diff options
Diffstat (limited to 'doc')
| -rw-r--r-- | doc/make.texi | 45 |
1 files changed, 43 insertions, 2 deletions
diff --git a/doc/make.texi b/doc/make.texi index 8f86c0c..52a56ef 100644 --- a/doc/make.texi +++ b/doc/make.texi @@ -4711,6 +4711,8 @@ they have particular specialized uses. @xref{Automatic Variables}. the user has set it with a command argument. * Multi-Line:: An alternate way to set a variable to a multi-line string. +* Undefine Directive:: How to undefine a variable so that it appears + as if it was never set. * Environment:: Variable values can come from the environment. * Target-specific:: Variable values can be defined on a per-target basis. @@ -5518,7 +5520,7 @@ See the next section for information about @code{define}. @xref{Multi-Line, ,Defining Multi-Line Variables}. @end ifnottex -@node Multi-Line, Environment, Override Directive, Using Variables +@node Multi-Line, Undefine Directive, Override Directive, Using Variables @section Defining Multi-Line Variables @findex define @findex endef @@ -5599,7 +5601,42 @@ endef @noindent @xref{Override Directive, ,The @code{override} Directive}. -@node Environment, Target-specific, Multi-Line, Using Variables +@node Undefine Directive, Environment, Multi-Line, Using Variables +@section Undefining Variables +@findex undefine +@cindex undefining variable + +If you want to clear a variable, setting its value to empty is usually +sufficient. Expanding such a variable will yield the same result (empty +string) regardless of whether it was set or not. However, if you are +using the @code{flavor} (@pxref{Flavor Function}) and +@code{origin} (@pxref{Origin Function}) functions, there is a difference +between a variable that was never set and a variable with an empty value. +In such situations you may want to use the @code{undefine} directive to +make a variable appear as if it was never set. For example: + +@example +foo := foo +bar = bar + +undefine foo +undefine bar + +$(info $(origin foo)) +$(info $(flavor bar)) +@end example + +This example will print ``undefined'' for both variables. + +If you want to undefine a command-line variable definition, you can use +the @code{override} directive together with @code{undefine}, similar to +how this is done for variable definitions: + +@example +override undefine CFLAGS +@end example + +@node Environment, Target-specific, Undefine Directive, Using Variables @section Variables from the Environment @cindex variables, environment @@ -10485,6 +10522,10 @@ Here is a summary of the directives GNU @code{make} recognizes: Define multi-line variables.@* @xref{Multi-Line}. +@item undefine @var{variable} +Undefining variables.@* +@xref{Undefine Directive}. + @item ifdef @var{variable} @itemx ifndef @var{variable} @itemx ifeq (@var{a},@var{b}) |