Fortran I/O Runtime Library Internal Design

This note is meant to be an overview of the design of the implementation of the f18 Fortran compiler’s runtime support library for I/O statements.

The interface to the I/O runtime support library is defined in the C++ header file runtime/io-api.h. This interface was designed to minimize the amount of complexity exposed to its clients, which are of course the sequences of calls generated by the compiler to implement each I/O statement. By keeping this interface as simple as possible, we hope that we have lowered the risk of future incompatible changes that would necessitate recompilation of Fortran codes in order to link with later versions of the runtime library. As one will see in io-api.h, the interface is also directly callable from C and C++ programs.

The I/O facilities of the Fortran 2018 language are specified in the language standard in its clauses 12 (I/O statements) and 13 (FORMAT). It’s a complicated collection of language features:

  • Files can comprise records or streams.
  • Records can be fixed-length or variable-length.
  • Record files can be accessed sequentially or directly (random access).
  • Files can be formatted, or unformatted raw bits.
  • CHARACTER scalars and arrays can be used as if they were fixed-length formatted sequential record files.
  • Formatted I/O can be under control of a FORMAT statement or FMT= specifier, list-directed with default formatting chosen by the runtime, or NAMELIST, in which a collection of variables can be given a name and passed as a group to the runtime library.
  • Sequential records of a file can be partially processed by one or more non-advancing I/O statements and eventually completed by another.
  • FORMAT strings can manipulate the position in the current record arbitrarily, causing re-reading or overwriting.
  • Floating-point output formatting supports more rounding modes than the IEEE standard for floating-point arithmetic.

The Fortran I/O runtime support library is written in C++17, and uses some C++17 standard library facilities, but it is intended to not have any link-time dependences on the C++ runtime support library or any LLVM libraries. This is important because there are at least two C++ runtime support libraries, and we don’t want Fortran application builders to have to build multiple versions of their codes; neither do we want to require them to ship LLVM libraries along with their products.

Consequently, dynamic memory allocation in the Fortran runtime uses only C’s malloc() and free() functions, and the few C++ standard class templates that we instantiate in the library have been modified with optional template arguments that override their allocators and deallocators.

Conversions between the many binary floating-point formats supported by f18 and their decimal representations are performed with the same template library of fast conversion algorithms used to interpret floating-point values in Fortran source programs and to emit them to module files.

Overview of Classes

A suite of C++ classes and class templates are composed to construct the Fortran I/O runtime support library. They (mostly) reside in the C++ namespace Fortran::runtime::io. They are summarized here in a bottom-up order of dependence.

The header and C++ implementation source file names of these classes are in the process of being vigorously rearranged and modified; use grep or an IDE to discover these classes in the source for now. (Sorry!)


A general facility for the entire library, Terminator latches a source program statement location in terms of an unowned pointer to its source file path name and line number and uses them to construct a fatal error message if needed. It is used for both user program errors and internal runtime library crashes.


When I/O error conditions arise at runtime that the Fortran program might have the privilege to handle itself via ERR=, END=, or EOR= labels and/or by an IOSTAT= variable, this subclass of Terminator is used to either latch the error indication or to crash. It sorts out priorities in the case of multiple errors and determines the final IOSTAT= value at the end of an I/O statement.


Fortran’s formatted I/O statements are affected by a suite of modes that can be configured by OPEN statements, overridden by data transfer I/O statement control lists, and further overridden between data items with control edit descriptors in a FORMAT string. These modes are represented with a MutableModes instance, and these are instantiated and copied where one would expect them to be in order to properly isolate their modifications. The modes in force at the time each data item is processed constitute a member of each DataEdit.


Represents a single data edit descriptor from a FORMAT statement or FMT= character value, with some hidden extensions to also support formatting of list-directed transfers. It holds an instance of MutableModes, and also has a repetition count for when an array appears as a data item in the io-list. For simplicity and efficiency, each data edit descriptor is encoded in the DataEdit as a simple capitalized character (or two) and some optional field widths.


This class template traverses a FORMAT statement’s contents (or FMT= character value) to extract data edit descriptors like E20.14 to serve each item in an I/O data transfer statement’s io-list, making callbacks to an instance of its class template argument along the way to effect character literal output and record positioning. The Fortran language standard defines formatted I/O as if the FORMAT string were driving the traversal of the data items in the io-list, but our implementation reverses that perspective to allow a more convenient (for the compiler) I/O runtime support library API design in which each data item is presented to the library with a distinct type-dependent call.

Clients of FormatControl instantiations call its GetNextDataEdit() member function to acquire the next data edit descriptor to be processed from the format, and FinishOutput() to flush out any remaining output strings or record positionings at the end of the io-list.

The DefaultFormatControlCallbacks structure summarizes the API expected by FormatControl from its class template actual arguments.


This class encapsulates all (I hope) the operating system interfaces used to interact with the host’s filesystems for operations on external units. Asynchronous I/O interfaces are faked for now with synchronous operations and deferred results.


An active connection to an external or internal unit maintains the common parts of its state in this subclass of ConnectionAttributes. The base class holds state that should not change during the lifetime of the connection, while the subclass maintains state that may change during I/O statement execution.


When I/O is being performed from/to a Fortran CHARACTER array rather than an external file, this class manages the standard interoperable descriptor used to access its elements as records. It has the necessary interfaces to serve as an actual argument to the FormatControl class template.


This CRTP class template isolates all of the complexity involved between an external unit’s OpenFile and the buffering requirements imposed by the capabilities of Fortran FORMAT control edit descriptors that allow repositioning within the current record. Its interface enables its clients to define a “frame” (my term, not Fortran’s) that is a contiguous range of bytes that are or may soon be in the file. This frame is defined as a file offset and a byte size. The FileFrame instance manages an internal circular buffer with two essential guarantees:

  1. The most recently requested frame is present in the buffer and contiguous in memory.
  2. Any extra data after the frame that may have been read from the external unit will be preserved, so that it’s safe to read from a socket, pipe, or tape and not have to worry about repositioning and rereading.

In end-of-file situations, it’s possible that a request to read a frame may come up short.

As a CRTP class template, FileFrame accesses the raw filesystem facilities it needs from *this.


This class mixes in ConnectionState, OpenFile, and FileFrame<ExternalFileUnit> to represent the state of an open (or soon to be opened) external file descriptor as a Fortran I/O unit. It has the contextual APIs required to serve as a template actual argument to FormatControl. And it contains a std::variant<> suitable for holding the state of the active I/O statement in progress on the unit (see below).

ExternalFileUnit instances reside in a Map that is allocated as a static variable and indexed by Fortran unit number. Static member functions LookUp(), LookUpOrCrash(), and LookUpOrCreate() probe the map to convert Fortran UNIT= numbers from I/O statements into references to active units.


The subclasses of IoStatementBase each encapsulate and maintain the state of one active Fortran I/O statement across the several I/O runtime library API function calls it may comprise. The subclasses handle the distinctions between internal vs. external I/O, formatted vs. list-directed vs. unformatted I/O, input vs. output, and so on.

IoStatementBase inherits default FORMAT processing callbacks and an IoErrorHandler. Each of the IoStatementBase classes that pertain to formatted I/O support the contextual callback interfaces needed by FormatControl, overriding the default callbacks of the base class, which crash if called inappropriately (e.g., if a CLOSE statement somehow passes a data item from an io-list).

The lifetimes of these subclasses’ instances each begin with a user program call to an I/O API routine with a name like BeginExternalListOutput() and persist until EndIoStatement() is called.

To reduce dynamic memory allocation, external I/O statements allocate their per-statement state class instances in space reserved in the ExternalFileUnit instance. Internal I/O statements currently use dynamic allocation, but the I/O API supports a means whereby the code generated for the Fortran program may supply stack space to the I/O runtime support library for this purpose.


F18’s Fortran I/O runtime support library defines and implements an API that uses a sequence of function calls to implement each Fortran I/O statement. The state of each I/O statement in progress is maintained in some subclass of IoStatementBase, as noted above. The purpose of IoStatementState is to provide generic access to the specific state classes without recourse to C++ virtual functions or function pointers, language features that may not be available to us in some important execution environments. IoStatementState comprises a std::variant<> of wrapped references to the various possibilities, and uses std::visit() to access them as needed by the I/O API calls that process each specifier in the I/O control-list and each item in the io-list.

Pointers to IoStatementState instances are the Cookie type returned in the I/O API for Begin... I/O statement calls, passed back for the control-list specifiers and io-list data items, and consumed by the EndIoStatement() call at the end of the statement.

Storage for IoStatementState is reserved in ExternalFileUnit for external I/O units, and in the various final subclasses for internal I/O statement states otherwise.

Since Fortran permits a CLOSE statement to reference a nonexistent unit, the library has to treat that (expected to be rare) situation as a weird variation of internal I/O since there’s no ExternalFileUnit available to hold its IoStatementBase subclass or IoStatementState.

A Narrative Overview Of PRINT *, 'HELLO, WORLD'

  1. When the compiled Fortran program begins execution at the main() entry point exported from its main program, it calls ProgramStart() with its arguments and environment.
  2. The generated code calls BeginExternalListOutput() to start the sequence of calls that implement the PRINT statement. Since the Fortran runtime I/O library has not yet been used in this process, its data structures are initialized on this first call, and Fortran I/O units 5 and 6 are connected with the stadard input and output file descriptors (respectively). The default unit code is converted to 6 and passed to ExternalFileUnit::LookUpOrCrash(), which returns a reference to unit 6’s instance.
  3. We check that the unit was opened for formatted I/O.
  4. ExternalFileUnit::BeginIoStatement<>() is called to initialize an instance of ExternalListIoStatementState<false> in the unit, point to it with an IoStatementState, and return a reference to that object whose address will be the Cookie for this statement.
  5. The generated code calls OutputAscii() with that cookie and the address and length of the string.
  6. OutputAscii() confirms that the cookie corresponds to an output statement and determines that it’s list-directed.
  7. ListDirectedStatementState<false>::EmitLeadingSpaceOrAdvance() emits the required initial space on the new current output record by calling IoStatementState::GetConnectionState() to locate the connection state, determining from the record position state that the space is necessary, and calling IoStatementState::Emit() to cough it out. That call is redirected to ExternalFileUnit::Emit(), which calls FileFrame<ExternalFileUnit>::WriteFrame() to extend the frame of the current record and then memcpy() to fill its first byte with the space.
  8. Back in OutputAscii(), the mutable modes and connection state of the IoStatementState are queried to see whether we’re in an WRITE(UNIT=,FMT=,DELIM=) statement with a delimited specifier. If we were, the library would emit the appropriate quote marks, double up any instances of that character in the text, and split the text over multiple records if it’s long.
  9. But we don’t have a delimiter, so OutputAscii() just carves up the text into record-sized chunks and emits them. There’s just one chunk for our short CHARACTER string value in this example. It’s passed to IoStatementState::Emit(), which (as above) is redirected to ExternalFileUnit::Emit(), which interacts with the frame to extend the frame and memcpy data into the buffer.
  10. A flag is set in ListDirectedStatementState<false> to remember that the last item emitted in this list-directed output statement was an undelimited CHARACTER value, so that if the next item is also an undelimited CHARACTER, no interposing space will be emitted between them.
  11. OutputAscii() return true to its caller.
  12. The generated code calls EndIoStatement(), which is redirected to ExternalIoStatementState<false>’s override of that function. As this is not a non-advancing I/O statement, ExternalFileUnit::AdvanceRecord() is called to end the record. Since this is a sequential formatted file, a newline is emitted.
  13. If unit 6 is connected to a terminal, the buffer is flushed. FileFrame<ExternalFileUnit>::Flush() drives ExternalFileUnit::Write() to push out the data in maximal contiguous chunks, dealing with any short writes that might occur, and collecting I/O errors along the way. This statement has no ERR= label or IOSTAT= specifier, so errors arriving at IoErrorHandler::SignalErrno() will cause an immediate crash.
  14. ExternalIoStatementBase::EndIoStatement() is called. It gets the final IOSTAT= value from IoStatementBase::EndIoStatement(), tells the ExternalFileUnit that no I/O statement remains active, and returns the I/O status value back to the program.
  15. Eventually, the program calls ProgramEndStatement(), which calls ExternalFileUnit::CloseAll(), which flushes and closes all open files. If the standard output were not a terminal, the output would be written now with the same sequence of calls as above.
  16. exit(EXIT_SUCCESS).