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(g77-300.info)Common Blocks

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Common Blocks (COMMON)
======================

   `g77' names and lays out `COMMON' areas the same way `f2c' does, for
compatibility with `f2c'.

   Currently, `g77' does not emit "true" debugging information for
members of a `COMMON' area, due to an apparent bug in the GBE.

   (As of Version 0.5.19, `g77' emits debugging information for such
members in the form of a constant string specifying the base name of
the aggregate area and the offset of the member in bytes from the start
of the area.  Use the `-fdebug-kludge' option to enable this behavior.
In `gdb', use `set language c' before printing the value of the member,
then `set language fortran' to restore the default language, since
`gdb' doesn't provide a way to print a readable version of a character
string in Fortran language mode.

   This kludge will be removed in a future version of `g77' that, in
conjunction with a contemporary version of `gdb', properly supports
Fortran-language debugging, including access to members of `COMMON'
areas.)

   Version 0.5.26 of `g77' is believed to provide correct and complete
debug information for COMMON BLOCK and EQUIVALENCE items - hence the
`-fdebug-kludge' option has been disabled.

   Note: Options for Code Generation Conventions, for
information on the `-fdebug-kludge' option.

   Moreover, `g77' currently implements a `COMMON' area such that its
type is an array of the C `char' data type.

   So, when debugging, you must know the offset into a `COMMON' area
for a particular item in that area, and you have to take into account
the appropriate multiplier for the respective sizes of the types (as
declared in your code) for the items preceding the item in question as
compared to the size of the `char' type.

   For example, using default implicit typing, the statement

     COMMON I(15), R(20), T

results in a public 144-byte `char' array named `_BLNK__' with `I'
placed at `_BLNK__[0]', `R' at `_BLNK__[60]', and `T' at `_BLNK__[140]'.
(This is assuming that the target machine for the compilation has
4-byte `INTEGER(KIND=1)' and `REAL(KIND=1)' types.)