1 //===-- runtime/unit.cpp ----------------------------------------*- C++ -*-===//
2 //
3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4 // See https://llvm.org/LICENSE.txt for license information.
5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6 //
7 //===----------------------------------------------------------------------===//
8 
9 #include "unit.h"
10 #include "environment.h"
11 #include "io-error.h"
12 #include "lock.h"
13 #include "unit-map.h"
14 #include <cstdio>
15 #include <utility>
16 
17 namespace Fortran::runtime::io {
18 
19 // The per-unit data structures are created on demand so that Fortran I/O
20 // should work without a Fortran main program.
21 static Lock unitMapLock;
22 static UnitMap *unitMap{nullptr};
23 static ExternalFileUnit *defaultInput{nullptr};
24 static ExternalFileUnit *defaultOutput{nullptr};
25 
26 void FlushOutputOnCrash(const Terminator &terminator) {
27   if (!defaultOutput) {
28     return;
29   }
30   CriticalSection critical{unitMapLock};
31   if (defaultOutput) {
32     IoErrorHandler handler{terminator};
33     handler.HasIoStat(); // prevent nested crash if flush has error
34     defaultOutput->Flush(handler);
35   }
36 }
37 
38 ExternalFileUnit *ExternalFileUnit::LookUp(int unit) {
39   return GetUnitMap().LookUp(unit);
40 }
41 
42 ExternalFileUnit &ExternalFileUnit::LookUpOrCrash(
43     int unit, const Terminator &terminator) {
44   ExternalFileUnit *file{LookUp(unit)};
45   if (!file) {
46     terminator.Crash("Not an open I/O unit number: %d", unit);
47   }
48   return *file;
49 }
50 
51 ExternalFileUnit &ExternalFileUnit::LookUpOrCreate(
52     int unit, const Terminator &terminator, bool &wasExtant) {
53   return GetUnitMap().LookUpOrCreate(unit, terminator, wasExtant);
54 }
55 
56 ExternalFileUnit &ExternalFileUnit::LookUpOrCreateAnonymous(
57     int unit, Direction dir, bool isUnformatted, const Terminator &terminator) {
58   bool exists{false};
59   ExternalFileUnit &result{
60       GetUnitMap().LookUpOrCreate(unit, terminator, exists)};
61   if (!exists) {
62     IoErrorHandler handler{terminator};
63     result.OpenAnonymousUnit(
64         dir == Direction::Input ? OpenStatus::Unknown : OpenStatus::Replace,
65         Action::ReadWrite, Position::Rewind, Convert::Native, handler);
66     result.isUnformatted = isUnformatted;
67   }
68   return result;
69 }
70 
71 ExternalFileUnit *ExternalFileUnit::LookUp(const char *path) {
72   return GetUnitMap().LookUp(path);
73 }
74 
75 ExternalFileUnit &ExternalFileUnit::CreateNew(
76     int unit, const Terminator &terminator) {
77   bool wasExtant{false};
78   ExternalFileUnit &result{
79       GetUnitMap().LookUpOrCreate(unit, terminator, wasExtant)};
80   RUNTIME_CHECK(terminator, !wasExtant);
81   return result;
82 }
83 
84 ExternalFileUnit *ExternalFileUnit::LookUpForClose(int unit) {
85   return GetUnitMap().LookUpForClose(unit);
86 }
87 
88 int ExternalFileUnit::NewUnit(const Terminator &terminator) {
89   return GetUnitMap().NewUnit(terminator).unitNumber();
90 }
91 
92 void ExternalFileUnit::OpenUnit(OpenStatus status, std::optional<Action> action,
93     Position position, OwningPtr<char> &&newPath, std::size_t newPathLength,
94     Convert convert, IoErrorHandler &handler) {
95   if (executionEnvironment.conversion != Convert::Unknown) {
96     convert = executionEnvironment.conversion;
97   }
98   swapEndianness_ = convert == Convert::Swap ||
99       (convert == Convert::LittleEndian && !isHostLittleEndian) ||
100       (convert == Convert::BigEndian && isHostLittleEndian);
101   if (IsOpen()) {
102     if (status == OpenStatus::Old &&
103         (!newPath.get() ||
104             (path() && pathLength() == newPathLength &&
105                 std::memcmp(path(), newPath.get(), newPathLength) == 0))) {
106       // OPEN of existing unit, STATUS='OLD', not new FILE=
107       newPath.reset();
108       return;
109     }
110     // Otherwise, OPEN on open unit with new FILE= implies CLOSE
111     DoImpliedEndfile(handler);
112     Flush(handler);
113     Close(CloseStatus::Keep, handler);
114   }
115   set_path(std::move(newPath), newPathLength);
116   Open(status, action, position, handler);
117   auto totalBytes{knownSize()};
118   if (access == Access::Direct) {
119     if (!isFixedRecordLength || !recordLength) {
120       handler.SignalError(IostatOpenBadRecl,
121           "OPEN(UNIT=%d,ACCESS='DIRECT'): record length is not known",
122           unitNumber());
123     } else if (*recordLength <= 0) {
124       handler.SignalError(IostatOpenBadRecl,
125           "OPEN(UNIT=%d,ACCESS='DIRECT',RECL=%jd): record length is invalid",
126           unitNumber(), static_cast<std::intmax_t>(*recordLength));
127     } else if (totalBytes && (*totalBytes % *recordLength != 0)) {
128       handler.SignalError(IostatOpenBadAppend,
129           "OPEN(UNIT=%d,ACCESS='DIRECT',RECL=%jd): record length is not an "
130           "even divisor of the file size %jd",
131           unitNumber(), static_cast<std::intmax_t>(*recordLength),
132           static_cast<std::intmax_t>(*totalBytes));
133     }
134   }
135   endfileRecordNumber.reset();
136   currentRecordNumber = 1;
137   if (totalBytes && recordLength && *recordLength) {
138     endfileRecordNumber = 1 + (*totalBytes / *recordLength);
139   }
140   if (position == Position::Append) {
141     if (!endfileRecordNumber) {
142       // Fake it so that we can backspace relative from the end
143       endfileRecordNumber = std::numeric_limits<std::int64_t>::max() - 2;
144     }
145     currentRecordNumber = *endfileRecordNumber;
146   }
147 }
148 
149 void ExternalFileUnit::OpenAnonymousUnit(OpenStatus status,
150     std::optional<Action> action, Position position, Convert convert,
151     IoErrorHandler &handler) {
152   // I/O to an unconnected unit reads/creates a local file, e.g. fort.7
153   std::size_t pathMaxLen{32};
154   auto path{SizedNew<char>{handler}(pathMaxLen)};
155   std::snprintf(path.get(), pathMaxLen, "fort.%d", unitNumber_);
156   OpenUnit(status, action, position, std::move(path), std::strlen(path.get()),
157       convert, handler);
158 }
159 
160 void ExternalFileUnit::CloseUnit(CloseStatus status, IoErrorHandler &handler) {
161   DoImpliedEndfile(handler);
162   Flush(handler);
163   Close(status, handler);
164 }
165 
166 void ExternalFileUnit::DestroyClosed() {
167   GetUnitMap().DestroyClosed(*this); // destroys *this
168 }
169 
170 bool ExternalFileUnit::SetDirection(
171     Direction direction, IoErrorHandler &handler) {
172   if (direction == Direction::Input) {
173     if (mayRead()) {
174       direction_ = Direction::Input;
175       return true;
176     } else {
177       handler.SignalError(IostatReadFromWriteOnly,
178           "READ(UNIT=%d) with ACTION='WRITE'", unitNumber());
179       return false;
180     }
181   } else {
182     if (mayWrite()) {
183       direction_ = Direction::Output;
184       return true;
185     } else {
186       handler.SignalError(IostatWriteToReadOnly,
187           "WRITE(UNIT=%d) with ACTION='READ'", unitNumber());
188       return false;
189     }
190   }
191 }
192 
193 UnitMap &ExternalFileUnit::GetUnitMap() {
194   if (unitMap) {
195     return *unitMap;
196   }
197   CriticalSection critical{unitMapLock};
198   if (unitMap) {
199     return *unitMap;
200   }
201   Terminator terminator{__FILE__, __LINE__};
202   IoErrorHandler handler{terminator};
203   unitMap = New<UnitMap>{terminator}().release();
204   ExternalFileUnit &out{ExternalFileUnit::CreateNew(6, terminator)};
205   out.Predefine(1);
206   out.SetDirection(Direction::Output, handler);
207   defaultOutput = &out;
208   ExternalFileUnit &in{ExternalFileUnit::CreateNew(5, terminator)};
209   in.Predefine(0);
210   in.SetDirection(Direction::Input, handler);
211   defaultInput = &in;
212   // TODO: Set UTF-8 mode from the environment
213   return *unitMap;
214 }
215 
216 void ExternalFileUnit::CloseAll(IoErrorHandler &handler) {
217   CriticalSection critical{unitMapLock};
218   if (unitMap) {
219     unitMap->CloseAll(handler);
220     FreeMemoryAndNullify(unitMap);
221   }
222   defaultOutput = nullptr;
223 }
224 
225 void ExternalFileUnit::FlushAll(IoErrorHandler &handler) {
226   CriticalSection critical{unitMapLock};
227   if (unitMap) {
228     unitMap->FlushAll(handler);
229   }
230 }
231 
232 static void SwapEndianness(
233     char *data, std::size_t bytes, std::size_t elementBytes) {
234   if (elementBytes > 1) {
235     auto half{elementBytes >> 1};
236     for (std::size_t j{0}; j + elementBytes <= bytes; j += elementBytes) {
237       for (std::size_t k{0}; k < half; ++k) {
238         std::swap(data[j + k], data[j + elementBytes - 1 - k]);
239       }
240     }
241   }
242 }
243 
244 bool ExternalFileUnit::Emit(const char *data, std::size_t bytes,
245     std::size_t elementBytes, IoErrorHandler &handler) {
246   auto furthestAfter{std::max(furthestPositionInRecord,
247       positionInRecord + static_cast<std::int64_t>(bytes))};
248   if (furthestAfter > recordLength.value_or(furthestAfter)) {
249     handler.SignalError(IostatRecordWriteOverrun,
250         "Attempt to write %zd bytes to position %jd in a fixed-size record of "
251         "%jd bytes",
252         bytes, static_cast<std::intmax_t>(positionInRecord),
253         static_cast<std::intmax_t>(*recordLength));
254     return false;
255   }
256   WriteFrame(frameOffsetInFile_, recordOffsetInFrame_ + furthestAfter, handler);
257   if (positionInRecord > furthestPositionInRecord) {
258     std::memset(Frame() + recordOffsetInFrame_ + furthestPositionInRecord, ' ',
259         positionInRecord - furthestPositionInRecord);
260   }
261   char *to{Frame() + recordOffsetInFrame_ + positionInRecord};
262   std::memcpy(to, data, bytes);
263   if (swapEndianness_) {
264     SwapEndianness(to, bytes, elementBytes);
265   }
266   positionInRecord += bytes;
267   furthestPositionInRecord = furthestAfter;
268   return true;
269 }
270 
271 bool ExternalFileUnit::Receive(char *data, std::size_t bytes,
272     std::size_t elementBytes, IoErrorHandler &handler) {
273   RUNTIME_CHECK(handler, direction_ == Direction::Input);
274   auto furthestAfter{std::max(furthestPositionInRecord,
275       positionInRecord + static_cast<std::int64_t>(bytes))};
276   if (furthestAfter > recordLength.value_or(furthestAfter)) {
277     handler.SignalError(IostatRecordReadOverrun,
278         "Attempt to read %zd bytes at position %jd in a record of %jd bytes",
279         bytes, static_cast<std::intmax_t>(positionInRecord),
280         static_cast<std::intmax_t>(*recordLength));
281     return false;
282   }
283   auto need{recordOffsetInFrame_ + furthestAfter};
284   auto got{ReadFrame(frameOffsetInFile_, need, handler)};
285   if (got >= need) {
286     std::memcpy(data, Frame() + recordOffsetInFrame_ + positionInRecord, bytes);
287     if (swapEndianness_) {
288       SwapEndianness(data, bytes, elementBytes);
289     }
290     positionInRecord += bytes;
291     furthestPositionInRecord = furthestAfter;
292     return true;
293   } else {
294     // EOF or error: can be handled & has been signaled
295     endfileRecordNumber = currentRecordNumber;
296     return false;
297   }
298 }
299 
300 std::optional<char32_t> ExternalFileUnit::GetCurrentChar(
301     IoErrorHandler &handler) {
302   RUNTIME_CHECK(handler, direction_ == Direction::Input);
303   if (const char *p{FrameNextInput(handler, 1)}) {
304     // TODO: UTF-8 decoding; may have to get more bytes in a loop
305     return *p;
306   }
307   return std::nullopt;
308 }
309 
310 const char *ExternalFileUnit::FrameNextInput(
311     IoErrorHandler &handler, std::size_t bytes) {
312   RUNTIME_CHECK(handler, !isUnformatted);
313   if (static_cast<std::int64_t>(positionInRecord + bytes) <=
314       recordLength.value_or(positionInRecord + bytes)) {
315     auto at{recordOffsetInFrame_ + positionInRecord};
316     auto need{static_cast<std::size_t>(at + bytes)};
317     auto got{ReadFrame(frameOffsetInFile_, need, handler)};
318     SetSequentialVariableFormattedRecordLength();
319     if (got >= need) {
320       return Frame() + at;
321     }
322     handler.SignalEnd();
323     endfileRecordNumber = currentRecordNumber;
324   }
325   return nullptr;
326 }
327 
328 bool ExternalFileUnit::SetSequentialVariableFormattedRecordLength() {
329   if (recordLength || access != Access::Sequential) {
330     return true;
331   }
332   if (FrameLength() > recordOffsetInFrame_) {
333     const char *record{Frame() + recordOffsetInFrame_};
334     if (const char *nl{reinterpret_cast<const char *>(
335             std::memchr(record, '\n', FrameLength() - recordOffsetInFrame_))}) {
336       recordLength = nl - record;
337       if (*recordLength > 0 && record[*recordLength - 1] == '\r') {
338         --*recordLength;
339       }
340       return true;
341     }
342   }
343   return false;
344 }
345 
346 void ExternalFileUnit::SetLeftTabLimit() {
347   leftTabLimit = furthestPositionInRecord;
348   positionInRecord = furthestPositionInRecord;
349 }
350 
351 void ExternalFileUnit::BeginReadingRecord(IoErrorHandler &handler) {
352   RUNTIME_CHECK(handler, direction_ == Direction::Input);
353   if (beganReadingRecord_) {
354     return;
355   }
356   beganReadingRecord_ = true;
357   if (access == Access::Sequential) {
358     if (endfileRecordNumber && currentRecordNumber >= *endfileRecordNumber) {
359       handler.SignalEnd();
360     } else if (isFixedRecordLength) {
361       RUNTIME_CHECK(handler, recordLength.has_value());
362       auto need{static_cast<std::size_t>(recordOffsetInFrame_ + *recordLength)};
363       auto got{ReadFrame(frameOffsetInFile_, need, handler)};
364       if (got < need) {
365         handler.SignalEnd();
366       }
367     } else if (isUnformatted) {
368       BeginSequentialVariableUnformattedInputRecord(handler);
369     } else { // formatted
370       BeginSequentialVariableFormattedInputRecord(handler);
371     }
372   }
373 }
374 
375 void ExternalFileUnit::FinishReadingRecord(IoErrorHandler &handler) {
376   RUNTIME_CHECK(handler, direction_ == Direction::Input && beganReadingRecord_);
377   beganReadingRecord_ = false;
378   if (handler.GetIoStat() != IostatOk) {
379     // avoid bogus crashes in END/ERR circumstances
380   } else if (access == Access::Sequential) {
381     RUNTIME_CHECK(handler, recordLength.has_value());
382     if (isFixedRecordLength) {
383       frameOffsetInFile_ += recordOffsetInFrame_ + *recordLength;
384       recordOffsetInFrame_ = 0;
385     } else if (isUnformatted) {
386       // Retain footer in frame for more efficient BACKSPACE
387       frameOffsetInFile_ += recordOffsetInFrame_ + *recordLength;
388       recordOffsetInFrame_ = sizeof(std::uint32_t);
389       recordLength.reset();
390     } else { // formatted
391       if (Frame()[recordOffsetInFrame_ + *recordLength] == '\r') {
392         ++recordOffsetInFrame_;
393       }
394       recordOffsetInFrame_ += *recordLength + 1;
395       RUNTIME_CHECK(handler, Frame()[recordOffsetInFrame_ - 1] == '\n');
396       recordLength.reset();
397     }
398   }
399   ++currentRecordNumber;
400   BeginRecord();
401 }
402 
403 bool ExternalFileUnit::AdvanceRecord(IoErrorHandler &handler) {
404   bool ok{true};
405   if (direction_ == Direction::Input) {
406     FinishReadingRecord(handler);
407     BeginReadingRecord(handler);
408   } else { // Direction::Output
409     if (!isUnformatted) {
410       if (isFixedRecordLength && recordLength) {
411         if (furthestPositionInRecord < *recordLength) {
412           WriteFrame(frameOffsetInFile_, *recordLength, handler);
413           std::memset(Frame() + recordOffsetInFrame_ + furthestPositionInRecord,
414               ' ', *recordLength - furthestPositionInRecord);
415         }
416       } else {
417         positionInRecord = furthestPositionInRecord;
418         ok &= Emit("\n", 1, 1, handler); // TODO: Windows CR+LF
419       }
420     }
421     frameOffsetInFile_ +=
422         recordOffsetInFrame_ + recordLength.value_or(furthestPositionInRecord);
423     recordOffsetInFrame_ = 0;
424     impliedEndfile_ = true;
425     ++currentRecordNumber;
426     BeginRecord();
427   }
428   return ok;
429 }
430 
431 void ExternalFileUnit::BackspaceRecord(IoErrorHandler &handler) {
432   if (access != Access::Sequential) {
433     handler.SignalError(IostatBackspaceNonSequential,
434         "BACKSPACE(UNIT=%d) on non-sequential file", unitNumber());
435   } else {
436     if (endfileRecordNumber && currentRecordNumber > *endfileRecordNumber) {
437       // BACKSPACE after ENDFILE
438     } else {
439       DoImpliedEndfile(handler);
440       if (frameOffsetInFile_ + recordOffsetInFrame_ > 0) {
441         --currentRecordNumber;
442         if (isFixedRecordLength) {
443           BackspaceFixedRecord(handler);
444         } else if (isUnformatted) {
445           BackspaceVariableUnformattedRecord(handler);
446         } else {
447           BackspaceVariableFormattedRecord(handler);
448         }
449       }
450     }
451     BeginRecord();
452   }
453 }
454 
455 void ExternalFileUnit::FlushIfTerminal(IoErrorHandler &handler) {
456   if (isTerminal()) {
457     Flush(handler);
458   }
459 }
460 
461 void ExternalFileUnit::Endfile(IoErrorHandler &handler) {
462   if (access != Access::Sequential) {
463     handler.SignalError(IostatEndfileNonSequential,
464         "ENDFILE(UNIT=%d) on non-sequential file", unitNumber());
465   } else if (!mayWrite()) {
466     handler.SignalError(IostatEndfileUnwritable,
467         "ENDFILE(UNIT=%d) on read-only file", unitNumber());
468   } else if (endfileRecordNumber &&
469       currentRecordNumber > *endfileRecordNumber) {
470     // ENDFILE after ENDFILE
471   } else {
472     DoEndfile(handler);
473     ++currentRecordNumber;
474   }
475 }
476 
477 void ExternalFileUnit::Rewind(IoErrorHandler &handler) {
478   if (access == Access::Direct) {
479     handler.SignalError(IostatRewindNonSequential,
480         "REWIND(UNIT=%d) on non-sequential file", unitNumber());
481   } else {
482     DoImpliedEndfile(handler);
483     SetPosition(0);
484     currentRecordNumber = 1;
485   }
486 }
487 
488 void ExternalFileUnit::EndIoStatement() {
489   frameOffsetInFile_ += recordOffsetInFrame_;
490   recordOffsetInFrame_ = 0;
491   io_.reset();
492   u_.emplace<std::monostate>();
493   lock_.Drop();
494 }
495 
496 void ExternalFileUnit::BeginSequentialVariableUnformattedInputRecord(
497     IoErrorHandler &handler) {
498   std::int32_t header{0}, footer{0};
499   std::size_t need{recordOffsetInFrame_ + sizeof header};
500   std::size_t got{ReadFrame(frameOffsetInFile_, need, handler)};
501   // Try to emit informative errors to help debug corrupted files.
502   const char *error{nullptr};
503   if (got < need) {
504     if (got == recordOffsetInFrame_) {
505       handler.SignalEnd();
506     } else {
507       error = "Unformatted variable-length sequential file input failed at "
508               "record #%jd (file offset %jd): truncated record header";
509     }
510   } else {
511     std::memcpy(&header, Frame() + recordOffsetInFrame_, sizeof header);
512     recordLength = sizeof header + header; // does not include footer
513     need = recordOffsetInFrame_ + *recordLength + sizeof footer;
514     got = ReadFrame(frameOffsetInFile_, need, handler);
515     if (got < need) {
516       error = "Unformatted variable-length sequential file input failed at "
517               "record #%jd (file offset %jd): hit EOF reading record with "
518               "length %jd bytes";
519     } else {
520       std::memcpy(&footer, Frame() + recordOffsetInFrame_ + *recordLength,
521           sizeof footer);
522       if (footer != header) {
523         error = "Unformatted variable-length sequential file input failed at "
524                 "record #%jd (file offset %jd): record header has length %jd "
525                 "that does not match record footer (%jd)";
526       }
527     }
528   }
529   if (error) {
530     handler.SignalError(error, static_cast<std::intmax_t>(currentRecordNumber),
531         static_cast<std::intmax_t>(frameOffsetInFile_),
532         static_cast<std::intmax_t>(header), static_cast<std::intmax_t>(footer));
533     // TODO: error recovery
534   }
535   positionInRecord = sizeof header;
536 }
537 
538 void ExternalFileUnit::BeginSequentialVariableFormattedInputRecord(
539     IoErrorHandler &handler) {
540   if (this == defaultInput && defaultOutput) {
541     defaultOutput->Flush(handler);
542   }
543   std::size_t length{0};
544   do {
545     std::size_t need{recordOffsetInFrame_ + length + 1};
546     length = ReadFrame(frameOffsetInFile_, need, handler);
547     if (length < need) {
548       handler.SignalEnd();
549       break;
550     }
551   } while (!SetSequentialVariableFormattedRecordLength());
552 }
553 
554 void ExternalFileUnit::BackspaceFixedRecord(IoErrorHandler &handler) {
555   RUNTIME_CHECK(handler, recordLength.has_value());
556   if (frameOffsetInFile_ < *recordLength) {
557     handler.SignalError(IostatBackspaceAtFirstRecord);
558   } else {
559     frameOffsetInFile_ -= *recordLength;
560   }
561 }
562 
563 void ExternalFileUnit::BackspaceVariableUnformattedRecord(
564     IoErrorHandler &handler) {
565   std::int32_t header{0}, footer{0};
566   auto headerBytes{static_cast<std::int64_t>(sizeof header)};
567   frameOffsetInFile_ += recordOffsetInFrame_;
568   recordOffsetInFrame_ = 0;
569   if (frameOffsetInFile_ <= headerBytes) {
570     handler.SignalError(IostatBackspaceAtFirstRecord);
571     return;
572   }
573   // Error conditions here cause crashes, not file format errors, because the
574   // validity of the file structure before the current record will have been
575   // checked informatively in NextSequentialVariableUnformattedInputRecord().
576   std::size_t got{
577       ReadFrame(frameOffsetInFile_ - headerBytes, headerBytes, handler)};
578   RUNTIME_CHECK(handler, got >= sizeof footer);
579   std::memcpy(&footer, Frame(), sizeof footer);
580   recordLength = footer;
581   RUNTIME_CHECK(handler, frameOffsetInFile_ >= *recordLength + 2 * headerBytes);
582   frameOffsetInFile_ -= *recordLength + 2 * headerBytes;
583   if (frameOffsetInFile_ >= headerBytes) {
584     frameOffsetInFile_ -= headerBytes;
585     recordOffsetInFrame_ = headerBytes;
586   }
587   auto need{static_cast<std::size_t>(
588       recordOffsetInFrame_ + sizeof header + *recordLength)};
589   got = ReadFrame(frameOffsetInFile_, need, handler);
590   RUNTIME_CHECK(handler, got >= need);
591   std::memcpy(&header, Frame() + recordOffsetInFrame_, sizeof header);
592   RUNTIME_CHECK(handler, header == *recordLength);
593 }
594 
595 // There's no portable memrchr(), unfortunately, and strrchr() would
596 // fail on a record with a NUL, so we have to do it the hard way.
597 static const char *FindLastNewline(const char *str, std::size_t length) {
598   for (const char *p{str + length}; p-- > str;) {
599     if (*p == '\n') {
600       return p;
601     }
602   }
603   return nullptr;
604 }
605 
606 void ExternalFileUnit::BackspaceVariableFormattedRecord(
607     IoErrorHandler &handler) {
608   // File offset of previous record's newline
609   auto prevNL{
610       frameOffsetInFile_ + static_cast<std::int64_t>(recordOffsetInFrame_) - 1};
611   if (prevNL < 0) {
612     handler.SignalError(IostatBackspaceAtFirstRecord);
613     return;
614   }
615   while (true) {
616     if (frameOffsetInFile_ < prevNL) {
617       if (const char *p{
618               FindLastNewline(Frame(), prevNL - 1 - frameOffsetInFile_)}) {
619         recordOffsetInFrame_ = p - Frame() + 1;
620         *recordLength = prevNL - (frameOffsetInFile_ + recordOffsetInFrame_);
621         break;
622       }
623     }
624     if (frameOffsetInFile_ == 0) {
625       recordOffsetInFrame_ = 0;
626       *recordLength = prevNL;
627       break;
628     }
629     frameOffsetInFile_ -= std::min<std::int64_t>(frameOffsetInFile_, 1024);
630     auto need{static_cast<std::size_t>(prevNL + 1 - frameOffsetInFile_)};
631     auto got{ReadFrame(frameOffsetInFile_, need, handler)};
632     RUNTIME_CHECK(handler, got >= need);
633   }
634   RUNTIME_CHECK(handler, Frame()[recordOffsetInFrame_ + *recordLength] == '\n');
635   if (*recordLength > 0 &&
636       Frame()[recordOffsetInFrame_ + *recordLength - 1] == '\r') {
637     --*recordLength;
638   }
639 }
640 
641 void ExternalFileUnit::DoImpliedEndfile(IoErrorHandler &handler) {
642   if (impliedEndfile_) {
643     impliedEndfile_ = false;
644     if (access == Access::Sequential && mayPosition()) {
645       DoEndfile(handler);
646     }
647   }
648 }
649 
650 void ExternalFileUnit::DoEndfile(IoErrorHandler &handler) {
651   endfileRecordNumber = currentRecordNumber;
652   Truncate(frameOffsetInFile_ + recordOffsetInFrame_, handler);
653   BeginRecord();
654   impliedEndfile_ = false;
655 }
656 } // namespace Fortran::runtime::io
657