a55e30b12c
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 https://github.com/llvm/llvm-project/issues/167569.
69 lines
2.2 KiB
Markdown
69 lines
2.2 KiB
Markdown
<!--===- docs/RuntimeEnvironment.md
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Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
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See https://llvm.org/LICENSE.txt for license information.
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SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
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-->
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```{contents}
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---
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local:
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---
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```
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# Environment variables of significance to Fortran execution
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A few environment variables are queried by the Fortran runtime support
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library.
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The following environment variables can affect the behavior of
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Fortran programs during execution.
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## `DEFAULT_UTF8=1`
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Set `DEFAULT_UTF8` to cause formatted external input to assume UTF-8
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encoding on input and use UTF-8 encoding on formatted external output.
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## `FORT_CONVERT`
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Determines data conversions applied to unformatted I/O.
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* `NATIVE`: no conversions (default)
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* `LITTLE_ENDIAN`: assume input is little-endian; emit little-endian output
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* `BIG_ENDIAN`: assume input is big-endian; emit big-endian output
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* `SWAP`: reverse endianness (always convert)
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## `FORT_CHECK_POINTER_DEALLOCATION`
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Fortran requires that a pointer that appears in a `DEALLOCATE` statement
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must have been allocated in an `ALLOCATE` statement with the same declared
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type.
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The runtime support library validates this requirement by checking the
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size of the allocated data, and will fail with an error message if
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the deallocated pointer is not valid.
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Set `FORT_CHECK_POINTER_DEALLOCATION=0` to disable this check.
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## `FORT_FMT_RECL`
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Set to an integer value to specify the record length for list-directed
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and `NAMELIST` output.
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The default is 72.
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## `NO_STOP_MESSAGE`
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Set `NO_STOP_MESSAGE=1` to disable the extra information about
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IEEE floating-point exception flags that the Fortran language
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standard requires for `STOP` and `ERROR STOP` statements.
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## `FORT_TRUNCATE_STREAM`
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Set `FORT_TRUNCATE_STREAM=1` to make output to a formatted unit
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with `ACCESS="STREAM"` truncate the file when the unit has been
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repositioned via `POS=` to an earlier point in the file.
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This behavior is analogous to the implicit writing of an ENDFILE record
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when output takes place to a sequential unit after
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executing a `BACKSPACE` or `REWIND` statement.
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Truncation of a stream-access unit is common to several other
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compilers, but it is not mentioned in the standard.
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