"ret (void*)-1" from mmap64 doesn't work?

[Jim Meyering]

Hi,

I reported what I thought was a bug in glibc:

    http://bugzilla.redhat.com/365111

in which I tried to make gdb simulate a mmap64 failure.
However, using "ret (void*)-1" appears not to do what I expected.

In comment #6, Ulrich showed that setting an arch-specific register
did what I wanted:

    http://bugzilla.redhat.com/365111#c6

Is there a way to make gdb do that portably?

Jim