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</P>
<P>
A <EM>rule</EM> appears in the makefile and says when and how to remake
certain files, called the rule's <EM>targets</EM> (most often only one per rule).
It lists the other files that are the <EM>dependencies</EM> of the target, and
<EM>commands</EM> to use to create or update the target.

</P>
<P>
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The order of rules is not significant, except for determining the
<EM>default goal</EM>: the target for <CODE>make</CODE> to consider, if you do
not otherwise specify one.  The default goal is the target of the first
rule in the first makefile.  If the first rule has multiple targets,
only the first target is taken as the default.  There are two
exceptions: a target starting with a period is not a default unless it
contains one or more slashes, <SAMP>`/'</SAMP>, as well; and, a target that
defines a pattern rule has no effect on the default goal.
(See section <A HREF="make.html#SEC91">Defining and Redefining Pattern Rules</A>.)

</P>
<P>
Therefore, we usually write the makefile so that the first rule is the
one for compiling the entire program or all the programs described by
the makefile (often with a target called <SAMP>`all'</SAMP>).
See section <A HREF="make.html#SEC80">Arguments to Specify the Goals</A>.

</P>



<H2><A NAME="SEC20" HREF="make_toc.html#TOC20">Rule Syntax</A></H2>

<P>
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In general, a rule looks like this:

</P>

<PRE>
<VAR>targets</VAR> : <VAR>dependencies</VAR>
        <VAR>command</VAR>
        ...
</PRE>

<P>
or like this:

</P>

<PRE>
<VAR>targets</VAR> : <VAR>dependencies</VAR> ; <VAR>command</VAR>
        <VAR>command</VAR>
        ...
</PRE>

<P>
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The <VAR>targets</VAR> are file names, separated by spaces.  Wildcard
characters may be used (see section <A HREF="make.html#SEC21">Using Wildcard Characters in File Names</A>) and a name of the form <TT>`<VAR>a</VAR>(<VAR>m</VAR>)'</TT>
represents member <VAR>m</VAR> in archive file <VAR>a</VAR> 
(see section <A HREF="make.html#SEC102">Archive Members as Targets</A>).  
Usually there is only one
target per rule, but occasionally there is a reason to have more
(see section <A HREF="make.html#SEC35">Multiple Targets in a Rule</A>).
</P>
<P>
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The <VAR>command</VAR> lines start with a tab character.  The first command may
appear on the line after the dependencies, with a tab character, or may
appear on the same line, with a semicolon.  Either way, the effect is the
same.  See section <A HREF="make.html#SEC42">Writing the Commands in Rules</A>.

</P>
<P>
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Because dollar signs are used to start variable references, if you really
want a dollar sign in a rule you must write two of them, <SAMP>`$$'</SAMP>
(see section <A HREF="make.html#SEC55">How to Use Variables</A>).  
You may split a long line by inserting a backslash
followed by a newline, but this is not required, as <CODE>make</CODE> places no
limit on the length of a line in a makefile.

</P>
<P>
A rule tells <CODE>make</CODE> two things: when the targets are out of date,
and how to update them when necessary.

</P>
<P>
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The criterion for being out of date is specified in terms of the
<VAR>dependencies</VAR>, which consist of file names separated by spaces.
(Wildcards and archive members (see section <A HREF="make.html#SEC101">Using <CODE>make</CODE> to Update Archive Files</A>) are allowed here too.)
A target is out of date if it does not exist or if it is older than any
of the dependencies (by comparison of last-modification times).  The
idea is that the contents of the target file are computed based on
information in the dependencies, so if any of the dependencies changes,
the contents of the existing target file are no longer necessarily
valid.

</P>
<P>
How to update is specified by <VAR>commands</VAR>.  These are lines to be
executed by the shell (normally <SAMP>`sh'</SAMP>), but with some extra features
(see section <A HREF="make.html#SEC42">Writing the Commands in Rules</A>).

</P>


<H2><A NAME="SEC21" HREF="make_toc.html#TOC21">Using Wildcard Characters in File Names</A></H2>
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</P>
<P>
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A single file name can specify many files using <EM>wildcard characters</EM>.
The wildcard characters in <CODE>make</CODE> are <SAMP>`*'</SAMP>, <SAMP>`?'</SAMP> and
<SAMP>`[...]'</SAMP>, the same as in the Bourne shell.  For example, <TT>`*.c'</TT>
specifies a list of all the files (in the working directory) whose names
end in <SAMP>`.c'</SAMP>.
</P>
<P>
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The character <SAMP>`~'</SAMP> at the beginning of a file name also has special
significance.  If alone, or followed by a slash, it represents your home
directory.  For example <TT>`~/bin'</TT> expands to <TT>`/home/you/bin'</TT>.
If the <SAMP>`~'</SAMP> is followed by a word, the string represents the home
directory of the user named by that word.  For example <TT>`~john/bin'</TT>
expands to <TT>`/home/john/bin'</TT>.
</P>
<P>
Wildcard expansion happens automatically in targets, in dependencies,
and in commands (where the shell does the expansion).  In other
contexts, wildcard expansion happens only if you request it explicitly
with the <CODE>wildcard</CODE> function.

</P>
<P>
The special significance of a wildcard character can be turned off by
preceding it with a backslash.  Thus, <TT>`foo\*bar'</TT> would refer to a
specific file whose name consists of <SAMP>`foo'</SAMP>, an asterisk, and
<SAMP>`bar'</SAMP>.
</P>



<H3><A NAME="SEC22" HREF="make_toc.html#TOC22">Wildcard Examples</A></H3>

<P>
Wildcards can be used in the commands of a rule, where they are expanded
by the shell.  For example, here is a rule to delete all the object files:

</P>

<PRE>
clean:
        rm -f *.o
</PRE>

<P>
<A NAME="IDX122"></A>

</P>
<P>
Wildcards are also useful in the dependencies of a rule.  With the
following rule in the makefile, <SAMP>`make print'</SAMP> will print all the
<SAMP>`.c'</SAMP> files that have changed since the last time you printed them:

</P>

<PRE>
print: *.c
        lpr -p $?
        touch print
</PRE>

<P>
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This rule uses <TT>`print'</TT> as an empty target file; see section <A HREF="make.html#SEC33">Empty Target Files to Record Events</A>.  (The automatic variable
<SAMP>`$?'</SAMP> is used to print only those files that have changed; see
section <A HREF="make.html#SEC94">Automatic Variables</A>.)
</P>
<P>
Wildcard expansion does not happen when you define a variable.  Thus, if
you write this:

</P>

<PRE>
objects = *.o
</PRE>

<P>
then the value of the variable <CODE>objects</CODE> is the actual string
<SAMP>`*.o'</SAMP>.  However, if you use the value of <CODE>objects</CODE> in a target,
dependency or command, wildcard expansion will take place at that time.
To set <CODE>objects</CODE> to the expansion, instead use:

</P>

<PRE>
objects := $(wildcard *.o)
</PRE>

<P>
See section <A HREF="make.html#SEC24">The Function <CODE>wildcard</CODE></A>.

</P>


<H3><A NAME="SEC23" HREF="make_toc.html#TOC23">Pitfalls of Using Wildcards</A></H3>
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</P>
<P>
Now here is an example of a naive way of using wildcard expansion, that
does not do what you would intend.  Suppose you would like to say that the
executable file <TT>`foo'</TT> is made from all the object files in the
directory, and you write this:

</P>

<PRE>
objects = *.o

foo : $(objects)
        cc -o foo $(CFLAGS) $(objects)
</PRE>

<P>
The value of <CODE>objects</CODE> is the actual string <SAMP>`*.o'</SAMP>.  Wildcard
expansion happens in the rule for <TT>`foo'</TT>, so that each <EM>existing</EM>
<SAMP>`.o'</SAMP> file becomes a dependency of <TT>`foo'</TT> and will be recompiled if
necessary.

</P>
<P>
But what if you delete all the <SAMP>`.o'</SAMP> files?  When a wildcard matches
no files, it is left as it is, so then <TT>`foo'</TT> will depend on the
oddly-named file <TT>`*.o'</TT>.  Since no such file is likely to exist,
<CODE>make</CODE> will give you an error saying it cannot figure out how to
make <TT>`*.o'</TT>.  This is not what you want!

</P>
<P>
Actually it is possible to obtain the desired result with wildcard
expansion, but you need more sophisticated techniques, including the
<CODE>wildcard</CODE> function and string substitution.
These are described in the following section.

</P>


<H3><A NAME="SEC24" HREF="make_toc.html#TOC24">The Function <CODE>wildcard</CODE></A></H3>
<P>
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</P>
<P>
Wildcard expansion happens automatically in rules.  But wildcard expansion
does not normally take place when a variable is set, or inside the
arguments of a function.  If you want to do wildcard expansion in such
places, you need to use the <CODE>wildcard</CODE> function, like this:

</P>

<PRE>
$(wildcard <VAR>pattern</VAR>...)
</PRE>

<P>
This string, used anywhere in a makefile, is replaced by a
space-separated list of names of existing files that match one of the
given file name patterns.  If no existing file name matches a pattern,
then that pattern is omitted from the output of the <CODE>wildcard</CODE>
function.  Note that this is different from how unmatched wildcards
behave in rules, where they are used verbatim rather than ignored
(see section <A HREF="make.html#SEC23">Pitfalls of Using Wildcards</A>).

</P>
<P>
One use of the <CODE>wildcard</CODE> function is to get a list of all the C source
files in a directory, like this:

</P>

<PRE>
$(wildcard *.c)
</PRE>

<P>
We can change the list of C source files into a list of object files by
replacing the <SAMP>`.o'</SAMP> suffix with <SAMP>`.c'</SAMP> in the result, like this:

</P>

<PRE>
$(patsubst %.c,%.o,$(wildcard *.c))
</PRE>

<P>
(Here we have used another function, <CODE>patsubst</CODE>.
See section <A HREF="make.html#SEC73">Functions for String Substitution<BR> and Analysis</A>.)
</P>
<P>
Thus, a makefile to compile all C source files in the directory and then
link them together could be written as follows:

</P>

<PRE>
objects := $(patsubst %.c,%.o,$(wildcard *.c))

foo : $(objects)
        cc -o foo $(objects)
</PRE>

<P>
(This takes advantage of the implicit rule for compiling C programs, so
there is no need to write explicit rules for compiling the files.
See section <A HREF="make.html#SEC57">The Two Flavors of Variables</A>, for an explanation of
<SAMP>`:='</SAMP>, which is a variant of <SAMP>`='</SAMP>.)

</P>


<H2><A NAME="SEC25" HREF="make_toc.html#TOC25">Searching Directories for Dependencies</A></H2>
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</P>
<P>
For large systems, it is often desirable to put sources in a separate
directory from the binaries.  The <EM>directory search</EM> features of
<CODE>make</CODE> facilitate this by searching several directories
automatically to find a dependency.  When you redistribute the files
among directories, you do not need to change the individual rules,
just the search paths.

</P>



<H3><A NAME="SEC26" HREF="make_toc.html#TOC26"><CODE>VPATH</CODE>: Search Path for All Dependencies</A></H3>
<P>
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</P>
<P>
The value of the <CODE>make</CODE> variable <CODE>VPATH</CODE> specifies a list of
directories that <CODE>make</CODE> should search.  Most often, the
directories are expected to contain dependency files that are not in the
current directory; however, <CODE>VPATH</CODE> specifies a search list that
<CODE>make</CODE> applies for all files, including files which are targets of
rules.

</P>
<P>
Thus, if a file that is listed as a target or dependency does not exist
in the current directory, <CODE>make</CODE> searches the directories listed in
<CODE>VPATH</CODE> for a file with that name.  If a file is found in one of
them, that file becomes the dependency.  Rules may then specify the
names of source files in the dependencies as if they all existed in the
current directory.  See section <A HREF="make.html#SEC28">Writing Shell Commands with Directory Search</A>.

</P>
<P>
In the <CODE>VPATH</CODE> variable, directory names are separated by colons or
blanks.  The order in which directories are listed is the order followed
by <CODE>make</CODE> in its search.

</P>
<P>
For example,