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Chapter 8  Rules

Rules are used by OMake to specify how to build files. At its simplest, a rule has the following form.

    <target>: <dependencies>

The <target> is the name of a file to be built. The <dependencies> are a list of files that are needed before the <target> can be built. The <commands> are a list of indented lines specifying commands to build the target. For example, the following rule specifies how to compile a file hello.c.

    hello.o: hello.c
        $(CC) $(CFLAGS) -c -o hello.o hello.c

This rule states that the hello.o file depends on the hello.c file. If the hello.c file has changed, the command $(CC) $(CFLAGS) -c -o hello.o hello.c is to be executed to update the target file hello.o.

A rule can have an arbitrary number of commands. The individual command lines are executed independently by the command shell. The commands do not have to begin with a tab, but they must be indented from the dependency line.

In addition to normal variables, the following special variables may be used in the body of a rule.

  • $*: the target name, without a suffix.
  • $@: the target name.
  • $^: a list of the sources, in alphabetical order, with duplicates removed.
  • $+: all the sources, in the original order.
  • $<: the first source.

For example, the above hello.c rule may be simplified as follows.

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

Unlike normal values, the variables in a rule body are expanded lazily, and binding is dynamic. The following function definition illustrates some of the issues.

    CLibrary(name, files) =
        OFILES = $(addsuffix .o, $(files))

        $(name).a: $(OFILES)
            $(AR) cq $@ $(OFILES)

This function defines a rule to build a program called $(name) from a list of .o files. The files in the argument are specified without a suffix, so the first line of the function definition defines a variable OFILES that adds the .o suffix to each of the file names. The next step defines a rule to build a target library $(name).a from the $(OFILES) files. The expression $(AR) is evaluated when the function is called, and the value of the variable AR is taken from the caller’s scope (see also the section on Scoping).

8.1  Implicit rules

Rules may also be implicit. That is, the files may be specified by wildcard patterns. The wildcard character is %. For example, the following rule specifies a default rule for building .o files.

    %.o: %.c
        $(CC) $(CFLAGS) -c -o $@ $*.c

This rule is a template for building an arbitrary .o file from a .c file.

By default, implicit rules are only used for the targets in the current directory. However subdirectories included via the .SUBDIRS rules inherit all the implicit rules that are in scope (see also the section on Scoping).

8.2  Bounded implicit rules

Implicit rules may specify the set of files they apply to. The following syntax is used.

    <targets>: <pattern>: <dependencies>

For example, the following rule applies only to the files a.o and b.o.

   a.o b.o: %.o: %.c
        $(CC) $(CFLAGS) -DSPECIAL -c $*.c

8.3  section

Frequently, the commands in a rule body are expressions to be evaluated by the shell. omake also allows expressions to be evaluated by omake itself.

The syntax of these “computed rules” uses the section expression. The following rule uses the omake IO functions to produce the target hello.c.

            FP = fopen(hello.c, w)
            fprintln($(FP), $""#include <stdio.h> int main() { printf("Hello world\n"); }"")

This example uses the quotation $""..."" (see also Section B.1.6) to quote the text being printed. These quotes are not included in the output file. The fopen, fprintln, and close functions perform file IO as discussed in the IO section.

In addition, commands that are function calls, or special expressions, are interpreted correctly. Since the fprintln function can take a file directly, the above rule can be abbreviated as follows.

       fprintln($@, $""#include <stdio.h> int main() { printf("Hello world\n"); }"")

8.4  section rule

Rules can also be computed using the section rule form, where a rule body is expected instead of an expression. In the following rule, the file a.c is copied onto the hello.c file if it exists, otherwise hello.c is created from the file default.c.

        section rule
           if $(target-exists a.c)
              hello.c: a.c
                 cat a.c > hello.c
              hello.c: default.c
                 cp default.c hello.c

8.5  Special dependencies

8.5.1  :exists:

In some cases, the contents of a dependency do not matter, only whether the file exists or not. In this case, the :exists: qualifier can be used for the dependency.

    foo.c: a.c :exists: .flag
       if $(test -e .flag)
           $(CP) a.c $@

8.5.2  :effects:

Some commands produce files by side-effect. For example, the latex(1) command produces a .aux file as a side-effect of producing a .dvi file. In this case, the :effects: qualifier can be used to list the side-effect explicitly. omake is careful to avoid simultaneously running programs that have overlapping side-effects.

    paper.dvi: paper.tex :effects: paper.aux
        latex paper

8.5.3  :value:

The :value: dependency is used to specify that the rule execution depends on the value of an expression. For example, the following rule

    a: b c :value: $(X)

specifies that “a” should be recompiled if the value of $(X) changes (X does not have to be a filename). This is intended to allow greater control over dependencies.

In addition, it can be used instead of other kinds of dependencies. For example, the following rule:

    a: b :exists: c

is the same as

    a: b :value: $(target-exists c)


  • The values are arbitrary (they are not limited to variables)
  • The values are evaluated at rule expansion time, so expressions containing variables like $@, $^, etc are legal.

8.6  .SCANNER rules

Scanner rules define a way to specify automatic dependency scanning. A .SCANNER rule has the following form.

    .SCANNER: target: dependencies

The rule is used to compute additional dependencies that might be defined in the source files for the specified target. The result of executing the scanner commands must be a sequence of dependencies in OMake format, printed to the standard output. For example, on GNU systems the gcc -MM foo.c produces dependencies for the file foo.c (based on #include information).

We can use this to specify a scanner for C files that adds the scanned dependencies for the .o file. The following scanner specifies that dependencies for a file, say foo.o can be computed by running gcc -MM foo.c. Furthermore, foo.c is a dependency, so the scanner should be recomputed whenever the foo.c file changes.

    .SCANNER: %.o: %.c
        gcc -MM $<

Let’s suppose that the command gcc -MM foo.c prints the following line.

    foo.o: foo.h /usr/include/stdio.h

The result is that the files foo.h and /usr/include/stdio.h are considered to be dependencies of foo.o—that is, foo.o should be rebuilt if either of these files changes.

This works, to an extent. One nice feature is that the scanner will be re-run whenever the foo.c file changes. However, one problem is that dependencies in C are recursive. That is, if the file foo.h is modified, it might include other files, establishing further dependencies. What we need is to re-run the scanner if foo.h changes too.

We can do this with a value dependency. The variable $& is defined as the dependency results from any previous scan. We can add these as dependencies using the digest function, which computes an MD5 digest of the files.

    .SCANNER: %.o: %.c :value: $(digest $&)
        gcc -MM $<

Now, when the file foo.h changes, its digest will also change, and the scanner will be re-run because of the value dependency (since $& will include foo.h).

This still is not quite right. The problem is that the C compiler uses a search-path for include files. There may be several versions of the file foo.h, and the one that is chosen depends on the include path. What we need is to base the dependencies on the search path.

The $(digest-in-path-optional ...) function computes the digest based on a search path, giving us a solution that works.

    .SCANNER: %.o: %.c :value: $(digest-in-path-optional $(INCLUDES), $&)
       gcc -MM $(addprefix -I, $(INCLUDES)) $<

The standard output of the scanner rules will be captured by OMake and is not allowed to contain any content that OMake will not be able to parse as a dependency. The output is allowed to contain dependency specifications for unrelated targets, however such dependencies will be ignored. The scanner rules are allowed to produce arbitrary output on the standard error channel — such output will be handled in the same way as the output of the ordinary rules (in other words, it will be presented to the user, when dictated by the --output-… options enabled).

Additional examples of the .SCANNER rules can be found in Section 3.4.3.

8.6.1  Named scanners, and the :scanner: dependencies

Sometimes it may be useful to specify explicitly which scanner should be used in a rule. For example, we might compile .c files with different options, or (heaven help us) we may be using both gcc and the Microsoft Visual C++ compiler cl. In general, the target of a .SCANNER is not tied to a particular target, and we may name it as we like.

    .SCANNER: scan-gcc-%.c: %.c :value: $(digest-in-path-optional $(INCLUDES), $&)
        gcc -MM $(addprefix -I, $(INCLUDES)) $<

    .SCANNER: scan-cl-%.c: %.c :value: $(digest-in-path-optional $(INCLUDES), $&)
        cl --scan-dependencies-or-something $(addprefix /I, $(INCLUDES)) $<

The next step is to define explicit scanner dependencies. The :scanner: dependency is used for this. In this case, the scanner dependencies are specified explicitly.

    $(GCC_FILES): %.o: %.c :scanner: scan-gcc-%.c
        gcc ...

    $(CL_FILES): %.obj: %.c :scanner: scan-cl-%.c
        cl ...

Explicit :scanner: scanner specification may also be used to state that a single .SCANNER rule should be used to generate dependencies for more than one target. For example,

    .SCANNER: scan-all-c: $(GCC_FILES) :value: $(digest-in-path-optional $(INCLUDES), $&)
        gcc -MM $(addprefix -I, $(INCLUDES)) $(GCC_FILES)

    $(GCC_FILES): %.o: %.c :scanner: scan-all-c

The above has the advantage of only running gcc once and a disadvantage that when a single source file changes, all the files will end up being re-scanned.

8.6.2  Notes

In most cases, you won’t need to define scanners of your own. The standard installation includes default scanners (both explicitly and implicitly named ones) for C, OCaml, and LATEX files.

The SCANNER_MODE variable controls the usage of implicit scanner dependencies.

The explicit :scanner: dependencies reduce the chances of scanner mis-specifications. In large complicated projects it might be a good idea to set SCANNER_MODE to error and use only the named .SCANNER rules and explicit :scanner: specifications.


The .DEFAULT target specifies a target to be built by default if omake is run without explicit targets. The following rule instructs omake to build the program hello by default

   .DEFAULT: hello


The .SUBDIRS target is used to specify a set of subdirectories that are part of the project. Each subdirectory should have its own OMakefile, which is evaluated in the context of the current environment.

   .SUBDIRS: src doc tests

This rule specifies that the OMakefiles in each of the src, doc, and tests directories should be read.

In some cases, especially when the OMakefiles are very similar in a large number of subdirectories, it is inconvenient to have a separate OMakefile for each directory. If the .SUBDIRS rule has a body, the body is used instead of the OMakefile.

   .SUBDIRS: src1 src2 src3
      println(Subdirectory $(CWD))
      .DEFAULT: lib.a

In this case, the src1, src2, and src3 files do not need OMakefiles. Furthermore, if one exists, it is ignored. The following includes the file if it exists.

   .SUBDIRS: src1 src2 src3
       if $(file-exists OMakefile)
          include OMakefile
       .DEFAULT: lib.a


The .INCLUDE target is like the include directive, but it specifies a rule to build the file if it does not exist.

   .INCLUDE: config
       echo "CONFIG_READ = true" > config


You may also specify dependencies to an .INCLUDE rule.

   .INCLUDE: config: config.defaults
      cp config.defaults config

A word of caution is in order here. The usual policy is used for determining when the rule is out-of-date. The rule is executed if any of the following hold.

  • the target does not exist,
  • the rule has never been executed before,
  • any of the following have changed since the last time the rule was executed,
    • the target,
    • the dependencies,
    • the commands-text.

In some of the cases, this will mean that the rule is executed even if the target file already exists. If the target is a file that you expect to edit by hand (and therefore you don’t want to overwrite it), you should make the rule evaluation conditional on whether the target already exists.

   .INCLUDE: config: config.defaults
       # Don't overwrite my carefully hand-edited file
       if $(not $(file-exists config))
          cp config.defaults config

8.10  .PHONY

A “phony” target is a target that is not a real file, but exists to collect a set of dependencies. Phony targets are specified with the .PHONY rule. In the following example, the install target does not correspond to a file, but it corresponds to some commands that should be run whenever the install target is built (for example, by running omake install).

   .PHONY: install

   install: myprogram.exe
      cp myprogram.exe /usr/bin

8.11  Rule scoping

As we have mentioned before, omake is a scoped language. This provides great flexibility—different parts of the project can define different configurations without interfering with one another (for example, one part of the project might be compiled with CFLAGS=-O3 and another with CFLAGS=-g).

But how is the scope for a target file selected? Suppose we are building a file dir/foo.o. omake uses the following rules to determine the scope.

  • First, if there is an explicit rule for building dir/foo.o (a rule with no wildcards), the context for that rule determines the scope for building the target.
  • Otherwise, the directory dir/ must be part of the project. This normally means that a configuration file dir/OMakefile exists (although, see the .SUBDIRS section for another way to specify the OMakefile). In this case, the scope of the target is the scope at the end of the dir/OMakefile.

To illustrate rule scoping, let’s go back to the example of a “Hello world” program with two files. Here is an example OMakefile (the two definitions of CFLAGS are for illustration).

    # The executable is compiled with debugging
    CFLAGS = -g
    hello: hello_code.o hello_lib.o
       $(CC) $(CFLAGS) -o $@ $+

    # Redefine CFLAGS
    CFLAGS += -O3

In this project, the target hello is explicit. The scope of the hello target is the line beginning with hello:, where the value of CFLAGS is -g. The other two targets, hello_code.o and hello_lib.o do not appear as explicit targets, so their scope is at the end of the OMakefile, where the CFLAGS variable is defined to be -g -O3. That is, hello will be linked with CFLAGS=-g and the .o files will be compiled with CFLAGS=-g -O3.

We can change this behavior for any of the targets by specifying them as explicit targets. For example, suppose we wish to compile hello_lib.o with a preprocessor variable LIBRARY.

    # The executable is compiled with debugging
    CFLAGS = -g
    hello: hello_code.o hello_lib.o
       $(CC) $(CFLAGS) -o $@ $+

    # Compile hello_lib.o with CFLAGS = -g -DLIBRARY

    # Redefine CFLAGS
    CFLAGS += -O3

In this case, hello_lib.o is also mentioned as an explicit target, in a scope where CFLAGS=-g -DLIBRARY. Since no rule body is specified, it is compiled using the usual implicit rule for building .o files (in a context where CFLAGS=-g -DLIBRARY).

8.11.1  Scoping of implicit rules

Implicit rules (rules containing wildcard patterns) are not global, they follow the normal scoping convention. This allows different parts of a project to have different sets of implicit rules. If we like, we can modify the example above to provide a new implicit rule for building hello_lib.o.

    # The executable is compiled with debugging
    CFLAGS = -g
    hello: hello_code.o hello_lib.o
       $(CC) $(CFLAGS) -o $@ $+

    # Compile hello_lib.o with CFLAGS = -g -DLIBRARY
        %.o: %.c
            $(CC) $(CFLAGS) -DLIBRARY -c $<

    # Redefine CFLAGS
    CFLAGS += -O3

In this case, the target hello_lib.o is built in a scope with a new implicit rule for building %.o files. The implicit rule adds the -DLIBRARY option. This implicit rule is defined only for the target hello_lib.o; the target hello_code.o is built as normal.

8.11.2  Scoping of .SCANNER rules

Scanner rules are scoped the same way as normal rules. If the .SCANNER rule is explicit (containing no wildcard patterns), then the scope of the scan target is the same as the the rule. If the .SCANNER rule is implicit, then the environment is taken from the :scanner: dependency.

    # The executable is compiled with debugging
    CFLAGS = -g
    hello: hello_code.o hello_lib.o
       $(CC) $(CFLAGS) -o $@ $+

    # scanner for .c files
    .SCANNER: scan-c-%.c: %.c
       $(CC) $(CFLAGS) -MM $<

    # Compile hello_lib.o with CFLAGS = -g -DLIBRARY
        hello_lib.o: hello_lib.c :scanner: scan-c-hello_lib.c
           $(CC) $(CFLAGS) -c $<

    # Compile hello_code.c with CFLAGS = -g -O3
        CFLAGS += -O3
        hello_code.o: hello_code.c :scanner: scan-c-hello_code.c
           $(CC) $(CFLAGS) -c $<

Again, this is for illustration—it is unlikely you would need to write a complicated configuration like this! In this case, the .SCANNER rule specifies that the C-compiler should be called with the -MM flag to compute dependencies. For the target hello_lib.o, the scanner is called with CFLAGS=-g -DLIBRARY, and for hello_code.o it is called with CFLAGS=-g -O3.

8.11.3  Scoping for .PHONY targets

Phony targets (targets that do not correspond to files) are defined with a .PHONY: rule. Phony targets are scoped as usual. The following illustrates a common mistake, where the .PHONY target is declared after it is used.

    # !!This example is broken!!
    all: hello

    hello: hello_code.o hello_lib.o
        $(CC) $(CFLAGS) -o $@ $+

    .PHONY: all

This doesn’t work as expected because the .PHONY declaration occurs too late. The proper way to write this example is to place the .PHONY declaration first.

    # Phony targets must be declared before being used
    .PHONY: all

    all: hello

    hello: hello_code.o hello_lib.o
        $(CC) $(CFLAGS) -o $@ $+

Phony targets are passed to subdirectories. As a practical matter, it is wise to declare all .PHONY targets in your root OMakefile, before any .SUBDIRS. This will ensure that 1) they are considered as phony targets in each of the subdirectories, and 2) you can build them from the project root.

    .PHONY: all install clean

    .SUBDIRS: src lib clib

Note that when a .PHONY target is inherited by a subdirectory via a .SUBDIRS, a whole hierarchy of .PHONY targets (that are a part of the global one) is created, as described in Section 8.12.2 below.

8.12  Running OMake from a subdirectory

Running omake foo asks OMake to build the file foo in context of the whole project, even when running from a subdirectory of the project. Therefore, if bar/baz is a regular target (not a .PHONY one), then running omake bar/baz and running (cd bar; omake baz) are usually equivalent.

There are two noteworthy exceptions to the above rule:

  • If the subdirectory is not a part of the project (there is no .SUBDIRS) for it, then OMake will complain if you try to run it in that directory.
  • If a subdirectory contains an OMakeroot of its own, this would designate the subdirectory as a separate project (which is usually a bad idea and is not recommended).

8.12.1  Phony targets in a subdirectory

Suppose you have a .PHONY: clean declared in your root OMakefile and both the root OMakefile and the OMakefile in some of the subdirectories contain clean: rules. In this case

  • Running omake clean in the root directory will execute all the rules (each in the appropriate directory);
  • Running omake clean in the subdirectory will execute just its local one, as well as the ones from the subdirectories of the current directory.

The above equally applies to the built-in .PHONY targets, including .DEFAULT. Namely, if OMake is executed (without argument) in the root directory of a project, all the .DEFAULT targets in the project will be built. On the other hand, when OMake is executed (without argument) in a subdirectory, only the .DEFAULT targets defined in and under that subdirectory will be built.

The following Section explains the underlying semantics that gives rise to the above behavior.

8.12.2  Hierarchy of .PHONY targets

When the the root OMakefile contains a .PHONY: clean directive, it creates:

  • A “global” phony target /.PHONY/clean (note the leading “/”);
  • A “relative” phony target attached to the current directory — .PHONY/clean (note the lack of the leading “/”);
  • A dependency /.PHONY/clean: .PHONY/clean.

All the clean: ... rules in the root OMakefile following this .PHONY: clean declaration would be interpreted as rules for the .PHONY/clean target.

Now when OMake then comes across a .SUBDIRS: foo directive (when it is in scope of the above .PHONY: clean declaration), it does the following:

  • Creates a new .PHONY/foo/clean “relative” phony target;
  • Creates the dependency .PHONY/clean: .PHONY/foo/clean;
  • Processes the body of the .SUBDIRS: foo directive, or reads the foo/OMakefile file, if the body is empty. While doing that, it interprets its instructions relative to the foo directory. In particular, all the clean: ... rules will be taken to apply to .PHONY/foo/clean.

Now when you run omake clean in the root directory of the project, it is interpreted as omake .PHONY/clean (similar to how it happens with the normal targets), so both the rules for .PHONY/clean are executed and the rules for its dependency .PHONY/foo/clean. Running (cd foo; omake clean) is, as for normal targets, equivalent to running omake .PHONY/foo/clean and only those rules that apply to .PHONY/foo/clean will be executed.

8.13  Pathnames in rules

In rules, the targets and dependencies are first translated to file values (as in the file function). They are then translated to strings for the command line. This can cause some unexpected behavior. In the following example, the absname function is the absolute pathname for the file a, but the rule still prints the relative pathname.

    .PHONY: demo
    demo: $(absname a)
        echo $<

    # omake demo

There is arguably a good reason for this. On Win32 systems, the / character is viewed as an “option specifier.” The pathname separator is the \ character. OMake translates the filenames automatically so that things work as expected on both systems.

   demo: a/b
       echo $<

   # omake demo (on a Unix system)
   # omake demo (on a Win32 system)

Sometimes you may wish that target strings to be passed literally to the commands in the rule. One way to do this is to specify them literally.

    SRC = a/b $(absname c/d)
    demo: $(SRC)
        echo $(SRC)

    # omake demo (on a Win32 system)
    a/b c:\...\c\d

Alternately, you might wish that filenames be automatically expanded to absolute pathnames. For example, this might be useful when parsing the OMake output to look for errors. For this, you can use the --absname option (Section A.3.20). If you call omake with the --absname option, all filenames will be expanded to absolute names.

    # omake --absname demo (on a Unix system)
    /home/.../a/b /home/.../c/d

Alternately, the --absname option is scoped. If you want to use it for only a few rules, you can use the OMakeFlags function to control how it is applied.

      demo: a
          echo $<

   # omake demo

N.B. The --absname option is currently an experimental feature.

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