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20.9 Debugging configure scripts

While in general, configure scripts generated by Autoconf strive to be fairly portable to various systems, compilers, shells, and other tools, it may still be necessary to debug a failing test, broken script or makefile, or fix or override an incomplete, faulty, or erroneous test, especially during macro development. Failures can occur at all levels, in M4 syntax or semantics, shell script issues, or due to bugs in the test or the tools invoked by configure. Together with the rather arcane error message that m4 and make may produce when their input contains syntax errors, this can make debugging rather painful.

Nevertheless, here is a list of hints and strategies that may help:

• When autoconf fails, common causes for error include:
  • mismatched or unbalanced parentheses or braces (see Dealing with unbalanced parentheses),
  • under- or over-quoted macro arguments (see The Autoconf Language, see Quoting and Parameters, see Quotation and Nested Macros),
  • spaces between macro name and opening parenthesis (see The Autoconf Language).

  Typically, it helps to go back to the last working version of the input and compare the differences for each of these errors. Another possibility is to sprinkle pairs of m4_traceon and m4_traceoff judiciously in the code, either without a parameter or listing some macro names and watch m4 expand its input verbosely (see Debugging via autom4te).

• Sometimes autoconf succeeds but the generated configure script has invalid shell syntax. You can detect this case by running ‘bash -n configure’ or ‘sh -n configure’. If this command fails, the same tips apply, as if autoconf had failed.
• Debugging configure script execution may be done by sprinkling pairs of set -x and set +x into the shell script before and after the region that contains a bug. Running the whole script with ‘shell -vx ./configure 2>&1 | tee log-file’ with a decent shell may work, but produces lots of output. Here, it can help to search for markers like ‘checking for’ a particular test in the log-file.
• Alternatively, you might use a shell with debugging capabilities like bashdb.
• When configure tests produce invalid results for your system, it may be necessary to override them:
  • For programs, tools or libraries variables, preprocessor, compiler, or linker flags, it is often sufficient to override them at make run time with some care (see make macro=value and Submakes). Since this normally won’t cause configure to be run again with these changed settings, it may fail if the changed variable would have caused different test results from configure, so this may work only for simple differences.
  • Most tests which produce their result in a substituted variable allow to override the test by setting the variable on the configure command line (see Compilers and Options, see Defining Variables, see Particular systems).
  • Many tests store their result in a cache variable (see Caching Results). This lets you override them either on the configure command line as above, or through a primed cache or site file (see Cache Files, see Setting Site Defaults). The name of a cache variable is documented with a test macro or may be inferred from Cache Variable Names; the precise semantics of undocumented variables are often internal details, subject to change.
• Alternatively, configure may produce invalid results because of uncaught programming errors, in your package or in an upstream library package. For example, when AC_CHECK_LIB fails to find a library with a specified function, always check config.log. This will reveal the exact error that produced the failing result: the library linked by AC_CHECK_LIB probably has a fatal bug.

Conversely, as macro author, you can make it easier for users of your macro:

• by minimizing dependencies between tests and between test results as far as possible,
• by using make variables to factorize and allow override of settings at make run time,
• by honoring the GNU Coding Standards and not overriding flags reserved for the user except temporarily during configure tests,
• by not requiring users of your macro to use the cache variables. Instead, expose the result of the test via run-if-true and run-if-false parameters. If the result is not a boolean, then provide it through documented shell variables.