Coarray execution mode detection in Fortran 2018 compilers

Goal

Determine at compile time whether a Fortran 2018 compiler build will launch a program with true multi‑image coarray execution or fall back to a single‑image serial run.

Constraints and Uncertainty

• Coarray syntax is part of the language, but the runtime that actually creates multiple images is supplied by the compiler or an external library.
• Many compilers accept coarray constructs without requiring a runtime flag, yet the program may still execute with one image unless a specific flag or environment variable is set.
• There is no standardized compile‑time query or pragma that reports the availability of multi‑image support.

Unresolved Decision

Should a Fortran 2018 conforming compiler provide a portable way—via a query intrinsic, compiler option, or environment variable—to indicate whether the build will support true coarray parallelism?

Specific Questions

1. Is there an intrinsic or compiler flag in any Fortran 2018 implementation that reliably reports multi‑image support at compile time?
2. What is the recommended practice for detecting and handling the single‑image fallback in portable coarray code?
3. How do current compilers differ in their handling of this detection, and what impact does that have on portability across vendor toolchains?