--- make-doc-non-dfsg-3.81.orig/doc/make/make_12.html +++ make-doc-non-dfsg-3.81/doc/make/make_12.html @@ -0,0 +1,352 @@ + + + + + + +GNU make: 12. Features of GNU make + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

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 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 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 This feature is obsolete in GNU make, because the +general feature of rule chaining (see section Chains of Implicit Rules) allows one pattern rule for installing members in an +archive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 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 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. + +

+ +

+ + + + + + + + + + + + +

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