diff options
Diffstat (limited to 'doc/make.texi')
| -rw-r--r-- | doc/make.texi | 50 |
1 files changed, 47 insertions, 3 deletions
diff --git a/doc/make.texi b/doc/make.texi index 2965540..b62fee0 100644 --- a/doc/make.texi +++ b/doc/make.texi @@ -1305,7 +1305,7 @@ specified by the existing contents of @file{mfile}. Sometimes it is useful to have a makefile that is mostly just like another makefile. You can often use the @samp{include} directive to include one in the other, and add more targets or variable definitions. -However, it is illegal for two makefiles to give different recipes for +However, it is invalid for two makefiles to give different recipes for the same target. But there is another way. @cindex match-anything rule, used to override @@ -1379,6 +1379,7 @@ chapters. @cindex =, expansion @cindex ?=, expansion @cindex +=, expansion +@cindex !=, expansion @cindex define, expansion Variable definitions are parsed as follows: @@ -1388,6 +1389,7 @@ Variable definitions are parsed as follows: @var{immediate} ?= @var{deferred} @var{immediate} := @var{immediate} @var{immediate} += @var{deferred} or @var{immediate} +@var{immediate} != @var{immediate} define @var{immediate} @var{deferred} @@ -1408,12 +1410,21 @@ endef define @var{immediate} += @var{deferred} or @var{immediate} endef + +define @var{immediate} != + @var{immediate} +endef @end example For the append operator, @samp{+=}, the right-hand side is considered immediate if the variable was previously set as a simple variable (@samp{:=}), and deferred otherwise. +For the shell assignment operator, @samp{!=}, the right-hand side is +evaluated immediately and handed to the shell. The result is stored in the +variable named on the left, and that variable becomes a simple variable +(and will thus be re-evaluated on each reference). + @subheading Conditional Directives @cindex ifdef, expansion @cindex ifeq, expansion @@ -5402,6 +5413,7 @@ Several variables have constant initial values. @cindex = @cindex := @cindex ?= +@cindex != To set a variable from the makefile, write a line starting with the variable name followed by @samp{=} or @samp{:=}. Whatever follows the @@ -5457,6 +5469,33 @@ FOO = bar endif @end example +The shell assignment operator @samp{!=} can be used to execute a +program and set a variable to its output. This operator first +evaluates the right-hand side, then passes that result to the shell +for execution. If the result of the execution ends in a newline, that +one newline is removed; all other newlines are replaced by spaces. +The resulting string is then placed into the named +recursively-expanded variable. For example: + +@example +hash != printf '\043' +file_list != find . -name '*.c' +@end example + +If the result of the execution could produce a @code{$}, and you don't +intend what follows that to be interpreted as a make variable or +function reference, then you must replace every @code{$} with +@code{$$} as part of the execution. Alternatively, you can set a +simply expanded variable to the result of running a program using the +@code{shell} function call. @xref{Shell Function, , The @code{shell} +Function}. For example: + +@example +hash := $(shell printf '\043') +var := $(shell find . -name "*.c") +@end example + + @node Appending, Override Directive, Setting, Using Variables @section Appending More Text to Variables @cindex += @@ -5977,7 +6016,7 @@ prog: a.o b.o Due to the @code{private} modifier, @code{a.o} and @code{b.o} will not inherit the @code{EXTRA_CFLAGS} variable assignment from the -@code{progs} target. +@code{prog} target. @node Special Variables, , Suppressing Inheritance, Using Variables @comment node-name, next, previous, up @@ -6073,7 +6112,7 @@ foo @end example Note that assigning more than one target name to @code{.DEFAULT_GOAL} is -illegal and will result in an error. +invalid and will result in an error. @vindex MAKE_RESTARTS @r{(number of times @code{make} has restarted)} @item MAKE_RESTARTS @@ -10444,6 +10483,11 @@ nonexistent file comes from SunOS 4 @code{make}. (But note that SunOS 4 @code{make} does not allow multiple makefiles to be specified in one @code{-include} directive.) The same feature appears with the name @code{sinclude} in SGI @code{make} and perhaps others. + +@item +The @code{!=} shell assignment operator exists in many BSD of +@code{make} and is purposefully implemented here to behave identically +to those implementations. @end itemize The remaining features are inventions new in GNU @code{make}: |