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8.5 File Descriptor Macros
The following macros define file descriptors used to output messages
(or input values) from configure scripts.
For example:
echo "$wombats found" >&AS_MESSAGE_LOG_FD
echo 'Enter desired kangaroo count:' >&AS_MESSAGE_FD
read kangaroos <&AS_ORIGINAL_STDIN_FD`
However doing so is seldom needed, because Autoconf provides higher
level macros as described below.
— Macro: AS_MESSAGE_FD
The file descriptor for ‘checking for...’ messages and results.
Normally this directs messages to the standard output, however when
configure is run with the -q option, messages sent to
AS_MESSAGE_FD are discarded.
If you want to display some messages, consider using one of the printing
macros (see Printing Messages) instead. Copies of messages output
via these macros are also recorded in config.log.
— Macro: AS_MESSAGE_LOG_FD
The file descriptor for messages logged to config.log. Macros
that run tools, like AC_COMPILE_IFELSE (see Running the Compiler), redirect all output to this descriptor. You may want to do
so if you develop such a low-level macro.
— Macro: AS_ORIGINAL_STDIN_FD
The file descriptor for the original standard input.
When configure runs, it may accidentally execute an
interactive command that has the same name as the non-interactive meant
to be used or checked. If the standard input was the terminal, such
interactive programs would cause configure to stop, pending
some user input. Therefore configure redirects its standard
input from /dev/null during its initialization. This is not
normally a problem, since configure normally does not need
user input.
In the extreme case where your configure script really needs to
obtain some values from the original standard input, you can read them
explicitly from AS_ORIGINAL_STDIN_FD.
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