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(g77-300.info)Output Assumed To Flush

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Output Assumed To Flush
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   For several versions prior to 0.5.20, `g77' configured its version
of the `libf2c' run-time library so that one of its configuration
macros, `ALWAYS_FLUSH', was defined.

   This was done as a result of a belief that many programs expected
output to be flushed to the operating system (under UNIX, via the
`fflush()' library call) with the result that errors, such as disk
full, would be immediately flagged via the relevant `ERR=' and
`IOSTAT=' mechanism.

   Because of the adverse effects this approach had on the performance
of many programs, `g77' no longer configures `libf2c' (now named
`libg2c' in its `g77' incarnation) to always flush output.

   If your program depends on this behavior, either insert the
appropriate `CALL FLUSH' statements, or modify the sources to the
`libg2c', rebuild and reinstall `g77', and relink your programs with
the modified library.

   (Ideally, `libg2c' would offer the choice at run-time, so that a
compile-time option to `g77' or `f2c' could result in generating the
appropriate calls to flushing or non-flushing library routines.)

   Some Fortran programs require output (writes) to be flushed to the
operating system (under UNIX, via the `fflush()' library call) so that
errors, such as disk full, are immediately flagged via the relevant
`ERR=' and `IOSTAT=' mechanism, instead of such errors being flagged
later as subsequent writes occur, forcing the previously written data
to disk, or when the file is closed.

   Essentially, the difference can be viewed as synchronous error
reporting (immediate flagging of errors during writes) versus
asynchronous, or, more precisely, buffered error reporting (detection
of errors might be delayed).

   `libg2c' supports flagging write errors immediately when it is built
with the `ALWAYS_FLUSH' macro defined.  This results in a `libg2c' that
runs slower, sometimes quite a bit slower, under certain
circumstances--for example, accessing files via the networked file
system NFS--but the effect can be more reliable, robust file I/O.

   If you know that Fortran programs requiring this level of precision
of error reporting are to be compiled using the version of `g77' you
are building, you might wish to modify the `g77' source tree so that
the version of `libg2c' is built with the `ALWAYS_FLUSH' macro defined,
enabling this behavior.

   To do this, find this line in `gcc/libf2c/f2c.h' in your `g77'
source tree:

     /* #define ALWAYS_FLUSH */

   Remove the leading `/* ', so the line begins with `#define', and the
trailing ` */'.

   Then build or rebuild `g77' as appropriate.