precise command used is ``$(GET) $(GFLAGS)'`. The rules for SCCS are terminal (see section Match-Anything Pattern Rules), so SCCS files cannot be generated from another source; they must actually exist. For the benefit of SCCS, a file ``n'` is copied from ``n.sh'` and made executable (by everyone). This is for shell scripts that are checked into SCCS. Since RCS preserves the execution permission of a file, you do not need to use this feature with RCS. We recommend that you avoid using of SCCS. RCS is widely held to be superior, and is also free. By choosing free software in place of comparable (or inferior) proprietary software, you support the free software movement.

Usually, you want to change only the variables listed in the table above, which are documented in the following section.

However, the commands in built-in implicit rules actually use variables such as `COMPILE.c`, `LINK.p`, and `PREPROCESS.S`, whose values contain the commands listed above.

`make` follows the convention that the rule to compile a ``.x'` source file uses the variable `COMPILE.x`. Similarly, the rule to produce an executable from a ``.x'` file uses `LINK.x`; and the rule to preprocess a ``.x'` file uses `PREPROCESS.x`.

Every rule that produces an object file uses the variable `OUTPUT_OPTION`. `make` defines this variable either to contain ``-o $@'`, or to be empty, depending on a compile-time option. You need the ``-o'` option to ensure that the output goes into the right file when the source file is in a different directory, as when using `VPATH` (see section Searching Directories for Dependencies). However, compilers on some systems do not accept a ``-o'` switch for object files. If you use such a system, and use `VPATH`, some compilations will put their output in the wrong place. A possible workaround for this problem is to give `OUTPUT_OPTION` the value ``; mv $*.o $@'`.

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