Fortran deployment failure: diagnosing implicit typing default and STOP semantics ambiguity

Answer to the exact question

The implicit INTEGER*4 default is controlled by the compiler option or source‑file directive that enforces IMPLICIT NONE. In gfortran this is -fimplicit-none; in ifort it is -implicitnone. When neither flag nor a module‑level IMPLICIT NONE is present, the compiler falls back to the Fortran 77 rule that variables beginning with I–N are INTEGER (typically 4 bytes on 64‑bit targets).

STOP semantics vary: STOP without an argument is implementation‑defined; some compilers return the default error code 0, others abort with a diagnostic. STOP n requests a specific exit status, but even this can be treated differently (e.g., some treat STOP 0 as a normal exit while others treat any non‑zero as an error). The safest portable form in modern Fortran is ERROR STOP, which always terminates with a diagnostic and an implementation‑defined non‑zero exit code.

Undefined deallocation timing of allocatable arrays is generally not a cause of deployment failure unless the program relies on a specific order of finalization. Most compilers deallocate at program exit, but the order can differ between modules, leading to subtle crashes if a finalizer accesses a deallocated array. This is a secondary concern compared to the implicit typing and STOP ambiguity.

Likely explanation

Missing IMPLICIT NONE causes silent type changes, leading to miscompiled code that may crash or produce wrong results. Ambiguous STOP semantics can cause the log to show a non‑zero exit status even when the program intended a normal termination.

Confirmed facts and practical steps

• Add IMPLICIT NONE to every module, program, and subroutine. Re‑compile and check for implicit‑typing warnings.
• Replace all STOP statements with ERROR STOP or STOP n where is a known exit code. Verify the exit code in the logs matches the expectation.
• If allocatable finalizers are used, review their order or add explicit DEALLOCATE calls before program termination.

Verification checklist

1. Compile with -Wall (gfortran) or -warn all (ifort) to surface implicit‑typing warnings.
2. Run the program under a debugger or with valgrind to catch invalid memory accesses that may stem from wrong types.
3. Capture the program’s exit status (e.g., echo $? on Unix) and compare it to the STOP value recorded in the deployment log.
4. Consult the compiler’s manual for the exact behavior of STOP in the deployed version.

Missing diagnostic detail

Which Fortran compiler and version are you using for deployment? The default implicit typing rules and STOP semantics differ between gfortran, ifort, flang, and others, and the exact exit code mapping can change with compiler updates.