each thread in a process or core file.

Program exits with return code 0 if all frames were shown without any errors.  If some frames were shown, but there were some non-fatal errors, possibly causing an incomplete backtrace, the program exits with return code 1.  If no frames could be shown, or a fatal error occurred the program exits with return code 2.  If the program was invoked with bad or missing arguments it will exit with return code 64.