[flang][runtime] Don't write implied ENDFILE for REC=/POS=

[klausler]

An implied ENDFILE record, which truncates an external file, should be written to a sequential unit whenever the file is repositioned for a BACKSPACE or REWIND statement if a WRITE statement has executed since the last OPEN/BACKSPACE/REWIND.

But the REC= and POS= positioning specifiers don't apply to sequential units (they're for direct and stream units, resp.), so don't truncate the file when they're used.

[llvmbot]

@llvm/pr-subscribers-flang-runtime

Author: Peter Klausler (klausler)

Changes

An implied ENDFILE record, which truncates an external file, should be written to a sequential unit whenever the file is repositioned for a BACKSPACE or REWIND statement if a WRITE statement has executed since the last OPEN/BACKSPACE/REWIND.

But the REC= and POS= positioning specifiers don't apply to sequential units (they're for direct and stream units, resp.), so don't truncate the file when they're used.

---

Full diff: https://github.com/llvm/llvm-project/pull/79637.diff

1 Files Affected:

• (modified) flang/runtime/unit.cpp (+1-1)

diff --git a/flang/runtime/unit.cpp b/flang/runtime/unit.cpp
index 18590567c65eb0a..d927d012225672d 100644
--- a/flang/runtime/unit.cpp
+++ b/flang/runtime/unit.cpp
@@ -679,6 +679,7 @@ void ExternalFileUnit::Rewind(IoErrorHandler &handler) {
     handler.SignalError(IostatRewindNonSequential,
         "REWIND(UNIT=%d) on non-sequential file", unitNumber());
   } else {
+    DoImpliedEndfile(handler);
     SetPosition(0, handler);
     currentRecordNumber = 1;
     leftTabLimit.reset();
@@ -687,7 +688,6 @@ void ExternalFileUnit::Rewind(IoErrorHandler &handler) {
 }
 
 void ExternalFileUnit::SetPosition(std::int64_t pos, IoErrorHandler &handler) {
-  DoImpliedEndfile(handler);
   frameOffsetInFile_ = pos;
   recordOffsetInFrame_ = 0;
   if (access == Access::Direct) {