$ file app
app: Mach-O universal binary with 2 architectures
app (for architecture i386): Mach-O executable i386
app (for architecture x86_64): Mach-O 64-bit executable x86_64
$ gdb app
GNU gdb (GDB) 7.6
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-apple-darwin13.0.0".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
"app": not in executable format: File format not recognized
$ file test
test: Mach-O 64-bit executable x86_64
$ gdb test
GNU gdb (GDB) 7.6
Copyright (C) 2013 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law. Type "show copying"
and "show warranty" for details.
This GDB was configured as "x86_64-apple-darwin13.0.0".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from /Users/dmulder/test...Reading symbols from /Users/dmulder/test.dSYM/Contents/Resources/DWARF/test...done.
done.
Why would the 64bit binary succeed, but the 64+32 binary fail?
Unfortunately, the non-Apple version of GNU gdb is currently unable to debug universal (or 'fat') binaries (ones that contain both 32-bit and 64-bit executables).
One option is to use lipo to extract a single architecture and run gdb on that:
lipo -thin x86_64 -output app-x86_64 ./app
or
lipo -thin i386 -output app-i386 ./app
If you'd prefer to debug the combined executable, you could try using LLDB, or an Apple version of gdb.