[flang][runtime] Control stream truncation via runtime environment #168415
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The ISO Fortran standards don't say whether a WRITE to a formatted stream unit should truncate the unit if there has been any repositioning (via POS= control list specifiers) to an earlier point in the stream. But units with sequential records do truncate on writes after BACKSPACE and REWIND statements, and many compilers (including this one) truncate stream units too. Since some compilers don't truncate streams, this patch adds an environment variable FORT_TRUNCATE_STREAM that can be set to 0 to disable truncation and ease porting to flang-new of codes that depend on that behavior. Fixes llvm#167569.
🐧 Linux x64 Test Results
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eugeneepshteyn
approved these changes
Nov 17, 2025
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The ISO Fortran standards don't say whether a WRITE to a formatted stream unit should truncate the unit if there has been any repositioning (via POS= control list specifiers) to an earlier point in the stream. But units with sequential records do truncate on writes after BACKSPACE and REWIND statements, and many compilers (including this one) truncate stream units too. Since some compilers don't truncate streams, this patch adds an environment variable FORT_TRUNCATE_STREAM that can be set to 0 to disable truncation and ease porting to flang-new of codes that depend on that behavior.
Fixes #167569.