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6.6 Checking Runtime Behavior

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 cross-compiling.

— Macro: AC_RUN_IFELSE (input, [action-if-true], [action-if-false], [action-if-cross-compiling = ‘AC_MSG_FAILURE])

Run the compiler (and compilation flags) and the linker of the current language (see Language Choice) on the input, then execute the resulting program. If the program 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 AC_LANG_PROGRAM and friends. LDFLAGS and LIBS are used for linking, in addition to the compilation flags of the current language (see Language Choice). Additionally, action-if-true can run ./conftest$EXEEXT for further testing.

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.

If cross-compilation mode is enabled (this is the case if either the compiler being used does not produce executables that run on the system where configure is being run, or if the options --build and --host were both specified and their values are different), then the test program is not run. If the optional shell commands action-if-cross-compiling are given, those commands are run instead; typically these commands provide pessimistic defaults that allow cross-compilation to work even if the guess was wrong. If the fourth argument is empty or omitted, but cross-compilation is detected, then configure prints an error message and exits. If you want your package to be useful in a cross-compilation scenario, you should provide a non-empty action-if-cross-compiling clause, as well as wrap the AC_RUN_IFELSE compilation inside an AC_CACHE_CHECK (see Caching Results) which allows the user to override the pessimistic default if needed.

It is customary to report unexpected failures with AC_MSG_FAILURE.

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. If you are not concerned about users configuring your package for cross-compilation, you may ignore the warning. A few of the macros distributed with Autoconf produce this warning message; but if this is a problem for you, please report it as a bug, along with an appropriate pessimistic guess to use instead.

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.

It is also permissible to temporarily assign to cross_compiling in order to force tests to behave as though they are in a cross-compilation environment, particularly since this provides a way to test your action-if-cross-compiling even when you are not using a cross-compiler.

     # We temporarily set cross-compile mode to force AC_COMPUTE_INT
     # to use the slow link-only method

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 halt/1 function: the given status code is used to determine the success of the test (status is 0) or its failure (status is different than 0), as explained above. It must be noted that data output through the standard output (e.g., using 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 file. The 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], [])
       [AC_LANG_PROGRAM([], [dnl
         file:write_file("conftest.out", code:lib_dir()),
       [echo "code:lib_dir() returned: `cat conftest.out`"],
       [AC_MSG_FAILURE([test Erlang program execution failed])])