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(g77-300.info)Large File Unit Numbers

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Large File Unit Numbers
-----------------------

   If your program crashes at run time with a message including the
text `illegal unit number', that probably is a message from the
run-time library, `libg2c'.

   The message means that your program has attempted to use a file unit
number that is out of the range accepted by `libg2c'.  Normally, this
range is 0 through 99, and the high end of the range is controlled by a
`libg2c' source-file macro named `MXUNIT'.

   If you can easily change your program to use unit numbers in the
range 0 through 99, you should do so.

   As distributed, whether as part of `f2c' or `g77', `libf2c' accepts
file unit numbers only in the range 0 through 99.  For example, a
statement such as `WRITE (UNIT=100)' causes a run-time crash in
`libf2c', because the unit number, 100, is out of range.

   If you know that Fortran programs at your installation require the
use of unit numbers higher than 99, you can change the value of the
`MXUNIT' macro, which represents the maximum unit number, to an
appropriately higher value.

   To do this, edit the file `gcc/libf2c/libI77/fio.h' in your `g77'
source tree, changing the following line:

     #define MXUNIT 100

   Change the line so that the value of `MXUNIT' is defined to be at
least one _greater_ than the maximum unit number used by the Fortran
programs on your system.

   (For example, a program that does `WRITE (UNIT=255)' would require
`MXUNIT' set to at least 256 to avoid crashing.)

   Then build or rebuild `g77' as appropriate.

   _Note:_ Changing this macro has _no_ effect on other limits your
system might place on the number of files open at the same time.  That
is, the macro might allow a program to do `WRITE (UNIT=100)', but the
library and operating system underlying `libf2c' might disallow it if
many other files have already been opened (via `OPEN' or implicitly via
`READ', `WRITE', and so on).  Information on how to increase these
other limits should be found in your system's documentation.


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