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Certain standard ways of remaking target files are used very often. For
example, one customary way to make an object file is from a C source file
using the C compiler, cc.
Implicit rules tell make how to use customary techniques so
that you do not have to specify them in detail when you want to use
them. For example, there is an implicit rule for C compilation. File
names determine which implicit rules are run. For example, C
compilation typically takes a `.c' file and makes a `.o' file.
So make applies the implicit rule for C compilation when it sees
this combination of file name endings.
A chain of implicit rules can apply in sequence; for example, make
will remake a `.o' file from a `.y' file by way of a `.c' file.
The built-in implicit rules use several variables in their commands so
that, by changing the values of the variables, you can change the way the
implicit rule works. For example, the variable CFLAGS controls the
flags given to the C compiler by the implicit rule for C compilation.
You can define your own implicit rules by writing pattern rules.
Suffix rules are a more limited way to define implicit rules. Pattern rules are more general and clearer, but suffix rules are retained for compatibility.
10.1 Using Implicit Rules How to use an existing implicit rule to get the commands for updating a file. 10.2 Catalogue of Implicit Rules A list of built-in implicit rules. 10.3 Variables Used by Implicit Rules How to change what predefined rules do. 10.4 Chains of Implicit Rules How to use a chain of implicit rules. 10.5 Defining and Redefining Pattern Rules How to define new implicit rules. 10.6 Defining Last-Resort Default Rules How to defining commands for rules which cannot find any. 10.7 Old-Fashioned Suffix Rules The old-fashioned style of implicit rule. 10.8 Implicit Rule Search Algorithm The precise algorithm for applying implicit rules.
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To allow make to find a customary method for updating a target file,
all you have to do is refrain from specifying commands yourself. Either
write a rule with no command lines, or don't write a rule at all. Then
make will figure out which implicit rule to use based on which
kind of source file exists or can be made.
For example, suppose the makefile looks like this:
foo : foo.o bar.o
cc -o foo foo.o bar.o $(CFLAGS) $(LDFLAGS)
|
Because you mention `foo.o' but do not give a rule for it, make
will automatically look for an implicit rule that tells how to update it.
This happens whether or not the file `foo.o' currently exists.
If an implicit rule is found, it can supply both commands and one or more prerequisites (the source files). You would want to write a rule for `foo.o' with no command lines if you need to specify additional prerequisites, such as header files, that the implicit rule cannot supply.
Each implicit rule has a target pattern and prerequisite patterns. There may
be many implicit rules with the same target pattern. For example, numerous
rules make `.o' files: one, from a `.c' file with the C compiler;
another, from a `.p' file with the Pascal compiler; and so on. The rule
that actually applies is the one whose prerequisites exist or can be made.
So, if you have a file `foo.c', make will run the C compiler;
otherwise, if you have a file `foo.p', make will run the Pascal
compiler; and so on.
Of course, when you write the makefile, you know which implicit rule you
want make to use, and you know it will choose that one because you
know which possible prerequisite files are supposed to exist.
See section Catalogue of Implicit Rules,
for a catalogue of all the predefined implicit rules.
Above, we said an implicit rule applies if the required prerequisites "exist or can be made". A file "can be made" if it is mentioned explicitly in the makefile as a target or a prerequisite, or if an implicit rule can be recursively found for how to make it. When an implicit prerequisite is the result of another implicit rule, we say that chaining is occurring. See section Chains of Implicit Rules.
In general, make searches for an implicit rule for each target, and
for each double-colon rule, that has no commands. A file that is mentioned
only as a prerequisite is considered a target whose rule specifies nothing,
so implicit rule search happens for it. See section Implicit Rule Search Algorithm, for the
details of how the search is done.
Note that explicit prerequisites do not influence implicit rule search. For example, consider this explicit rule:
foo.o: foo.p |
The prerequisite on `foo.p' does not necessarily mean that
make will remake `foo.o' according to the implicit rule to
make an object file, a `.o' file, from a Pascal source file, a
`.p' file. For example, if `foo.c' also exists, the implicit
rule to make an object file from a C source file is used instead,
because it appears before the Pascal rule in the list of predefined
implicit rules (see section Catalogue of Implicit Rules).
If you do not want an implicit rule to be used for a target that has no commands, you can give that target empty commands by writing a semicolon (see section Defining Empty Commands).
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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 prerequisites 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 prerequisites 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.
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) $(CPPFLAGS)'.
ld) via the C compiler. The precise
command used is `$(CC) $(LDFLAGS) n.o $(LOADLIBES) $(LDLIBS)'.
This rule does the right thing for a simple program with only one source file. It will also do the right thing if there are multiple object files (presumably coming from various other source files), one of which has a name matching that of the executable file. Thus,
x: y.o z.o |
when `x.c', `y.c' and `z.c' all exist will execute:
cc -c x.c -o x.o cc -c y.c -o y.o cc -c z.c -o z.o cc x.o y.o z.o -o x rm -f x.o rm -f y.o rm -f z.o |
In more complicated cases, such as when there is no object file whose name derives from the executable file name, you must write an explicit command for linking.
Each kind of file automatically made into `.o' object files will be automatically linked by using the compiler (`$(CC)', `$(FC)' or `$(PC)'; the C compiler `$(CC)' is used to assemble `.s' files) without the `-c' option. This could be done by using the `.o' object files as intermediates, but it is faster to do the compiling and linking in one step, so that's how it's done.
The convention of using the same suffix `.l' for all Lex files
regardless of whether they produce C code or Ratfor code makes it
impossible for make to determine automatically which of the two
languages you are using in any particular case. If make is
called upon to remake an object file from a `.l' file, it must
guess which compiler to use. It will guess the C compiler, because
that is more common. If you are using Ratfor, make sure make
knows this by mentioning `n.r' in the makefile. Or, if you
are using Ratfor exclusively, with no C files, remove `.c' from
the list of implicit rule suffixes with:
.SUFFIXES: .SUFFIXES: .o .r .f .l ... |
lint.
The precise command is `$(LINT) $(LINTFLAGS) $(CPPFLAGS) -i'.
The same command is used on the C code produced from
`n.y' or `n.l'.
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 4.5 Searching Directories for Prerequisites). 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 $@'.
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The commands in built-in implicit rules make liberal use of certain
predefined variables. You can alter these variables in the makefile,
with arguments to make, or in the environment to alter how the
implicit rules work without redefining the rules themselves. You can
cancel all variables used by implicit rules with the `-R' or
`--no-builtin-variables' option.
For example, the command used to compile a C source file actually says `$(CC) -c $(CFLAGS) $(CPPFLAGS)'. The default values of the variables used are `cc' and nothing, resulting in the command `cc -c'. By redefining `CC' to `ncc', you could cause `ncc' to be used for all C compilations performed by the implicit rule. By redefining `CFLAGS' to be `-g', you could pass the `-g' option to each compilation. All implicit rules that do C compilation use `$(CC)' to get the program name for the compiler and all include `$(CFLAGS)' among the arguments given to the compiler.
The variables used in implicit rules fall into two classes: those that are
names of programs (like CC) and those that contain arguments for the
programs (like CFLAGS). (The "name of a program" may also contain
some command arguments, but it must start with an actual executable program
name.) If a variable value contains more than one argument, separate them
with spaces.
Here is a table of variables used as names of programs in built-in rules:
AR
AS
CC
CXX
CO
CPP
FC
GET
LEX
PC
YACC
YACCR
MAKEINFO
TEX
TEXI2DVI
WEAVE
CWEAVE
TANGLE
CTANGLE
RM
Here is a table of variables whose values are additional arguments for the programs above. The default values for all of these is the empty string, unless otherwise noted.
ARFLAGS
ASFLAGS
CFLAGS
CXXFLAGS
COFLAGS
co program.
CPPFLAGS
FFLAGS
GFLAGS
get program.
LDFLAGS
LFLAGS
PFLAGS
RFLAGS
YFLAGS
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Sometimes a file can be made by a sequence of implicit rules. For example,
a file `n.o' could be made from `n.y' by running
first Yacc and then cc. Such a sequence is called a chain.
If the file `n.c' exists, or is mentioned in the makefile, no
special searching is required: make finds that the object file can
be made by C compilation from `n.c'; later on, when considering
how to make `n.c', the rule for running Yacc is
used. Ultimately both `n.c' and `n.o' are
updated.
However, even if `n.c' does not exist and is not mentioned,
make knows how to envision it as the missing link between
`n.o' and `n.y'! In this case, `n.c' is
called an intermediate file. Once make has decided to use the
intermediate file, it is entered in the data base as if it had been
mentioned in the makefile, along with the implicit rule that says how to
create it.
Intermediate files are remade using their rules just like all other files. But intermediate files are treated differently in two ways.
The first difference is what happens if the intermediate file does not
exist. If an ordinary file b does not exist, and make
considers a target that depends on b, it invariably creates
b and then updates the target from b. But if b is an
intermediate file, then make can leave well enough alone. It
won't bother updating b, or the ultimate target, unless some
prerequisite of b is newer than that target or there is some other
reason to update that target.
The second difference is that if make does create b
in order to update something else, it deletes b later on after it
is no longer needed. Therefore, an intermediate file which did not
exist before make also does not exist after make.
make reports the deletion to you by printing a `rm -f'
command showing which file it is deleting.
Ordinarily, a file cannot be intermediate if it is mentioned in the
makefile as a target or prerequisite. However, you can explicitly mark a
file as intermediate by listing it as a prerequisite of the special target
.INTERMEDIATE. This takes effect even if the file is mentioned
explicitly in some other way.
You can prevent automatic deletion of an intermediate file by marking it
as a secondary file. To do this, list it as a prerequisite of the
special target .SECONDARY. When a file is secondary, make
will not create the file merely because it does not already exist, but
make does not automatically delete the file. Marking a file as
secondary also marks it as intermediate.
You can list the target pattern of an implicit rule (such as `%.o')
as a prerequisite of the special target .PRECIOUS to preserve
intermediate files made by implicit rules whose target patterns match
that file's name; see 5.5 Interrupting or Killing make.
A chain can involve more than two implicit rules. For example, it is
possible to make a file `foo' from `RCS/foo.y,v' by running RCS,
Yacc and cc. Then both `foo.y' and `foo.c' are
intermediate files that are deleted at the end.
No single implicit rule can appear more than once in a chain. This means
that make will not even consider such a ridiculous thing as making
`foo' from `foo.o.o' by running the linker twice. This
constraint has the added benefit of preventing any infinite loop in the
search for an implicit rule chain.
There are some special implicit rules to optimize certain cases that would
otherwise be handled by rule chains. For example, making `foo' from
`foo.c' could be handled by compiling and linking with separate
chained rules, using `foo.o' as an intermediate file. But what
actually happens is that a special rule for this case does the compilation
and linking with a single cc command. The optimized rule is used in
preference to the step-by-step chain because it comes earlier in the
ordering of rules.
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You define an implicit rule by writing a pattern rule. A pattern rule looks like an ordinary rule, except that its target contains the character `%' (exactly one of them). The target is considered a pattern for matching file names; the `%' can match any nonempty substring, while other characters match only themselves. The prerequisites likewise use `%' to show how their names relate to the target name.
Thus, a pattern rule `%.o : %.c' says how to make any file `stem.o' from another file `stem.c'.
Note that expansion using `%' in pattern rules occurs after any variable or function expansions, which take place when the makefile is read. See section How to Use Variables, and Functions for Transforming Text.
10.5.1 Introduction to Pattern Rules An introduction to pattern rules. 10.5.2 Pattern Rule Examples Examples of pattern rules. 10.5.3 Automatic Variables How to use automatic variables in the commands of implicit rules. 10.5.4 How Patterns Match How patterns match. 10.5.5 Match-Anything Pattern Rules Precautions you should take prior to defining rules that can match any target file whatever. 10.5.6 Canceling Implicit Rules How to override or cancel built-in rules.
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A pattern rule contains the character `%' (exactly one of them) in the target; otherwise, it looks exactly like an ordinary rule. The target is a pattern for matching file names; the `%' matches any nonempty substring, while other characters match only themselves.
For example, `%.c' as a pattern matches any file name that ends in `.c'. `s.%.c' as a pattern matches any file name that starts with `s.', ends in `.c' and is at least five characters long. (There must be at least one character to match the `%'.) The substring that the `%' matches is called the stem.
`%' in a prerequisite of a pattern rule stands for the same stem that was matched by the `%' in the target. In order for the pattern rule to apply, its target pattern must match the file name under consideration, and its prerequisite patterns must name files that exist or can be made. These files become prerequisites of the target.
Thus, a rule of the form
%.o : %.c ; command... |
specifies how to make a file `n.o', with another file `n.c' as its prerequisite, provided that `n.c' exists or can be made.
There may also be prerequisites that do not use `%'; such a prerequisite attaches to every file made by this pattern rule. These unvarying prerequisites are useful occasionally.
A pattern rule need not have any prerequisites that contain `%', or in fact any prerequisites at all. Such a rule is effectively a general wildcard. It provides a way to make any file that matches the target pattern. See section 10.6 Defining Last-Resort Default Rules.
Pattern rules may have more than one target. Unlike normal rules, this
does not act as many different rules with the same prerequisites and
commands. If a pattern rule has multiple targets, make knows that
the rule's commands are responsible for making all of the targets. The
commands are executed only once to make all the targets. When searching
for a pattern rule to match a target, the target patterns of a rule other
than the one that matches the target in need of a rule are incidental:
make worries only about giving commands and prerequisites to the file
presently in question. However, when this file's commands are run, the
other targets are marked as having been updated themselves.
The order in which pattern rules appear in the makefile is important since this is the order in which they are considered. Of equally applicable rules, only the first one found is used. The rules you write take precedence over those that are built in. Note however, that a rule whose prerequisites actually exist or are mentioned always takes priority over a rule with prerequisites that must be made by chaining other implicit rules.
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Here are some examples of pattern rules actually predefined in
make. First, the rule that compiles `.c' files into `.o'
files:
%.o : %.c
$(CC) -c $(CFLAGS) $(CPPFLAGS) $< -o $@
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defines a rule that can make any file `x.o' from `x.c'. The command uses the automatic variables `$@' and `$<' to substitute the names of the target file and the source file in each case where the rule applies (see section Automatic Variables).
Here is a second built-in rule:
% :: RCS/%,v
$(CO) $(COFLAGS) $<
|
defines a rule that can make any file `x' whatsoever from a corresponding file `x,v' in the subdirectory `RCS'. Since the target is `%', this rule will apply to any file whatever, provided the appropriate prerequisite file exists. The double colon makes the rule terminal, which means that its prerequisite may not be an intermediate file (see section Match-Anything Pattern Rules).
This pattern rule has two targets:
%.tab.c %.tab.h: %.y
bison -d $<
|
This tells make that the command `bison -d x.y' will
make both `x.tab.c' and `x.tab.h'. If the file
`foo' depends on the files `parse.tab.o' and `scan.o'
and the file `scan.o' depends on the file `parse.tab.h',
when `parse.y' is changed, the command `bison -d parse.y'
will be executed only once, and the prerequisites of both
`parse.tab.o' and `scan.o' will be satisfied. (Presumably
the file `parse.tab.o' will be recompiled from `parse.tab.c'
and the file `scan.o' from `scan.c', while `foo' is
linked from `parse.tab.o', `scan.o', and its other
prerequisites, and it will execute happily ever after.)
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Suppose you are writing a pattern rule to compile a `.c' file into a `.o' file: how do you write the `cc' command so that it operates on the right source file name? You cannot write the name in the command, because the name is different each time the implicit rule is applied.
What you do is use a special feature of make, the automatic
variables. These variables have values computed afresh for each rule that
is executed, based on the target and prerequisites of the rule. In this
example, you would use `$@' for the object file name and `$<'
for the source file name.
Here is a table of automatic variables:
$@
$%
make to Update Archive Files. For example, if the target is `foo.a(bar.o)' then
`$%' is `bar.o' and `$@' is `foo.a'. `$%' is
empty when the target is not an archive member.
$<
$?
make to Update Archive Files).
$^
make to Update Archive Files). A target has only one prerequisite on each other file
it depends on, no matter how many times each file is listed as a
prerequisite. So if you list a prerequisite more than once for a target,
the value of $^ contains just one copy of the name.
$+
$*
In a static pattern rule, the stem is part of the file name that matched the `%' in the target pattern.
In an explicit rule, there is no stem; so `$*' cannot be determined
in that way. Instead, if the target name ends with a recognized suffix
(see section Old-Fashioned Suffix Rules), `$*' is set to
the target name minus the suffix. For example, if the target name is
`foo.c', then `$*' is set to `foo', since `.c' is a
suffix. GNU make does this bizarre thing only for compatibility
with other implementations of make. You should generally avoid
using `$*' except in implicit rules or static pattern rules.
If the target name in an explicit rule does not end with a recognized suffix, `$*' is set to the empty string for that rule.
`$?' is useful even in explicit rules when you wish to operate on only the prerequisites that have changed. For example, suppose that an archive named `lib' is supposed to contain copies of several object files. This rule copies just the changed object files into the archive:
lib: foo.o bar.o lose.o win.o
ar r lib $?
|
Of the variables listed above, four have values that are single file
names, and three have values that are lists of file names. These seven
have variants that get just the file's directory name or just the file
name within the directory. The variant variables' names are formed by
appending `D' or `F', respectively. These variants are
semi-obsolete in GNU make since the functions dir and
notdir can be used to get a similar effect (see section Functions for File Names). Note, however, that the
`D' variants all omit the trailing slash which always appears in
the output of the dir function. Here is a table of the variants:
Note that we use a special stylistic convention when we talk about these
automatic variables; we write "the value of `$<'", rather than
"the variable <" as we would write for ordinary variables
such as objects and CFLAGS. We think this convention
looks more natural in this special case. Please do not assume it has a
deep significance; `$<' refers to the variable named < just
as `$(CFLAGS)' refers to the variable named CFLAGS.
You could just as well use `$(<)' in place of `$<'.
GNU make provides support for the SysV make feature that
allows special variable references $$@, $$(@D), and
$$(@F) (note the required double-"$"!) to appear with the
prerequisites list (normal automatic variables are available
only within a command script). When appearing in a prerequisites
list, these variables are expanded to the name of the target, the
directory component of the target, and the file component of the
target, respectively.
Note that these variables are available only within explicit and
static pattern (see section Static Pattern Rules) rules;
they have no special significance within implicit (suffix or pattern)
rules. Also note that while SysV make actually expands its
entire prerequisite list twice, GNU make does not behave
this way: instead it simply expands these special variables without
re-expanding any other part of the prerequisites list.
This somewhat bizarre feature is included only to provide some
compatibility with SysV makefiles. In a native GNU make file
there are other ways to accomplish the same results. This feature is
disabled if the special pseudo target .POSIX is defined.
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A target pattern is composed of a `%' between a prefix and a suffix, either or both of which may be empty. The pattern matches a file name only if the file name starts with the prefix and ends with the suffix, without overlap. The text between the prefix and the suffix is called the stem. Thus, when the pattern `%.o' matches the file name `test.o', the stem is `test'. The pattern rule prerequisites are turned into actual file names by substituting the stem for the character `%'. Thus, if in the same example one of the prerequisites is written as `%.c', it expands to `test.c'.
When the target pattern does not contain a slash (and it usually does not), directory names in the file names are removed from the file name before it is compared with the target prefix and suffix. After the comparison of the file name to the target pattern, the directory names, along with the slash that ends them, are added on to the prerequisite file names generated from the pattern rule's prerequisite patterns and the file name. The directories are ignored only for the purpose of finding an implicit rule to use, not in the application of that rule. Thus, `e%t' matches the file name `src/eat', with `src/a' as the stem. When prerequisites are turned into file names, the directories from the stem are added at the front, while the rest of the stem is substituted for the `%'. The stem `src/a' with a prerequisite pattern `c%r' gives the file name `src/car'.
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When a pattern rule's target is just `%', it matches any file name
whatever. We call these rules match-anything rules. They are very
useful, but it can take a lot of time for make to think about them,
because it must consider every such rule for each file name listed either
as a target or as a prerequisite.
Suppose the makefile mentions `foo.c'. For this target, make
would have to consider making it by linking an object file `foo.c.o',
or by C compilation-and-linking in one step from `foo.c.c', or by
Pascal compilation-and-linking from `foo.c.p', and many other
possibilities.
We know these possibilities are ridiculous since `foo.c' is a C source
file, not an executable. If make did consider these possibilities,
it would ultimately reject them, because files such as `foo.c.o' and
`foo.c.p' would not exist. But these possibilities are so
numerous that make would run very slowly if it had to consider
them.
To gain speed, we have put various constraints on the way make
considers match-anything rules. There are two different constraints that
can be applied, and each time you define a match-anything rule you must
choose one or the other for that rule.
One choice is to mark the match-anything rule as terminal by defining it with a double colon. When a rule is terminal, it does not apply unless its prerequisites actually exist. Prerequisites that could be made with other implicit rules are not good enough. In other words, no further chaining is allowed beyond a terminal rule.
For example, the built-in implicit rules for extracting sources from RCS
and SCCS files are terminal; as a result, if the file `foo.c,v' does
not exist, make will not even consider trying to make it as an
intermediate file from `foo.c,v.o' or from `RCS/SCCS/s.foo.c,v'.
RCS and SCCS files are generally ultimate source files, which should not be
remade from any other files; therefore, make can save time by not
looking for ways to remake them.
If you do not mark the match-anything rule as terminal, then it is nonterminal. A nonterminal match-anything rule cannot apply to a file name that indicates a specific type of data. A file name indicates a specific type of data if some non-match-anything implicit rule target matches it.
For example, the file name `foo.c' matches the target for the pattern
rule `%.c : %.y' (the rule to run Yacc). Regardless of whether this
rule is actually applicable (which happens only if there is a file
`foo.y'), the fact that its target matches is enough to prevent
consideration of any nonterminal match-anything rules for the file
`foo.c'. Thus, make will not even consider trying to make
`foo.c' as an executable file from `foo.c.o', `foo.c.c',
`foo.c.p', etc.
The motivation for this constraint is that nonterminal match-anything rules are used for making files containing specific types of data (such as executable files) and a file name with a recognized suffix indicates some other specific type of data (such as a C source file).
Special built-in dummy pattern rules are provided solely to recognize certain file names so that nonterminal match-anything rules will not be considered. These dummy rules have no prerequisites and no commands, and they are ignored for all other purposes. For example, the built-in implicit rule
%.p : |
exists to make sure that Pascal source files such as `foo.p' match a specific target pattern and thereby prevent time from being wasted looking for `foo.p.o' or `foo.p.c'.
Dummy pattern rules such as the one for `%.p' are made for every suffix listed as valid for use in suffix rules (see section Old-Fashioned Suffix Rules).
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You can override a built-in implicit rule (or one you have defined yourself) by defining a new pattern rule with the same target and prerequisites, but different commands. When the new rule is defined, the built-in one is replaced. The new rule's position in the sequence of implicit rules is determined by where you write the new rule.
You can cancel a built-in implicit rule by defining a pattern rule with the same target and prerequisites, but no commands. For example, the following would cancel the rule that runs the assembler:
%.o : %.s |
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You can define a last-resort implicit rule by writing a terminal match-anything pattern rule with no prerequisites (see section 10.5.5 Match-Anything Pattern Rules). This is just like any other pattern rule; the only thing special about it is that it will match any target. So such a rule's commands are used for all targets and prerequisites that have no commands of their own and for which no other implicit rule applies.
For example, when testing a makefile, you might not care if the source files contain real data, only that they exist. Then you might do this:
%::
touch $@
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to cause all the source files needed (as prerequisites) to be created automatically.
You can instead define commands to be used for targets for which there
are no rules at all, even ones which don't specify commands. You do
this by writing a rule for the target .DEFAULT. Such a rule's
commands are used for all prerequisites which do not appear as targets in
any explicit rule, and for which no implicit rule applies. Naturally,
there is no .DEFAULT rule unless you write one.
If you use .DEFAULT with no commands or prerequisites:
.DEFAULT: |
the commands previously stored for .DEFAULT are cleared.
Then make acts as if you had never defined .DEFAULT at all.
If you do not want a target to get the commands from a match-anything
pattern rule or .DEFAULT, but you also do not want any commands
to be run for the target, you can give it empty commands (see section Defining Empty Commands).
You can use a last-resort rule to override part of another makefile. See section Overriding Part of Another Makefile.
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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 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 $@ $<
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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 $@ $<
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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 commands are also meaningless. They do not remove previous rules as do pattern rules with no commands (see section 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 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.
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Here is the procedure make uses for searching for an implicit rule
for a target t. This procedure is followed for each double-colon
rule with no commands, for each target of ordinary rules none of which have
commands, and for each prerequisite that is not the target of any rule. It
is also followed recursively for prerequisites that come from implicit
rules, in the search for a chain of rules.
Suffix rules are not mentioned in this algorithm because suffix rules are converted to equivalent pattern rules once the makefiles have been read in.
For an archive member target of the form `archive(member)', the following algorithm is run twice, first using the entire target name t, and second using `(member)' as the target t if the first run found no rule.
If all prerequisites exist or ought to exist, or there are no prerequisites, then this rule applies.
.DEFAULT, if any,
applies. In that case, give t the same commands that
.DEFAULT has. Otherwise, there are no commands for t.
Once a rule that applies has been found, for each target pattern of the rule other than the one that matched t or n, the `%' in the pattern is replaced with s and the resultant file name is stored until the commands to remake the target file t are executed. After these commands are executed, each of these stored file names are entered into the data base and marked as having been updated and having the same update status as the file t.
When the commands of a pattern rule are executed for t, the automatic variables are set corresponding to the target and prerequisites. See section Automatic Variables.
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