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Sometimes you need to find out how a system performs at runtime, such as whether a given function has a certain capability or bug. If you can, make such checks when your program runs instead of when it is configured. You can check for things like the machine's endianness when your program initializes itself.
If you really need to test for a runtime behavior while configuring,
you can write a test program to determine the result, and compile and
run it using
AC_RUN_IFELSE. Avoid running test programs if
possible, because this prevents people from configuring your package for
If program compiles and links successfully and returns an exit status of 0 when executed, run shell commands action-if-true. Otherwise, run shell commands action-if-false.
The input can be made by
LIBSare used for linking, in addition to the compilation flags of the current language (see Language Choice).
If the compiler being used does not produce executables that run on the system where configure is being run, then the test program is not run. If the optional shell commands action-if-cross-compiling are given, they are run instead. Otherwise, configure prints an error message and exits.
In the action-if-false section, the failing exit status is available in the shell variable ‘$?’. This exit status might be that of a failed compilation, or it might be that of a failed program execution.
It is customary to report unexpected failures with
Try to provide a pessimistic default value to use when cross-compiling
makes runtime tests impossible. You do this by passing the optional
last argument to
AC_RUN_IFELSE. autoconf prints a
warning message when creating configure each time it
encounters a call to
AC_RUN_IFELSE with no
action-if-cross-compiling argument given. You may ignore the
warning, though users cannot configure your package for
cross-compiling. A few of the macros distributed with Autoconf produce
this warning message.
To configure for cross-compiling you can also choose a value for those parameters based on the canonical system name (see Manual Configuration). Alternatively, set up a test results cache file with the correct values for the host system (see Caching Results).
To provide a default for calls of
AC_RUN_IFELSE that are embedded
in other macros, including a few of the ones that come with Autoconf,
you can test whether the shell variable
cross_compiling is set to
‘yes’, and then use an alternate method to get the results instead
of calling the macros.
A C or C++ runtime test should be portable. See Portable C and C++.
Erlang tests must exit themselves the Erlang VM by calling the
function: the given status code is used to determine the success of the test
0) or its failure (status is different than
explained above. It must be noted that data output through the standard output
io:format/2) may be truncated when halting the VM.
Therefore, if a test must output configuration information, it is recommended
to create and to output data into the temporary file named conftest.out,
using the functions of module
conftest.out file is
automatically deleted by the
AC_RUN_IFELSE macro. For instance, a
simplified implementation of Autoconf's
AC_ERLANG_SUBST_LIB_DIR macro is:
AC_INIT([LibdirTest], [1.0], [firstname.lastname@example.org]) AC_ERLANG_NEED_ERL AC_LANG(Erlang) AC_RUN_IFELSE( [AC_LANG_PROGRAM(, [dnl file:write_file("conftest.out", code:lib_dir()), halt(0)])], [echo "code:lib_dir() returned: `cat conftest.out`"], [AC_MSG_FAILURE([test Erlang program execution failed])])