Error Handling in Fortran

Unlike languages with try/catch exception handling, Fortran uses explicit status-reporting arguments attached directly to the statements that can fail: IOSTAT for I/O statements, STAT for ALLOCATE/DEALLOCATE, and error-handling specifiers on other intrinsics, each returning an INTEGER that is zero on success and nonzero on failure. This means every potentially-failing operation must be checked immediately at the call site rather than relying on control flow unwinding to a distant handler, which keeps error paths visible in the code but places the discipline of checking every status code squarely on the programmer.

READ and WRITE statements accept an IOSTAT= integer variable and optionally IOMSG= for a descriptive string; IOSTAT returns zero on success, a negative value for end-of-file or end-of-record conditions (distinguishable via the IS_IOSTAT_END and IS_IOSTAT_EOR intrinsic functions), and a positive value for a genuine I/O error such as malformed data or a missing file. Combining IOSTAT with an explicit loop is the standard idiom for reading an unknown number of records from a file until EOF, replacing the older and less portable ERR= and END= label-based branching still found in legacy codebases.

ALLOCATE and DEALLOCATE accept STAT= and ERRMSG= specifiers analogous to IOSTAT/IOMSG, so ALLOCATE(bigArray(n), STAT=ierr, ERRMSG=msg) lets a program gracefully handle an out-of-memory condition instead of crashing, which matters in large simulations where n might exceed available memory. For genuinely unrecoverable conditions, ERROR STOP immediately terminates all images in a coarray program (or the single image in a non-coarray program) with a nonzero exit status and an optional message, distinct from the plain STOP statement, and is the idiomatic way to fail loudly and immediately when a precondition the rest of the program depends on has been violated.

A common idiom wraps a fallible operation and its status check in a small subroutine that returns its own STAT-like output argument, propagating failure information up through several layers of calls without exceptions, similar in spirit to Result-returning functions in Rust or Go's error return values. For numerical robustness, the IEEE_EXCEPTIONS intrinsic module (IEEE_GET_FLAG, IEEE_SET_HALTING_MODE) lets a program detect or trap floating-point conditions like division by zero and overflow, giving Fortran a standardized way to handle both I/O-level and arithmetic-level failure modes within the same status-code philosophy rather than mixing paradigms.