(I'm not subscribed to the list, so please CC me on any responses.) For the context, see
https://lists.gnu.org/archive/html/emacs-devel/2012-11/msg00441.html I never before had to do anything serious in Autoconf, so please take the below with a grain of salt and consider it as a first impression of a naive newbie. I find the Autoconf manual too terse and cryptic to penetrate for someone, such as myself, who needs to do a relatively simple Autoconf job for the first time. The manual describes the Autoconf facilities in a way that is too abstract, using vague language, and includes too few examples to help the reader understand that vague language. I found myself grepping configure.ac files on my disk for examples, to be able to understand what to do. Allow me to show a few examples. -- Macro: AC_CHECK_HEADER (HEADER-FILE, [ACTION-IF-FOUND], [ACTION-IF-NOT-FOUND], [INCLUDES]) If the system header file HEADER-FILE is compilable, execute shell commands ACTION-IF-FOUND, otherwise execute ACTION-IF-NOT-FOUND. If you just want to define a symbol if the header file is available, consider using `AC_CHECK_HEADERS' instead. INCLUDES is decoded to determine the appropriate include directives. My problem here was with the last sentence: what does it mean "decoded to determine"? What kind of stuff can I put in that argument, and in what form? I found this example in Emacs's configure.ac: AC_CHECK_HEADERS(net/if.h, , , [AC_INCLUDES_DEFAULT #if HAVE_SYS_SOCKET_H #include <sys/socket.h> #endif]) which looks like I can put any series of preprocessor directives there. (A similar example is where AC_CHECK_HEADERS is described in the manual.) But it still leaves me in the dark wrt whether C code other than preprocessor directives can be used, or even whether AC_CHECK_HEADER can also use such directives. For the record, what I needed to do is this: AC_CHECK_HEADERS(X11/xpm.h, HAVE_XPM=yes, HAVE_XPM=no, [ #define FOR_MSW 1]) Another example: -- Macro: AC_MSG_RESULT (RESULT-DESCRIPTION) Notify the user of the results of a check. RESULT-DESCRIPTION is almost always the value of the cache variable for the check, typically `yes', `no', or a file name. It says "almost always", but doesn't describe what else, in addition to a cache variable, can be used as RESULT-DESCRIPTION. Again, grepping the uses, I find things like this: [if $PAXCTL -v conftest$EXEEXT >/dev/null 2>&1; then AC_MSG_RESULT(yes) else AC_MSG_RESULT(no); PAXCTL=""; fi]) or this: AC_MSG_RESULT([yes CFLAGS='$$1_CFLAGS' LIBS='$$1_LIBS']) or this: AC_MSG_RESULT([$HAVE_GNU_MAKE]) or this: AC_MSG_RESULT(6 or newer) Another example, with embedded comments: -- Macro: AC_DEFINE (VARIABLE, VALUE, [DESCRIPTION]) -- Macro: AC_DEFINE (VARIABLE) Define VARIABLE to VALUE (verbatim), by defining a C preprocessor What is the significance of "(verbatim)" here? macro for VARIABLE. VARIABLE should be a C identifier, optionally suffixed by a parenthesized argument list to define a C preprocessor macro with arguments. The macro argument list, if present, should be a comma-separated list of C identifiers, possibly terminated by an ellipsis `...' if C99 syntax is employed. VARIABLE should not contain comments, white space, trigraphs, backslash-newlines, universal character names, or non-ASCII characters. VALUE may contain backslash-escaped newlines, which will be preserved if you use `AC_CONFIG_HEADERS' but flattened if passed via `@DEFS@' (with no effect on the compilation, since the preprocessor sees only one line in the first place). Several potentially important issues packed into a single complex sentence with no explanations or cross-references to help the confused. What does "use `AC_CONFIG_HEADERS'" mean? What is "@DEFS@" and how does it come into play? The meaning of the "flattened" part and the note in parens after that are left to guesswork. VALUE should not contain raw newlines. If you are not using `AC_CONFIG_HEADERS', VALUE should not contain any `#' characters, as `make' tends to eat them. To use a shell variable, use `AC_DEFINE_UNQUOTED' instead. DESCRIPTION is only useful if you are using `AC_CONFIG_HEADERS'. In this case, DESCRIPTION is put into the generated `config.h.in' as the comment before the macro define. The following example defines the C preprocessor variable `EQUATION' to be the string constant `"$a > $b"': AC_DEFINE([EQUATION], ["$a > $b"], [Equation string.]) If neither VALUE nor DESCRIPTION are given, then VALUE defaults to 1 instead of to the empty string. This is for backwards compatibility with older versions of Autoconf, but this usage is obsolescent and may be withdrawn in future versions of Autoconf. This doesn't say what happens if VALUE is not given, but DESCRIPTION is. I seem to see this in use, e.g.: AC_DEFINE(IRIX6_5, [], [Define if the system is IRIX.]) I also see this: AC_DEFINE(USG, []) which is also doesn't seem to be covered. I bump into such examples wherever I go in the manual. I understand that it can be hard to take a POV of a newbie, so I think adding more examples, with emphasis on non-trivial uses of the described facilities, will go a long way towards making the manual more useful to newbies. TIA