Thanks to Frank and Adriaan.
I was not able to use the general method Frank described; I seem to have no addr2line program. The OSX-specific method works very well. To help others using OSX, below is a more spelled-out description of how to use this. Anyone can feel free to cut and paste into their documentation:
------------------------------------------------------------------------ ------------------------------ FINDING LOCATION OF GPC RUNTIME ERRORS ON MAC OSX:
For programs that get errors, not crash such as "segmentation fault", use the following, E.G., for program myprog.pas:
compile with debug information gp -g myprog
open console, find the appropriate CrashReporter log open ...Applications/Utilities/Console locate under "Logs", e.g. open triangle for: ~Library/Logs/CrashReporter then click on: myprog.crash.log
Leave the log open, and run the program normally, e.g. ./myprog
The traceback shows in the crash.log, e.g. Thread 0 crashed: 0 myprog ... 1 myprog ... 2 myprog 0x00072800 _p_IORangeCheckError + 0 (error.pas:626) 3 myprog 0x00072c58 _p_M0_main_program + 1296 (myprog.pas:82) 4 myprog ... 5 myprog ...
Interpretation: the problem was a range check error, in the main program of myprog, on line 82 of file myprog.pas
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