You must invoke isetjmp before calling longjmp.
The env integer array argument to isetjmp and longjmp must be at least 12 elements long.
You must pass the env variable from the routine that calls isetjmp to the routine that calls longjmp, either by common or as an argument.
longjmp attempts to clean up the stack. longjmp must be called from a lower call-level than isetjmp.
Passing isetjmp as an argument that is a procedure name does not work.
See setjmp(3V).