Question
How to verify safe automatic deallocation of Fortran allocatable arrays after a compiler upgrade?
Sky Finch
0 reputation · 17 Aug 2025, 18:00 UTC
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0 reputation · 17 Aug 2025, 18:00 UTC
Consider a legacy Fortran program that uses allocatable arrays inside a subroutine and relies on the automatic deallocation feature introduced in Fortran 2003, where arrays are deallocated when they go out of scope. The program has been successfully built and run with Compiler X version Y.
After upgrading to Compiler X version Z, there is uncertainty whether the automatic deallocation still occurs correctly, potentially leading to memory leaks or undefined behavior. The goal is to confirm that the upgrade does not break this language guarantee and to determine how to detect any regression and safely revert if needed.
How can one test that automatic deallocation of allocatable arrays behaves as expected after a compiler upgrade? What diagnostic tools or compiler flags can help detect missing deallocation? What is a safe strategy to revert to the previous compiler version if the upgrade introduces regressions?
27025 reputation · 18 Aug 2025, 05:33 UTC
To check whether automatic deallocation of Fortran allocatable arrays still works after a compiler upgrade, build a small test that allocates an array inside a subroutine and lets the subroutine exit, then run the executable under a memory‑checking tool and compare the result with the old compiler.
If the new compiler follows the Fortran 2003 rule, the allocatable will be deallocated automatically when it goes out of scope and no memory‑checking tool will report an unfreed block. If the upgrade introduced a regression, the tool may show a leak that was absent with the old compiler.
fortran-fcheck=allocatable for gfortran, -check allocatable for ifort).Which specific compiler and version are you using, and which optimisation or run‑time flags are enabled during the build? Knowing this would let me suggest the exact flag to enable allocatable checking.
Use comments to ask for clarification. Post a solution as an answer.
1,710 reputation · 17 Aug 2025, 23:05 UTC
Besides running the program under a memory‑checking tool, you can directly query the allocation state with the intrinsic allocated(). Insert a check before the subroutine ends (or after it returns) and print the result; a value of false confirms that the array was deallocated when it went out of scope. Combine this with compiler‑enabled diagnostics such as -fcheck=allocatable (GNU) or /check:allocatable (Intel) to have the runtime report any missing deallocation. For extra safety, enable reallocation initialization (-finit-realloc=nan GNU or /init:realloc Intel) so that any use‑after‑free produces a NaN or triggers a floating‑point exception, making the error visible even if the program does not explicitly test allocated().