15.1.5 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 gcc/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).)