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(g77-295.info)NeXTStep Problems

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NeXTStep Problems
-----------------

   Developers of Fortran code on NeXTStep (all architectures) have to
watch out for the following problem when writing programs with large,
statically allocated (i.e. non-stack based) data structures (common
blocks, saved arrays).

   Due to the way the native loader (`/bin/ld') lays out data
structures in virtual memory, it is very easy to create an executable
wherein the `__DATA' segment overlaps (has addresses in common) with
the `UNIX STACK' segment.

   This leads to all sorts of trouble, from the executable simply not
executing, to bus errors.  The NeXTStep command line tool `ebadexec'
points to the problem as follows:

     % /bin/ebadexec a.out
     /bin/ebadexec: __LINKEDIT segment (truncated address = 0x3de000
     rounded size = 0x2a000) of executable file: a.out overlaps with UNIX
     STACK segment (truncated address = 0x400000 rounded size =
     0x3c00000) of executable file: a.out

   (In the above case, it is the `__LINKEDIT' segment that overlaps the
stack segment.)

   This can be cured by assigning the `__DATA' segment (virtual)
addresses beyond the stack segment.  A conservative estimate for this
is from address 6000000 (hexadecimal) onwards--this has always worked
for me [Toon Moene]:

     % g77 -segaddr __DATA 6000000 test.f
     % ebadexec a.out
     ebadexec: file: a.out appears to be executable
     %

   Browsing through `egcs/gcc/f/Makefile.in', you will find that the
`f771' program itself also has to be linked with these flags--it has
large statically allocated data structures.  (Version 0.5.18 reduces
this somewhat, but probably not enough.)

   (The above item was contributed by Toon Moene
(<toon@moene.indiv.nluug.nl>).)