File: autoconf.info,  Node: File Descriptor Macros,  Prev: Initialization Macros,  Up: Programming in M4sh

9.4 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.  By
     default, ‘AS_INIT’ sets this to ‘1’ for standalone M4sh clients.
     However, ‘AC_INIT’ shuffles things around to another file
     descriptor, in order to allow the ‘-q’ option of ‘configure’ to
     choose whether messages should go to the script's standard output
     or be 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
     This must either be empty, or expand to a file descriptor for log
     messages.  By default, ‘AS_INIT’ sets this macro to the empty
     string for standalone M4sh clients, thus disabling logging.
     However, ‘AC_INIT’ shuffles things around so that both ‘configure’
     and ‘config.status’ use ‘config.log’ for log messages.  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
     This must expand to a file descriptor for the original standard
     input.  By default, ‘AS_INIT’ sets this macro to ‘0’ for standalone
     M4sh clients.  However, ‘AC_INIT’ shuffles things around for
     safety.

     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’.