1 //===-- runtime/unit.cpp --------------------------------------------------===//
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 <limits>
16 #include <utility>
17 
18 namespace Fortran::runtime::io {
19 
20 // The per-unit data structures are created on demand so that Fortran I/O
21 // should work without a Fortran main program.
22 static Lock unitMapLock;
23 static UnitMap *unitMap{nullptr};
24 static ExternalFileUnit *defaultInput{nullptr};
25 static ExternalFileUnit *defaultOutput{nullptr};
26 
27 void FlushOutputOnCrash(const Terminator &terminator) {
28   if (!defaultOutput) {
29     return;
30   }
31   CriticalSection critical{unitMapLock};
32   if (defaultOutput) {
33     IoErrorHandler handler{terminator};
34     handler.HasIoStat(); // prevent nested crash if flush has error
35     defaultOutput->FlushOutput(handler);
36   }
37 }
38 
39 ExternalFileUnit *ExternalFileUnit::LookUp(int unit) {
40   return GetUnitMap().LookUp(unit);
41 }
42 
43 ExternalFileUnit &ExternalFileUnit::LookUpOrCrash(
44     int unit, const Terminator &terminator) {
45   ExternalFileUnit *file{LookUp(unit)};
46   if (!file) {
47     terminator.Crash("Not an open I/O unit number: %d", unit);
48   }
49   return *file;
50 }
51 
52 ExternalFileUnit &ExternalFileUnit::LookUpOrCreate(
53     int unit, const Terminator &terminator, bool &wasExtant) {
54   return GetUnitMap().LookUpOrCreate(unit, terminator, wasExtant);
55 }
56 
57 ExternalFileUnit &ExternalFileUnit::LookUpOrCreateAnonymous(int unit,
58     Direction dir, std::optional<bool> isUnformatted,
59     const Terminator &terminator) {
60   bool exists{false};
61   ExternalFileUnit &result{
62       GetUnitMap().LookUpOrCreate(unit, terminator, exists)};
63   if (!exists) {
64     IoErrorHandler handler{terminator};
65     result.OpenAnonymousUnit(
66         dir == Direction::Input ? OpenStatus::Unknown : OpenStatus::Replace,
67         Action::ReadWrite, Position::Rewind, Convert::Native, handler);
68     result.isUnformatted = isUnformatted;
69   }
70   return result;
71 }
72 
73 ExternalFileUnit *ExternalFileUnit::LookUp(const char *path) {
74   return GetUnitMap().LookUp(path);
75 }
76 
77 ExternalFileUnit &ExternalFileUnit::CreateNew(
78     int unit, const Terminator &terminator) {
79   bool wasExtant{false};
80   ExternalFileUnit &result{
81       GetUnitMap().LookUpOrCreate(unit, terminator, wasExtant)};
82   RUNTIME_CHECK(terminator, !wasExtant);
83   return result;
84 }
85 
86 ExternalFileUnit *ExternalFileUnit::LookUpForClose(int unit) {
87   return GetUnitMap().LookUpForClose(unit);
88 }
89 
90 ExternalFileUnit &ExternalFileUnit::NewUnit(
91     const Terminator &terminator, bool forChildIo) {
92   ExternalFileUnit &unit{GetUnitMap().NewUnit(terminator)};
93   unit.createdForInternalChildIo_ = forChildIo;
94   return unit;
95 }
96 
97 void ExternalFileUnit::OpenUnit(std::optional<OpenStatus> status,
98     std::optional<Action> action, Position position, OwningPtr<char> &&newPath,
99     std::size_t newPathLength, Convert convert, IoErrorHandler &handler) {
100   if (executionEnvironment.conversion != Convert::Unknown) {
101     convert = executionEnvironment.conversion;
102   }
103   swapEndianness_ = convert == Convert::Swap ||
104       (convert == Convert::LittleEndian && !isHostLittleEndian) ||
105       (convert == Convert::BigEndian && isHostLittleEndian);
106   if (IsOpen()) {
107     bool isSamePath{newPath.get() && path() && pathLength() == newPathLength &&
108         std::memcmp(path(), newPath.get(), newPathLength) == 0};
109     if (status && *status != OpenStatus::Old && isSamePath) {
110       handler.SignalError("OPEN statement for connected unit may not have "
111                           "explicit STATUS= other than 'OLD'");
112       return;
113     }
114     if (!newPath.get() || isSamePath) {
115       // OPEN of existing unit, STATUS='OLD' or unspecified, not new FILE=
116       newPath.reset();
117       return;
118     }
119     // Otherwise, OPEN on open unit with new FILE= implies CLOSE
120     DoImpliedEndfile(handler);
121     FlushOutput(handler);
122     Close(CloseStatus::Keep, handler);
123   }
124   set_path(std::move(newPath), newPathLength);
125   Open(status.value_or(OpenStatus::Unknown), action, position, handler);
126   auto totalBytes{knownSize()};
127   if (access == Access::Direct) {
128     if (!isFixedRecordLength || !recordLength) {
129       handler.SignalError(IostatOpenBadRecl,
130           "OPEN(UNIT=%d,ACCESS='DIRECT'): record length is not known",
131           unitNumber());
132     } else if (*recordLength <= 0) {
133       handler.SignalError(IostatOpenBadRecl,
134           "OPEN(UNIT=%d,ACCESS='DIRECT',RECL=%jd): record length is invalid",
135           unitNumber(), static_cast<std::intmax_t>(*recordLength));
136     } else if (totalBytes && (*totalBytes % *recordLength != 0)) {
137       handler.SignalError(IostatOpenBadAppend,
138           "OPEN(UNIT=%d,ACCESS='DIRECT',RECL=%jd): record length is not an "
139           "even divisor of the file size %jd",
140           unitNumber(), static_cast<std::intmax_t>(*recordLength),
141           static_cast<std::intmax_t>(*totalBytes));
142     }
143   }
144   endfileRecordNumber.reset();
145   currentRecordNumber = 1;
146   if (totalBytes && recordLength && *recordLength) {
147     endfileRecordNumber = 1 + (*totalBytes / *recordLength);
148   }
149   if (position == Position::Append) {
150     if (!endfileRecordNumber) {
151       // Fake it so that we can backspace relative from the end
152       endfileRecordNumber = std::numeric_limits<std::int64_t>::max() - 2;
153     }
154     currentRecordNumber = *endfileRecordNumber;
155   }
156 }
157 
158 void ExternalFileUnit::OpenAnonymousUnit(std::optional<OpenStatus> status,
159     std::optional<Action> action, Position position, Convert convert,
160     IoErrorHandler &handler) {
161   // I/O to an unconnected unit reads/creates a local file, e.g. fort.7
162   std::size_t pathMaxLen{32};
163   auto path{SizedNew<char>{handler}(pathMaxLen)};
164   std::snprintf(path.get(), pathMaxLen, "fort.%d", unitNumber_);
165   OpenUnit(status, action, position, std::move(path), std::strlen(path.get()),
166       convert, handler);
167 }
168 
169 void ExternalFileUnit::CloseUnit(CloseStatus status, IoErrorHandler &handler) {
170   DoImpliedEndfile(handler);
171   FlushOutput(handler);
172   Close(status, handler);
173 }
174 
175 void ExternalFileUnit::DestroyClosed() {
176   GetUnitMap().DestroyClosed(*this); // destroys *this
177 }
178 
179 bool ExternalFileUnit::SetDirection(
180     Direction direction, IoErrorHandler &handler) {
181   if (direction == Direction::Input) {
182     if (mayRead()) {
183       direction_ = Direction::Input;
184       return true;
185     } else {
186       handler.SignalError(IostatReadFromWriteOnly,
187           "READ(UNIT=%d) with ACTION='WRITE'", unitNumber());
188       return false;
189     }
190   } else {
191     if (mayWrite()) {
192       direction_ = Direction::Output;
193       return true;
194     } else {
195       handler.SignalError(IostatWriteToReadOnly,
196           "WRITE(UNIT=%d) with ACTION='READ'", unitNumber());
197       return false;
198     }
199   }
200 }
201 
202 UnitMap &ExternalFileUnit::GetUnitMap() {
203   if (unitMap) {
204     return *unitMap;
205   }
206   CriticalSection critical{unitMapLock};
207   if (unitMap) {
208     return *unitMap;
209   }
210   Terminator terminator{__FILE__, __LINE__};
211   IoErrorHandler handler{terminator};
212   UnitMap *newUnitMap{New<UnitMap>{terminator}().release()};
213   bool wasExtant{false};
214   ExternalFileUnit &out{newUnitMap->LookUpOrCreate(6, terminator, wasExtant)};
215   RUNTIME_CHECK(terminator, !wasExtant);
216   out.Predefine(1);
217   out.SetDirection(Direction::Output, handler);
218   defaultOutput = &out;
219   ExternalFileUnit &in{newUnitMap->LookUpOrCreate(5, terminator, wasExtant)};
220   RUNTIME_CHECK(terminator, !wasExtant);
221   in.Predefine(0);
222   in.SetDirection(Direction::Input, handler);
223   defaultInput = &in;
224   // TODO: Set UTF-8 mode from the environment
225   unitMap = newUnitMap;
226   return *unitMap;
227 }
228 
229 void ExternalFileUnit::CloseAll(IoErrorHandler &handler) {
230   CriticalSection critical{unitMapLock};
231   if (unitMap) {
232     unitMap->CloseAll(handler);
233     FreeMemoryAndNullify(unitMap);
234   }
235   defaultOutput = nullptr;
236 }
237 
238 void ExternalFileUnit::FlushAll(IoErrorHandler &handler) {
239   CriticalSection critical{unitMapLock};
240   if (unitMap) {
241     unitMap->FlushAll(handler);
242   }
243 }
244 
245 static void SwapEndianness(
246     char *data, std::size_t bytes, std::size_t elementBytes) {
247   if (elementBytes > 1) {
248     auto half{elementBytes >> 1};
249     for (std::size_t j{0}; j + elementBytes <= bytes; j += elementBytes) {
250       for (std::size_t k{0}; k < half; ++k) {
251         std::swap(data[j + k], data[j + elementBytes - 1 - k]);
252       }
253     }
254   }
255 }
256 
257 bool ExternalFileUnit::Emit(const char *data, std::size_t bytes,
258     std::size_t elementBytes, IoErrorHandler &handler) {
259   auto furthestAfter{std::max(furthestPositionInRecord,
260       positionInRecord + static_cast<std::int64_t>(bytes))};
261   if (recordLength) {
262     // It is possible for recordLength to have a value now for a
263     // variable-length output record if the previous operation
264     // was a BACKSPACE.
265     if (!isFixedRecordLength) {
266       recordLength.reset();
267     } else if (furthestAfter > *recordLength) {
268       handler.SignalError(IostatRecordWriteOverrun,
269           "Attempt to write %zd bytes to position %jd in a fixed-size record "
270           "of %jd bytes",
271           bytes, static_cast<std::intmax_t>(positionInRecord),
272           static_cast<std::intmax_t>(*recordLength));
273       return false;
274     }
275   }
276   WriteFrame(frameOffsetInFile_, recordOffsetInFrame_ + furthestAfter, handler);
277   if (positionInRecord > furthestPositionInRecord) {
278     std::memset(Frame() + recordOffsetInFrame_ + furthestPositionInRecord, ' ',
279         positionInRecord - furthestPositionInRecord);
280   }
281   char *to{Frame() + recordOffsetInFrame_ + positionInRecord};
282   std::memcpy(to, data, bytes);
283   if (swapEndianness_) {
284     SwapEndianness(to, bytes, elementBytes);
285   }
286   positionInRecord += bytes;
287   furthestPositionInRecord = furthestAfter;
288   return true;
289 }
290 
291 bool ExternalFileUnit::Receive(char *data, std::size_t bytes,
292     std::size_t elementBytes, IoErrorHandler &handler) {
293   RUNTIME_CHECK(handler, direction_ == Direction::Input);
294   auto furthestAfter{std::max(furthestPositionInRecord,
295       positionInRecord + static_cast<std::int64_t>(bytes))};
296   if (furthestAfter > recordLength.value_or(furthestAfter)) {
297     handler.SignalError(IostatRecordReadOverrun,
298         "Attempt to read %zd bytes at position %jd in a record of %jd bytes",
299         bytes, static_cast<std::intmax_t>(positionInRecord),
300         static_cast<std::intmax_t>(*recordLength));
301     return false;
302   }
303   auto need{recordOffsetInFrame_ + furthestAfter};
304   auto got{ReadFrame(frameOffsetInFile_, need, handler)};
305   if (got >= need) {
306     std::memcpy(data, Frame() + recordOffsetInFrame_ + positionInRecord, bytes);
307     if (swapEndianness_) {
308       SwapEndianness(data, bytes, elementBytes);
309     }
310     positionInRecord += bytes;
311     furthestPositionInRecord = furthestAfter;
312     return true;
313   } else {
314     // EOF or error: can be handled & has been signaled
315     endfileRecordNumber = currentRecordNumber;
316     return false;
317   }
318 }
319 
320 std::optional<char32_t> ExternalFileUnit::GetCurrentChar(
321     IoErrorHandler &handler) {
322   RUNTIME_CHECK(handler, direction_ == Direction::Input);
323   if (const char *p{FrameNextInput(handler, 1)}) {
324     // TODO: UTF-8 decoding; may have to get more bytes in a loop
325     return *p;
326   }
327   return std::nullopt;
328 }
329 
330 const char *ExternalFileUnit::FrameNextInput(
331     IoErrorHandler &handler, std::size_t bytes) {
332   RUNTIME_CHECK(handler, isUnformatted.has_value() && !*isUnformatted);
333   if (static_cast<std::int64_t>(positionInRecord + bytes) <=
334       recordLength.value_or(positionInRecord + bytes)) {
335     auto at{recordOffsetInFrame_ + positionInRecord};
336     auto need{static_cast<std::size_t>(at + bytes)};
337     auto got{ReadFrame(frameOffsetInFile_, need, handler)};
338     SetSequentialVariableFormattedRecordLength();
339     if (got >= need) {
340       return Frame() + at;
341     }
342     handler.SignalEnd();
343     endfileRecordNumber = currentRecordNumber;
344   }
345   return nullptr;
346 }
347 
348 bool ExternalFileUnit::SetSequentialVariableFormattedRecordLength() {
349   if (recordLength || access != Access::Sequential) {
350     return true;
351   } else if (FrameLength() > recordOffsetInFrame_) {
352     const char *record{Frame() + recordOffsetInFrame_};
353     std::size_t bytes{FrameLength() - recordOffsetInFrame_};
354     if (const char *nl{
355             reinterpret_cast<const char *>(std::memchr(record, '\n', bytes))}) {
356       recordLength = nl - record;
357       if (*recordLength > 0 && record[*recordLength - 1] == '\r') {
358         --*recordLength;
359       }
360       return true;
361     }
362   }
363   return false;
364 }
365 
366 void ExternalFileUnit::SetLeftTabLimit() {
367   leftTabLimit = furthestPositionInRecord;
368   positionInRecord = furthestPositionInRecord;
369 }
370 
371 bool ExternalFileUnit::BeginReadingRecord(IoErrorHandler &handler) {
372   RUNTIME_CHECK(handler, direction_ == Direction::Input);
373   if (!beganReadingRecord_) {
374     beganReadingRecord_ = true;
375     if (access == Access::Sequential) {
376       if (endfileRecordNumber && currentRecordNumber >= *endfileRecordNumber) {
377         handler.SignalEnd();
378       } else if (isFixedRecordLength) {
379         RUNTIME_CHECK(handler, recordLength.has_value());
380         auto need{
381             static_cast<std::size_t>(recordOffsetInFrame_ + *recordLength)};
382         auto got{ReadFrame(frameOffsetInFile_, need, handler)};
383         if (got < need) {
384           handler.SignalEnd();
385         }
386       } else {
387         RUNTIME_CHECK(handler, isUnformatted.has_value());
388         if (isUnformatted.value_or(false)) {
389           BeginSequentialVariableUnformattedInputRecord(handler);
390         } else { // formatted
391           BeginSequentialVariableFormattedInputRecord(handler);
392         }
393       }
394     }
395   }
396   RUNTIME_CHECK(handler,
397       access != Access::Sequential || recordLength.has_value() ||
398           handler.InError());
399   return !handler.InError();
400 }
401 
402 void ExternalFileUnit::FinishReadingRecord(IoErrorHandler &handler) {
403   RUNTIME_CHECK(handler, direction_ == Direction::Input && beganReadingRecord_);
404   beganReadingRecord_ = false;
405   if (handler.InError()) {
406     // avoid bogus crashes in END/ERR circumstances
407   } else if (access == Access::Sequential) {
408     RUNTIME_CHECK(handler, recordLength.has_value());
409     if (isFixedRecordLength) {
410       frameOffsetInFile_ += recordOffsetInFrame_ + *recordLength;
411       recordOffsetInFrame_ = 0;
412     } else {
413       RUNTIME_CHECK(handler, isUnformatted.has_value());
414       if (isUnformatted.value_or(false)) {
415         // Retain footer in frame for more efficient BACKSPACE
416         frameOffsetInFile_ += recordOffsetInFrame_ + *recordLength;
417         recordOffsetInFrame_ = sizeof(std::uint32_t);
418         recordLength.reset();
419       } else { // formatted
420         if (FrameLength() > recordOffsetInFrame_ + *recordLength &&
421             Frame()[recordOffsetInFrame_ + *recordLength] == '\r') {
422           ++recordOffsetInFrame_;
423         }
424         if (FrameLength() >= recordOffsetInFrame_ &&
425             Frame()[recordOffsetInFrame_ + *recordLength] == '\n') {
426           ++recordOffsetInFrame_;
427         }
428         if (!resumptionRecordNumber || mayPosition()) {
429           frameOffsetInFile_ += recordOffsetInFrame_ + *recordLength;
430           recordOffsetInFrame_ = 0;
431         }
432         recordLength.reset();
433       }
434     }
435   }
436   ++currentRecordNumber;
437   BeginRecord();
438 }
439 
440 bool ExternalFileUnit::AdvanceRecord(IoErrorHandler &handler) {
441   if (direction_ == Direction::Input) {
442     FinishReadingRecord(handler);
443     return BeginReadingRecord(handler);
444   } else { // Direction::Output
445     bool ok{true};
446     RUNTIME_CHECK(handler, isUnformatted.has_value());
447     if (isFixedRecordLength && recordLength) {
448       // Pad remainder of fixed length record
449       if (furthestPositionInRecord < *recordLength) {
450         WriteFrame(
451             frameOffsetInFile_, recordOffsetInFrame_ + *recordLength, handler);
452         std::memset(Frame() + recordOffsetInFrame_ + furthestPositionInRecord,
453             isUnformatted.value_or(false) ? 0 : ' ',
454             *recordLength - furthestPositionInRecord);
455       }
456     } else {
457       positionInRecord = furthestPositionInRecord;
458       if (isUnformatted.value_or(false)) {
459         // Append the length of a sequential unformatted variable-length record
460         // as its footer, then overwrite the reserved first four bytes of the
461         // record with its length as its header.  These four bytes were skipped
462         // over in BeginUnformattedIO<Output>().
463         // TODO: Break very large records up into subrecords with negative
464         // headers &/or footers
465         std::uint32_t length;
466         length = furthestPositionInRecord - sizeof length;
467         ok = ok &&
468             Emit(reinterpret_cast<const char *>(&length), sizeof length,
469                 sizeof length, handler);
470         positionInRecord = 0;
471         ok = ok &&
472             Emit(reinterpret_cast<const char *>(&length), sizeof length,
473                 sizeof length, handler);
474       } else {
475         // Terminate formatted variable length record
476         ok = ok && Emit("\n", 1, 1, handler); // TODO: Windows CR+LF
477       }
478     }
479     CommitWrites();
480     impliedEndfile_ = true;
481     ++currentRecordNumber;
482     if (endfileRecordNumber && currentRecordNumber >= *endfileRecordNumber) {
483       endfileRecordNumber.reset();
484     }
485     return ok;
486   }
487 }
488 
489 void ExternalFileUnit::BackspaceRecord(IoErrorHandler &handler) {
490   if (access != Access::Sequential) {
491     handler.SignalError(IostatBackspaceNonSequential,
492         "BACKSPACE(UNIT=%d) on non-sequential file", unitNumber());
493   } else {
494     if (endfileRecordNumber && currentRecordNumber > *endfileRecordNumber) {
495       // BACKSPACE after explicit ENDFILE
496       currentRecordNumber = *endfileRecordNumber;
497     } else {
498       DoImpliedEndfile(handler);
499       if (frameOffsetInFile_ + recordOffsetInFrame_ > 0) {
500         --currentRecordNumber;
501         if (isFixedRecordLength) {
502           BackspaceFixedRecord(handler);
503         } else {
504           RUNTIME_CHECK(handler, isUnformatted.has_value());
505           if (isUnformatted.value_or(false)) {
506             BackspaceVariableUnformattedRecord(handler);
507           } else {
508             BackspaceVariableFormattedRecord(handler);
509           }
510         }
511       }
512     }
513     BeginRecord();
514   }
515 }
516 
517 void ExternalFileUnit::FlushOutput(IoErrorHandler &handler) {
518   if (!mayPosition()) {
519     auto frameAt{FrameAt()};
520     if (frameOffsetInFile_ >= frameAt &&
521         frameOffsetInFile_ <
522             static_cast<std::int64_t>(frameAt + FrameLength())) {
523       // A Flush() that's about to happen to a non-positionable file
524       // needs to advance frameOffsetInFile_ to prevent attempts at
525       // impossible seeks
526       CommitWrites();
527     }
528   }
529   Flush(handler);
530 }
531 
532 void ExternalFileUnit::FlushIfTerminal(IoErrorHandler &handler) {
533   if (isTerminal()) {
534     FlushOutput(handler);
535   }
536 }
537 
538 void ExternalFileUnit::Endfile(IoErrorHandler &handler) {
539   if (access != Access::Sequential) {
540     handler.SignalError(IostatEndfileNonSequential,
541         "ENDFILE(UNIT=%d) on non-sequential file", unitNumber());
542   } else if (!mayWrite()) {
543     handler.SignalError(IostatEndfileUnwritable,
544         "ENDFILE(UNIT=%d) on read-only file", unitNumber());
545   } else if (endfileRecordNumber &&
546       currentRecordNumber > *endfileRecordNumber) {
547     // ENDFILE after ENDFILE
548   } else {
549     DoEndfile(handler);
550     // Explicit ENDFILE leaves position *after* the endfile record
551     RUNTIME_CHECK(handler, endfileRecordNumber.has_value());
552     currentRecordNumber = *endfileRecordNumber + 1;
553   }
554 }
555 
556 void ExternalFileUnit::Rewind(IoErrorHandler &handler) {
557   if (access == Access::Direct) {
558     handler.SignalError(IostatRewindNonSequential,
559         "REWIND(UNIT=%d) on non-sequential file", unitNumber());
560   } else {
561     DoImpliedEndfile(handler);
562     SetPosition(0);
563     currentRecordNumber = 1;
564   }
565 }
566 
567 void ExternalFileUnit::EndIoStatement() {
568   io_.reset();
569   u_.emplace<std::monostate>();
570   lock_.Drop();
571 }
572 
573 void ExternalFileUnit::BeginSequentialVariableUnformattedInputRecord(
574     IoErrorHandler &handler) {
575   std::int32_t header{0}, footer{0};
576   std::size_t need{recordOffsetInFrame_ + sizeof header};
577   std::size_t got{ReadFrame(frameOffsetInFile_, need, handler)};
578   // Try to emit informative errors to help debug corrupted files.
579   const char *error{nullptr};
580   if (got < need) {
581     if (got == recordOffsetInFrame_) {
582       handler.SignalEnd();
583     } else {
584       error = "Unformatted variable-length sequential file input failed at "
585               "record #%jd (file offset %jd): truncated record header";
586     }
587   } else {
588     std::memcpy(&header, Frame() + recordOffsetInFrame_, sizeof header);
589     recordLength = sizeof header + header; // does not include footer
590     need = recordOffsetInFrame_ + *recordLength + sizeof footer;
591     got = ReadFrame(frameOffsetInFile_, need, handler);
592     if (got < need) {
593       error = "Unformatted variable-length sequential file input failed at "
594               "record #%jd (file offset %jd): hit EOF reading record with "
595               "length %jd bytes";
596     } else {
597       std::memcpy(&footer, Frame() + recordOffsetInFrame_ + *recordLength,
598           sizeof footer);
599       if (footer != header) {
600         error = "Unformatted variable-length sequential file input failed at "
601                 "record #%jd (file offset %jd): record header has length %jd "
602                 "that does not match record footer (%jd)";
603       }
604     }
605   }
606   if (error) {
607     handler.SignalError(error, static_cast<std::intmax_t>(currentRecordNumber),
608         static_cast<std::intmax_t>(frameOffsetInFile_),
609         static_cast<std::intmax_t>(header), static_cast<std::intmax_t>(footer));
610     // TODO: error recovery
611   }
612   positionInRecord = sizeof header;
613 }
614 
615 void ExternalFileUnit::BeginSequentialVariableFormattedInputRecord(
616     IoErrorHandler &handler) {
617   if (this == defaultInput && defaultOutput) {
618     defaultOutput->FlushOutput(handler);
619   }
620   std::size_t length{0};
621   do {
622     std::size_t need{length + 1};
623     length =
624         ReadFrame(frameOffsetInFile_, recordOffsetInFrame_ + need, handler) -
625         recordOffsetInFrame_;
626     if (length < need) {
627       if (length > 0) {
628         // final record w/o \n
629         recordLength = length;
630       } else {
631         handler.SignalEnd();
632       }
633       break;
634     }
635   } while (!SetSequentialVariableFormattedRecordLength());
636 }
637 
638 void ExternalFileUnit::BackspaceFixedRecord(IoErrorHandler &handler) {
639   RUNTIME_CHECK(handler, recordLength.has_value());
640   if (frameOffsetInFile_ < *recordLength) {
641     handler.SignalError(IostatBackspaceAtFirstRecord);
642   } else {
643     frameOffsetInFile_ -= *recordLength;
644   }
645 }
646 
647 void ExternalFileUnit::BackspaceVariableUnformattedRecord(
648     IoErrorHandler &handler) {
649   std::int32_t header{0}, footer{0};
650   auto headerBytes{static_cast<std::int64_t>(sizeof header)};
651   frameOffsetInFile_ += recordOffsetInFrame_;
652   recordOffsetInFrame_ = 0;
653   if (frameOffsetInFile_ <= headerBytes) {
654     handler.SignalError(IostatBackspaceAtFirstRecord);
655     return;
656   }
657   // Error conditions here cause crashes, not file format errors, because the
658   // validity of the file structure before the current record will have been
659   // checked informatively in NextSequentialVariableUnformattedInputRecord().
660   std::size_t got{
661       ReadFrame(frameOffsetInFile_ - headerBytes, headerBytes, handler)};
662   RUNTIME_CHECK(handler, got >= sizeof footer);
663   std::memcpy(&footer, Frame(), sizeof footer);
664   recordLength = footer;
665   RUNTIME_CHECK(handler, frameOffsetInFile_ >= *recordLength + 2 * headerBytes);
666   frameOffsetInFile_ -= *recordLength + 2 * headerBytes;
667   if (frameOffsetInFile_ >= headerBytes) {
668     frameOffsetInFile_ -= headerBytes;
669     recordOffsetInFrame_ = headerBytes;
670   }
671   auto need{static_cast<std::size_t>(
672       recordOffsetInFrame_ + sizeof header + *recordLength)};
673   got = ReadFrame(frameOffsetInFile_, need, handler);
674   RUNTIME_CHECK(handler, got >= need);
675   std::memcpy(&header, Frame() + recordOffsetInFrame_, sizeof header);
676   RUNTIME_CHECK(handler, header == *recordLength);
677 }
678 
679 // There's no portable memrchr(), unfortunately, and strrchr() would
680 // fail on a record with a NUL, so we have to do it the hard way.
681 static const char *FindLastNewline(const char *str, std::size_t length) {
682   for (const char *p{str + length}; p-- > str;) {
683     if (*p == '\n') {
684       return p;
685     }
686   }
687   return nullptr;
688 }
689 
690 void ExternalFileUnit::BackspaceVariableFormattedRecord(
691     IoErrorHandler &handler) {
692   // File offset of previous record's newline
693   auto prevNL{
694       frameOffsetInFile_ + static_cast<std::int64_t>(recordOffsetInFrame_) - 1};
695   if (prevNL < 0) {
696     handler.SignalError(IostatBackspaceAtFirstRecord);
697     return;
698   }
699   while (true) {
700     if (frameOffsetInFile_ < prevNL) {
701       if (const char *p{
702               FindLastNewline(Frame(), prevNL - 1 - frameOffsetInFile_)}) {
703         recordOffsetInFrame_ = p - Frame() + 1;
704         recordLength = prevNL - (frameOffsetInFile_ + recordOffsetInFrame_);
705         break;
706       }
707     }
708     if (frameOffsetInFile_ == 0) {
709       recordOffsetInFrame_ = 0;
710       recordLength = prevNL;
711       break;
712     }
713     frameOffsetInFile_ -= std::min<std::int64_t>(frameOffsetInFile_, 1024);
714     auto need{static_cast<std::size_t>(prevNL + 1 - frameOffsetInFile_)};
715     auto got{ReadFrame(frameOffsetInFile_, need, handler)};
716     RUNTIME_CHECK(handler, got >= need);
717   }
718   RUNTIME_CHECK(handler, Frame()[recordOffsetInFrame_ + *recordLength] == '\n');
719   if (*recordLength > 0 &&
720       Frame()[recordOffsetInFrame_ + *recordLength - 1] == '\r') {
721     --*recordLength;
722   }
723 }
724 
725 void ExternalFileUnit::DoImpliedEndfile(IoErrorHandler &handler) {
726   if (impliedEndfile_) {
727     impliedEndfile_ = false;
728     if (access == Access::Sequential && mayPosition()) {
729       DoEndfile(handler);
730     }
731   }
732 }
733 
734 void ExternalFileUnit::DoEndfile(IoErrorHandler &handler) {
735   endfileRecordNumber = currentRecordNumber;
736   Truncate(frameOffsetInFile_ + recordOffsetInFrame_, handler);
737   BeginRecord();
738   impliedEndfile_ = false;
739 }
740 
741 void ExternalFileUnit::CommitWrites() {
742   frameOffsetInFile_ +=
743       recordOffsetInFrame_ + recordLength.value_or(furthestPositionInRecord);
744   recordOffsetInFrame_ = 0;
745   BeginRecord();
746 }
747 
748 ChildIo &ExternalFileUnit::PushChildIo(IoStatementState &parent) {
749   OwningPtr<ChildIo> current{std::move(child_)};
750   Terminator &terminator{parent.GetIoErrorHandler()};
751   OwningPtr<ChildIo> next{New<ChildIo>{terminator}(parent, std::move(current))};
752   child_.reset(next.release());
753   return *child_;
754 }
755 
756 void ExternalFileUnit::PopChildIo(ChildIo &child) {
757   if (child_.get() != &child) {
758     child.parent().GetIoErrorHandler().Crash(
759         "ChildIo being popped is not top of stack");
760   }
761   child_.reset(child.AcquirePrevious().release()); // deletes top child
762 }
763 
764 void ChildIo::EndIoStatement() {
765   io_.reset();
766   u_.emplace<std::monostate>();
767 }
768 
769 bool ChildIo::CheckFormattingAndDirection(Terminator &terminator,
770     const char *what, bool unformatted, Direction direction) {
771   bool parentIsUnformatted{!parent_.get_if<FormattedIoStatementState>()};
772   bool parentIsInput{!parent_.get_if<IoDirectionState<Direction::Output>>()};
773   if (unformatted != parentIsUnformatted) {
774     terminator.Crash("Child %s attempted on %s parent I/O unit", what,
775         parentIsUnformatted ? "unformatted" : "formatted");
776     return false;
777   } else if (parentIsInput != (direction == Direction::Input)) {
778     terminator.Crash("Child %s attempted on %s parent I/O unit", what,
779         parentIsInput ? "input" : "output");
780     return false;
781   } else {
782     return true;
783   }
784 }
785 
786 } // namespace Fortran::runtime::io
787