On Mac OS X (10.9.2, Mavericks) at any rate, the prototype for longjmperror()
is:
void longjmperror(void);
You write a function with that signature. It must not return (or, rather, if it does, the program will be abort()
ed). What you do in that function is your business, but bear in mind that things have gone moderately catastrophically wrong for the function to be called at all). It might log an error to your log file, or just write a more meaningful message before exiting (instead of aborting and perhaps core dumping).
You link the object file containing the function ahead of the system library. You are normally not expected to replace system functions, but this is one you are intended to override.