(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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