(autoconf.info)File Descriptor Macros
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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 (Note: 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' (Note: 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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