gdb on mac 10.9 fails with "not in executable format: File format not recognized" for 32+64 arch

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.

As OP commented, using Apple's gdb will fix the problem.

Here are instructions to build Apple gdb 6.3.50.20050815-cvs from source on OS 10.9:

NOTE: You will need to install Xcode and set up a build environment. If you have Homebrew installed, run brew doctor to see if "Your system is ready to brew."

1. Download the gdb-1822 source tarball from: http://opensource.apple.com/tarballs/gdb/gdb-1822.tar.gz

2. Extract this into a temporary directory. Open a terminal and cd into gdb-1822/src.

3. Run the configure script:

   ./configure --prefix="$HOME/.local/stow/gdb-1822" --disable-debug --disable-dependency-tracking --with-system-readline
   

   (The last three configure arguments are from the homebrew-dupes formula: https://github.com/Homebrew/homebrew-dupes/blob/master/gdb.rb )

4. Run make:

   make
   make install
   

5. Follow the instructions at https://sourceware.org/gdb/wiki/BuildingOnDarwin#Creating_a_certificate to create a gdb-cert code signing certificate.

6. cd into $HOME/.local/stow/gdb-1822/bin and sign the gdb executable:

   codesign -s gdb-cert gdb
   

7. cd into $HOME/.local/stow and stow the gdb-1822 folder:

   stow gdb-1822
   

8. Add $HOME/.local/bin to your PATH and either restart the terminal or clear Bash's cache to the location of gdb:

   hash -d gdb