File: make.info,  Node: Features,  Next: Missing,  Prev: Integrating make,  Up: Top

14 Features of GNU 'make'
*************************

Here is a summary of the features of GNU 'make', for comparison with and
credit to other versions of 'make'.  We consider the features of 'make'
in 4.2 BSD systems as a baseline.  If you are concerned with writing
portable makefiles, you should not use the features of 'make' listed
here, nor the ones in *note Missing::.

   Many features come from the version of 'make' in System V.

   * The 'VPATH' variable and its special meaning.  *Note Searching
     Directories for Prerequisites: Directory Search.  This feature
     exists in System V 'make', but is undocumented.  It is documented
     in 4.3 BSD 'make' (which says it mimics System V's 'VPATH'
     feature).

   * Included makefiles.  *Note Including Other Makefiles: Include.
     Allowing multiple files to be included with a single directive is a
     GNU extension.

   * Variables are read from and communicated via the environment.
     *Note Variables from the Environment: Environment.

   * Options passed through the variable 'MAKEFLAGS' to recursive
     invocations of 'make'.  *Note Communicating Options to a
     Sub-'make': Options/Recursion.

   * The automatic variable '$%' is set to the member name in an archive
     reference.  *Note Automatic Variables::.

   * The automatic variables '$@', '$*', '$<', '$%', and '$?' have
     corresponding forms like '$(@F)' and '$(@D)'.  We have generalized
     this to '$^' as an obvious extension.  *Note Automatic Variables::.

   * Substitution variable references.  *Note Basics of Variable
     References: Reference.

   * The command line options '-b' and '-m', accepted and ignored.  In
     System V 'make', these options actually do something.

   * Execution of recursive commands to run 'make' via the variable
     'MAKE' even if '-n', '-q' or '-t' is specified.  *Note Recursive
     Use of 'make': Recursion.

   * Support for suffix '.a' in suffix rules.  *Note Archive Suffix
     Rules::.  This feature is obsolete in GNU 'make', because the
     general feature of rule chaining (*note Chains of Implicit Rules:
     Chained Rules.) allows one pattern rule for installing members in
     an archive (*note Archive Update::) to be sufficient.

   * The arrangement of lines and backslash/newline combinations in
     recipes is retained when the recipes are printed, so they appear as
     they do in the makefile, except for the stripping of initial
     whitespace.

   The following features were inspired by various other versions of
'make'.  In some cases it is unclear exactly which versions inspired
which others.

   * Pattern rules using '%'.  This has been implemented in several
     versions of 'make'.  We're not sure who invented it first, but it's
     been spread around a bit.  *Note Defining and Redefining Pattern
     Rules: Pattern Rules.

   * Rule chaining and implicit intermediate files.  This was
     implemented by Stu Feldman in his version of 'make' for AT&T Eighth
     Edition Research Unix, and later by Andrew Hume of AT&T Bell Labs
     in his 'mk' program (where he terms it "transitive closure").  We
     do not really know if we got this from either of them or thought it
     up ourselves at the same time.  *Note Chains of Implicit Rules:
     Chained Rules.

   * The automatic variable '$^' containing a list of all prerequisites
     of the current target.  We did not invent this, but we have no idea
     who did.  *Note Automatic Variables::.  The automatic variable '$+'
     is a simple extension of '$^'.

   * The "what if" flag ('-W' in GNU 'make') was (as far as we know)
     invented by Andrew Hume in 'mk'.  *Note Instead of Executing
     Recipes: Instead of Execution.

   * The concept of doing several things at once (parallelism) exists in
     many incarnations of 'make' and similar programs, though not in the
     System V or BSD implementations.  *Note Recipe Execution:
     Execution.

   * A number of different build tools that support parallelism also
     support collecting output and displaying as a single block.  *Note
     Output During Parallel Execution: Parallel Output.

   * Modified variable references using pattern substitution come from
     SunOS 4.  *Note Basics of Variable References: Reference.  This
     functionality was provided in GNU 'make' by the 'patsubst' function
     before the alternate syntax was implemented for compatibility with
     SunOS 4.  It is not altogether clear who inspired whom, since GNU
     'make' had 'patsubst' before SunOS 4 was released.

   * The special significance of '+' characters preceding recipe lines
     (*note Instead of Executing Recipes: Instead of Execution.) is
     mandated by 'IEEE Standard 1003.2-1992' (POSIX.2).

   * The '+=' syntax to append to the value of a variable comes from
     SunOS 4 'make'.  *Note Appending More Text to Variables: Appending.

   * The syntax 'ARCHIVE(MEM1 MEM2...)' to list multiple members in a
     single archive file comes from SunOS 4 'make'.  *Note Archive
     Members::.

   * The '-include' directive to include makefiles with no error for a
     nonexistent file comes from SunOS 4 'make'.  (But note that SunOS 4
     'make' does not allow multiple makefiles to be specified in one
     '-include' directive.)  The same feature appears with the name
     'sinclude' in SGI 'make' and perhaps others.

   * The '!=' shell assignment operator exists in many BSD of 'make' and
     is purposefully implemented here to behave identically to those
     implementations.

   * Various build management tools are implemented using scripting
     languages such as Perl or Python and thus provide a natural
     embedded scripting language, similar to GNU 'make''s integration of
     GNU Guile.

   The remaining features are inventions new in GNU 'make':

   * Use the '-v' or '--version' option to print version and copyright
     information.

   * Use the '-h' or '--help' option to summarize the options to 'make'.

   * Simply-expanded variables.  *Note The Two Flavors of Variables:
     Flavors.

   * Pass command line variable assignments automatically through the
     variable 'MAKE' to recursive 'make' invocations.  *Note Recursive
     Use of 'make': Recursion.

   * Use the '-C' or '--directory' command option to change directory.
     *Note Summary of Options: Options Summary.

   * Make verbatim variable definitions with 'define'.  *Note Defining
     Multi-Line Variables: Multi-Line.

   * Declare phony targets with the special target '.PHONY'.

     Andrew Hume of AT&T Bell Labs implemented a similar feature with a
     different syntax in his 'mk' program.  This seems to be a case of
     parallel discovery.  *Note Phony Targets: Phony Targets.

   * Manipulate text by calling functions.  *Note Functions for
     Transforming Text: Functions.

   * Use the '-o' or '--old-file' option to pretend a file's
     modification-time is old.  *Note Avoiding Recompilation of Some
     Files: Avoiding Compilation.

   * Conditional execution.

     This feature has been implemented numerous times in various
     versions of 'make'; it seems a natural extension derived from the
     features of the C preprocessor and similar macro languages and is
     not a revolutionary concept.  *Note Conditional Parts of Makefiles:
     Conditionals.

   * Specify a search path for included makefiles.  *Note Including
     Other Makefiles: Include.

   * Specify extra makefiles to read with an environment variable.
     *Note The Variable 'MAKEFILES': MAKEFILES Variable.

   * Strip leading sequences of './' from file names, so that './FILE'
     and 'FILE' are considered to be the same file.

   * Use a special search method for library prerequisites written in
     the form '-lNAME'.  *Note Directory Search for Link Libraries:
     Libraries/Search.

   * Allow suffixes for suffix rules (*note Old-Fashioned Suffix Rules:
     Suffix Rules.) to contain any characters.  In other versions of
     'make', they must begin with '.' and not contain any '/'
     characters.

   * Keep track of the current level of 'make' recursion using the
     variable 'MAKELEVEL'.  *Note Recursive Use of 'make': Recursion.

   * Provide any goals given on the command line in the variable
     'MAKECMDGOALS'.  *Note Arguments to Specify the Goals: Goals.

   * Specify static pattern rules.  *Note Static Pattern Rules: Static
     Pattern.

   * Provide selective 'vpath' search.  *Note Searching Directories for
     Prerequisites: Directory Search.

   * Provide computed variable references.  *Note Basics of Variable
     References: Reference.

   * Update makefiles.  *Note How Makefiles Are Remade: Remaking
     Makefiles.  System V 'make' has a very, very limited form of this
     functionality in that it will check out SCCS files for makefiles.

   * Various new built-in implicit rules.  *Note Catalogue of Built-In
     Rules: Catalogue of Rules.

   * Load dynamic objects which can modify the behavior of 'make'.
     *Note Loading Dynamic Objects: Loading Objects.