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6.1 Language Choice
Autoconf-generated `configure' scripts check for the C compiler and its
features by default. Packages that use other programming languages
(maybe more than one, e.g., C and C++) need to test features of the
compilers for the respective languages. The following macros determine
which programming language is used in the subsequent tests in
-- Macro: AC_LANG (LANGUAGE)
Do compilation tests using the compiler, preprocessor, and file
extensions for the specified LANGUAGE.
Supported languages are:
Do compilation tests using `CC' and `CPP' and use extension
`.c' for test programs. Use compilation flags: `CPPFLAGS'
with `CPP', and both `CPPFLAGS' and `CFLAGS' with `CC'.
Do compilation tests using `CXX' and `CXXCPP' and use
extension `.C' for test programs. Use compilation flags:
`CPPFLAGS' with `CXXCPP', and both `CPPFLAGS' and `CXXFLAGS'
Do compilation tests using `F77' and use extension `.f' for
test programs. Use compilation flags: `FFLAGS'.
Do compilation tests using `FC' and use extension `.f' (or
whatever has been set by `AC_FC_SRCEXT') for test programs.
Use compilation flags: `FCFLAGS'.
Compile and execute tests using `ERLC' and `ERL' and use
extension `.erl' for test Erlang modules. Use compilation
Do compilation tests using `OBJC' and `OBJCPP' and use
extension `.m' for test programs. Use compilation flags:
`CPPFLAGS' with `OBJCPP', and both `CPPFLAGS' and `OBJCFLAGS'
-- Macro: AC_LANG_PUSH (LANGUAGE)
Remember the current language (as set by `AC_LANG') on a stack, and
then select the LANGUAGE. Use this macro and `AC_LANG_POP' in
macros that need to temporarily switch to a particular language.
-- Macro: AC_LANG_POP ([LANGUAGE])
Select the language that is saved on the top of the stack, as set
by `AC_LANG_PUSH', and remove it from the stack.
If given, LANGUAGE specifies the language we just _quit_. It is a
good idea to specify it when it's known (which should be the
case...), since Autoconf detects inconsistencies.
# Perform some tests on Fortran 77.
-- Macro: AC_LANG_ASSERT (LANGUAGE)
Check statically that the current language is LANGUAGE. You
should use this in your language specific macros to avoid that
they be called with an inappropriate language.
This macro runs only at `autoconf' time, and incurs no cost at
`configure' time. Sadly enough and because Autoconf is a two
layer language (1), the macros `AC_LANG_PUSH' and `AC_LANG_POP'
cannot be "optimizing", therefore as much as possible you ought to
avoid using them to wrap your code, rather, require from the user
to run the macro with a correct current language, and check it
with `AC_LANG_ASSERT'. And anyway, that may help the user
understand she is running a Fortran macro while expecting a result
about her Fortran 77 compiler...
-- Macro: AC_REQUIRE_CPP
Ensure that whichever preprocessor would currently be used for
tests has been found. Calls `AC_REQUIRE' (Note: Prerequisite
Macros) with an argument of either `AC_PROG_CPP' or
`AC_PROG_CXXCPP', depending on which language is current.
---------- Footnotes ----------
(1) Because M4 is not aware of Sh code, especially conditionals,
some optimizations that look nice statically may produce incorrect
results at runtime.
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