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To check for a library, a function, or a global variable, Autoconf
configure scripts try to compile and link a small program that
uses it. This is unlike Metaconfig, which by default uses nm or
ar on the C library to try to figure out which functions are
available. Trying to link with the function is usually a more reliable
approach because it avoids dealing with the variations in the options
and output formats of nm and ar and in the location of the
standard libraries. It also allows configuring for cross-compilation or
checking a function’s runtime behavior if needed. On the other hand,
it can be slower than scanning the libraries once, but accuracy is more
important than speed.
AC_LINK_IFELSE is used to compile test programs to test for
functions and global variables. It is also used by AC_CHECK_LIB
to check for libraries (see Library Files), by adding the library being
checked for to LIBS temporarily and trying to link a small
program.
Run the compiler (and compilation flags) and the linker of the current
language (see Language Choice) on the input, run the shell
commands action-if-true on success, action-if-false
otherwise. The input can be made by AC_LANG_PROGRAM and
friends. If needed, action-if-true can further access the
just-linked program file conftest$EXEEXT.
LDFLAGS and LIBS are used for linking, in addition to the
current compilation flags.
It is customary to report unexpected failures with
AC_MSG_FAILURE. This macro does not try to execute the program;
use AC_RUN_IFELSE if you need to do that (see Checking Runtime Behavior).
The AC_LINK_IFELSE macro cannot be used for Erlang tests, since Erlang
programs are interpreted and do not require linking.
Next: Checking Runtime Behavior, Previous: Running the Compiler, Up: Writing Tests [Contents][Index]