12. 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 13. Incompatibilities and Missing Features.

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

• The VPATH variable and its special meaning. See section Searching Directories for Prerequisites. 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. See section Including Other Makefiles. Allowing multiple files to be included with a single directive is a GNU extension.

• Variables are read from and communicated via the environment. See section Variables from the Environment.

• Options passed through the variable MAKEFLAGS to recursive invocations of make. See section Communicating Options to a Sub-make.

• The automatic variable $% is set to the member name in an archive reference. See section Automatic Variables.

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

• Substitution variable references. See section Basics of Variable References.

• 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. See section Recursive Use of make.

• Support for suffix `.a' in suffix rules. See section 11.4 Suffix Rules for Archive Files. This feature is obsolete in GNU make, because the general feature of rule chaining (see section Chains of Implicit Rules) allows one pattern rule for installing members in an archive (see section 11.2 Implicit Rule for Archive Member Targets) to be sufficient.

• The arrangement of lines and backslash-newline combinations in commands is retained when the commands 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. See section Defining and Redefining 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. See section Chains of Implicit 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. See section 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. See section Instead of Executing the Commands.

• 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. See section Command Execution.

• Modified variable references using pattern substitution come from SunOS 4. See section Basics of Variable References. 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 command lines (see section Instead of Executing the Commands) 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. See section Appending More Text to Variables.

• The syntax `archive(mem1 mem2...)' to list multiple members in a single archive file comes from SunOS 4 make. See section 11.1 Archive Members as Targets.

• 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 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. See section The Two Flavors of Variables.

• Pass command-line variable assignments automatically through the variable MAKE to recursive make invocations. See section Recursive Use of make.

• Use the `-C' or `--directory' command option to change directory. See section Summary of Options.

• Make verbatim variable definitions with define. See section Defining Variables Verbatim.

• 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. See section Phony Targets.

• Manipulate text by calling functions. See section Functions for Transforming Text.

• Use the `-o' or `--old-file' option to pretend a file's modification-time is old. See section Avoiding Recompilation of Some Files.

• 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. See section Conditional Parts of Makefiles.

• Specify a search path for included makefiles. See section Including Other Makefiles.

• Specify extra makefiles to read with an environment variable. See section The Variable MAKEFILES.

• 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'. See section Directory Search for Link Libraries.

• Allow suffixes for suffix rules (see section Old-Fashioned 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. See section Recursive Use of make.

• Provide any goals given on the command line in the variable MAKECMDGOALS. See section Arguments to Specify the Goals.

• Specify static pattern rules. See section Static Pattern Rules.

• Provide selective vpath search. See section Searching Directories for Prerequisites.

• Provide computed variable references. See section Basics of Variable References.

• Update makefiles. See section How Makefiles Are Remade. 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. See section Catalogue of Implicit Rules.

• The built-in variable `MAKE_VERSION' gives the version number of make.