(make.info.gz) Origin Function

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 8.9 The `origin' Function
 =========================
 
 The `origin' function is unlike most other functions in that it does
 not operate on the values of variables; it tells you something _about_
 a variable.  Specifically, it tells you where it came from.
 
    The syntax of the `origin' function is:
 
      $(origin VARIABLE)
 
    Note that VARIABLE is the _name_ of a variable to inquire about; not
 a _reference_ to that variable.  Therefore you would not normally use a
 `$' or parentheses when writing it.  (You can, however, use a variable
 reference in the name if you want the name not to be a constant.)
 
    The result of this function is a string telling you how the variable
 VARIABLE was defined:
 
 `undefined'
      if VARIABLE was never defined.
 
 `default'
      if VARIABLE has a default definition, as is usual with `CC' and so
      on.   Variables Used by Implicit Rules Implicit Variables.
      Note that if you have redefined a default variable, the `origin'
      function will return the origin of the later definition.
 
 `environment'
      if VARIABLE was defined as an environment variable and the `-e'
      option is _not_ turned on ( Summary of Options Options
      Summary.).
 
 `environment override'
      if VARIABLE was defined as an environment variable and the `-e'
      option _is_ turned on ( Summary of Options Options Summary.).
 
 `file'
      if VARIABLE was defined in a makefile.
 
 `command line'
      if VARIABLE was defined on the command line.
 
 `override'
      if VARIABLE was defined with an `override' directive in a makefile
      ( The `override' Directive Override Directive.).
 
 `automatic'
      if VARIABLE is an automatic variable defined for the execution of
      the commands for each rule ( Automatic Variables).
 
    This information is primarily useful (other than for your curiosity)
 to determine if you want to believe the value of a variable.  For
 example, suppose you have a makefile `foo' that includes another
 makefile `bar'.  You want a variable `bletch' to be defined in `bar' if
 you run the command `make -f bar', even if the environment contains a
 definition of `bletch'.  However, if `foo' defined `bletch' before
 including `bar', you do not want to override that definition.  This
 could be done by using an `override' directive in `foo', giving that
 definition precedence over the later definition in `bar';
 unfortunately, the `override' directive would also override any command
 line definitions.  So, `bar' could include:
 
      ifdef bletch
      ifeq "$(origin bletch)" "environment"
      bletch = barf, gag, etc.
      endif
      endif
 
 If `bletch' has been defined from the environment, this will redefine
 it.
 
    If you want to override a previous definition of `bletch' if it came
 from the environment, even under `-e', you could instead write:
 
      ifneq "$(findstring environment,$(origin bletch))" ""
      bletch = barf, gag, etc.
      endif
 
    Here the redefinition takes place if `$(origin bletch)' returns
 either `environment' or `environment override'.   Functions for
 String Substitution and Analysis Text Functions.
 

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