# IO¶

Usage

use IO;


or

import IO;


Submodules

Support for a variety of kinds of input and output.

Note

All Chapel programs automatically include write, writeln and writef.

Input/output (I/O) facilities in Chapel include the types file and channel; the constants stdin, stdout and stderr; the functions open, file.close, file.reader, file.writer, channel.read, channel.write, and many others.

## I/O Overview¶

A file in Chapel identifies a file in the underlying operating system. Reads and writes to a file are done via one or more channels associated with the file. Each channel uses a buffer to provide sequential read or write access to its file, optionally starting at an offset.

For example, the following program opens a file and writes an integer to it:

// open the file "test-file.txt" for writing, creating it if
// it does not exist yet.
var myFile = open("test-file.txt", iomode.cw);

// create a writing channel starting at file offset 0
// (start and end offsets can be specified when creating the
// channel)
var myWritingChannel = myFile.writer();

var x: int = 17;

// This function will write the human-readable text version of x;
// binary I/O is also possible.
myWritingChannel.write(x);

// Now test-file.txt contains:
// 17


Then, the following program can be used to read the integer:

// open the file "test-file.txt" for reading only
var myFile = open("test-file.txt", iomode.r);

// create a reading channel starting at file offset 0
// (start and end offsets can be specified when creating the
// channel)

var x: int;

// Now read a textual integer. Note that the
// channel.read function returns a bool to indicate
// if it read something or if the end of the file
// was reached before something could be read.

// prints out:
// 17


## Design Rationale¶

Since channels operate independently, concurrent I/O to the same open file is possible without contending for locks. Furthermore, since the channel (and not the file) stores the current file offset, it is straightforward to create programs that access the same open file in parallel. Note that such parallel access is not possible in C when multiple threads are using the same FILE* to write to different regions of a file because of the race condition between fseek and fwrite. Because of these issues, Chapel programmers wishing to perform I/O will need to know how to open files as well as create channels.

## I/O Styles¶

Reading and writing of Chapel’s basic types is regulated by an applicable iostyle. In particular, the I/O style controls whether binary or text I/O should be performed. For binary I/O it specifies, for example, byte order and string encoding. For text I/O it specifies string representation; the base, field width and precision for numeric types; and so on. Each channel has an associated I/O style. It applies to all read/write operations on that channel, except when the program specifies explicitly an I/O style for a particular read or write.

See the definition for the iostyle type. This type represents I/O styles and provides details on formatting and other representation choices.

The default value of the iostyle type is undefined. However, the compiler-generated constructor is available. It can be used to generate the default I/O style, with or without modifications. In addition, the function defaultIOStyle will return the default I/O style just as new iostyle() will.

The I/O style for an I/O operation can be provided through an optional style= argument in a variety of places:

Note that file.reader, or file.writer will copy the file’s I/O style if a style= argument is not provided. Also note that I/O functions on channels will by default use the I/O style stored with that channel.

A channel’s I/O style may be retrieved using channel._style and set using channel._set_style. These functions should only be called while the channel lock is held, however. See Synchronization of Channel Data and Avoiding Data Races for more information on channel locks.

Note

iostyle is work in progress: the fields and/or their types may change. Among other changes, we expect to be replacing the types of some multiple-choice fields from integral to enums.

As an example for specifying an I/O style, the code below specifies the minimum width for writing numbers so array elements are aligned in the output:

stdout.writeln(MyArray, new iostyle(min_width=10));


I/O facilities in Chapel also include several other ways to control I/O formatting. There is support for formatted I/O with FormattedIO.channel.readf and FormattedIO.channel.writef. Also note that record or class implementations can provide custom functions implementing read or write operations for that type (see The readThis(), writeThis(), and readWriteThis() Methods).

## Files¶

There are several functions that open a file and return a file including open, opentmp, openmem, openfd, and openfp.

Once a file is open, it is necessary to create associated channel(s) - see file.reader and file.writer - to write to and/or read from the file.

Use the file.fsync function to explicitly synchronize the file to ensure that file data is committed to the file’s underlying device for persistence.

To release any resources associated with a file, it is necessary to first close any channels using that file (with channel.close) and then the file itself (with file.close).

Note

Escaped strings can be used for paths on systems where UTF-8 file names are not enforced.

## Functions for Channel Creation¶

file.writer creates a channel for writing to a file, and file.reader create a channel for reading from a file.

## Synchronization of Channel Data and Avoiding Data Races¶

Channels (and files) contain locks in order to keep their operation safe for multiple tasks. When creating a channel, it is possible to disable the lock (for performance reasons) by passing locking=false to e.g. file.writer(). Some channel methods - in particular those beginning with the underscore - should only be called on locked channels. With these methods, it is possible to get or set the channel style, or perform I/O “transactions” (see channel.mark and channel._mark). To use these methods, first lock the channel with channel.lock, call the methods you need, then unlock the channel with channel.unlock. Note that in the future, we may move to alternative ways of calling these functions that guarantee that they are not called on a channel without the appropriate locking.

Besides data races that can occur if locking is not used in channels when it should be, it is also possible for there to be data races on file data that is buffered simultaneously in multiple channels. The main way to avoid such data races is the channel.flush synchronization operation. channel.flush will make all writes to the channel, if any, available to concurrent viewers of its associated file, such as other channels or other applications accessing this file concurrently. See the note below for more details on the situation in which this kind of data race can occur.

Note

Since channels can buffer data until channel.flush is called, it is possible to write programs that have undefined behaviour because of race conditions on channel buffers. In particular, the problem comes up for programs that make:

• concurrent operations on multiple channels that operate on overlapping regions of a file

• where at least one of the overlapping channels is a writing channel

• and where data could be stored more than one of the overlapping channel’s buffers at the same time (ie, write and read ordering are not enforced through channel.flush and other mean such as sync variables).

Note that it is possible in some cases to create a file that does not allow multiple channels at different offsets. Channels created on such files will not change the file’s position based on a start= offset arguments. Instead, each read or write operation will use the file descriptor’s current position. Therefore, only one channel should be created for files created in the following situations:

## Performing I/O with Channels¶

Channels contain read and write methods, which are generic methods that can read or write anything, and can also take optional arguments such as I/O style or. These functions generally take any number of arguments and throw if there was an error. See:

In addition, there are several convenient synonyms for channel.write and channel.read:

Sometimes it’s important to flush the buffer in a channel - to do that, use the .flush() method. Flushing the buffer will make all writes available to other applications or other views of the file (ie, it will call e.g. the OS call pwrite). It is also possible to close a channel, which will implicitly flush it and release any buffer memory used by the channel. Note that if you need to ensure that data from a channel is on disk, you’ll have to call channel.flush or channel.close and then file.fsync on the related file.

## Functions for Closing Channels¶

A channel must be closed in order to free the resources allocated for it, to ensure that data written to it is visible to other channels, or to allow the associated file to be closed.

It is an error to perform any I/O operations on a channel that has been closed. It is an error to close a file when it has channels that have not been closed.

Files and channels are reference counted. Each file and channel is closed automatically when no references to it remain. For example, if a local variable is the only reference to a channel, the channel will be closed when that variable goes out of scope. Programs may also close a file or channel explicitly.

## The stdin, stdout, and stderr Channels¶

Chapel provides the predefined channels stdin, stdout, and stderr to access the corresponding operating system streams standard input, standard output, and standard error.

stdin supports reading; stdout and stderr support writing.

All three channels are safe to use concurrently. Their types’ kind argument is dynamic

## Error Handling¶

Most I/O routines throw a SysError.SystemError, and can be handled appropriately with try and catch. For legacy reasons most I/O routines can also can accept an optional error= argument. In this documentation, SystemError may include both the SysError.SystemError class proper and its subclasses.

Some of these subclasses commonly used within the I/O implementation include:

Some of the legacy error codes used include:

An error code can be converted to a string using the function errorToString.

## Ensuring Successful I/O¶

It is possible - in some situations - for I/O to fail without returning an error. In cases where a programmer wants to be sure that there was no error writing the data to disk, it is important to call file.fsync to make sure that data has arrived on disk without an error. Many errors can be reported with typical operation, but some errors can only be reported by the system during file.close or even file.fsync.

When a file (or channel) is closed, data written to that file will be written to disk eventually by the operating system. If an application needs to be sure that the data is immediately written to persistent storage, it should use file.fsync prior to closing the file.

## Correspondence to C I/O¶

It is not possible to seek, read, or write to a file directly. Create a channel to proceed.

channel.flush in Chapel has the same conceptual meaning as fflush() in C. However, fflush() is not necessarily called in channel.flush(). Unlike fsync(), which is actually called in file.fsync() in Chapel.

The iomode constants in Chapel have the same meaning as the following strings passed to fopen() in C:

• iomode.r “r”

• iomode.rw “r+”

• iomode.cw “w”

• iomode.cwr “w+”

However, open() in Chapel does not necessarily invoke fopen().

## IO Functions and Types¶

enum iomode { r = 1, cw = 2, rw = 3, cwr = 4 }

The iomode type is an enum. When used as arguments when opening files, its constants have the following meaning:

• iomode.r - open an existing file for reading.

• iomode.rw - open an existing file for reading and writing.

• iomode.cw - create a new file for writing. If the file already exists, its contents are removed when the file is opened in this mode.

• iomode.cwr - as with iomode.cw but reading from the file is also allowed.

enum iokind { dynamic = 0, native = 1, big = 2, little = 3 }

The iokind type is an enum. When used as arguments to the channel type, its constants have the following meaning:

• iokind.big means binary I/O with big-endian byte order is performed when writing/reading basic types from the channel.

• iokind.little means binary I/O with little-endian byte order (similar to iokind.big but with little-endian byte order).

• iokind.native means binary I/O in native byte order (similar to iokind.big but with the byte order that is native to the target platform).

• iokind.dynamic means that the applicable I/O style has full effect and as a result the kind varies at runtime.

In the case of iokind.big, iokind.little, and iokind.native the applicable iostyle is consulted when writing/reading strings, but not for other basic types.

There are synonyms available for these values:

param iodynamic = iokind.dynamic

A synonym for iokind.dynamic; see iokind

param ionative = iokind.native

A synonym for iokind.native; see iokind

param iobig = iokind.big

A synonym for iokind.big; see iokind

param iolittle = iokind.little

A synonym for iokind.little; see iokind

enum iostringstyle { len1b_data = -1, len2b_data = -2, len4b_data = -4, len8b_data = -8, lenVb_data = -10, data_toeof = -65280, data_null = -256 }

This enum contains values used to control binary I/O with strings via the str_style field in iostyle.

• iostringstyle.len1b_data indicates a string format of 1 byte of length followed by length bytes of string data.

• iostringstyle.len2b_data indicates a string format of 2 bytes of length followed by length bytes of string data.

• iostringstyle.len4b_data indicates a string format of 4 bytes of length followed by length bytes of string data.

• iostringstyle.len8b_data indicates a string format of 8 bytes of length followed by length bytes of string data.

• iostringstyle.lenVb_data indicates a string format of a variable number of bytes of length, encoded with high-bit meaning more bytes of length follow, and where the 7-bits of length from each byte store the 7-bit portions of the length in order from least-significant to most-significant. This way of encoding a variable-byte length matches Google Protocol Buffers.

• iostringstyle.data_toeof indicates a string format that contains only the string data without any length or terminator. When reading, this format will read a string until the end of the file is reached.

• iostringstyle.data_null indicates a string that is terminated by a zero byte. It can be combined with other numeric values to indicate a string terminated by a particular byte. For example, to indicate a string terminated by \$ (which in ASCII has byte value 0x24), one would use the value iostringstyle.data_null|0x24.

• A positive and nonzero value indicates that a string of exactly that many bytes should be read or written.

enum iostringformat { word = 0, basic = 1, chpl = 2, json = 3, toend = 4, toeof = 5 }

This enum contains values used to control text I/O with strings via the string_format field in iostyle.

• iostringformat.word means string is as-is; reading reads until whitespace. This is the default.

• iostringformat.basic means only escape string_end and \ with \

• iostringformat.chpl means escape string_end \ ' " \n with \ and nonprinting characters c = 0xXY with \xXY

• iostringformat.json means escape string_end " and \ with \, and nonprinting characters c = \uABCD

• iostringformat.toend means string is as-is; reading reads until string_end

• iostringformat.toeof means string is as-is; reading reads until end of file

proc stringStyleTerminated(terminator: uint(8))

This method returns the appropriate iostyle str_style value to indicate a string format where strings are terminated by a particular byte.

Arguments

terminator – a byte value that the strings will be terminated by

Returns

a value that indicates a string format where strings are terminated by the terminator byte. This value is appropriate to store in iostyle.str_style.

proc stringStyleNullTerminated()

This method returns the appropriate iostyle str_style value to indicate a string format where strings are terminated by a zero byte.

proc stringStyleWithLength(lengthBytes: int) throws

Return the appropriate iostyle str_style value to indicate a string format where string data is preceded by a lengthBytes of length. Only lengths of 1, 2, 4, or 8 are supported. When lengthBytes is 0, the returned value indicates variable-byte length.

Throws

SystemError – Thrown for an unsupported value of lengthBytes.

const IOHINT_NONE = 0: c_int

IOHINT_NONE means normal operation, nothing special to hint. Expect to use NONE most of the time. The other hints can be bitwise-ORed in.

const IOHINT_RANDOM = QIO_HINT_RANDOM

const IOHINT_SEQUENTIAL = QIO_HINT_SEQUENTIAL

IOHINT_SEQUENTIAL means expect sequential access. On Linux, this should double the readahead.

const IOHINT_CACHED = QIO_HINT_CACHED

IOHINT_CACHED means we expect the entire file to be cached and/or we pull it in all at once. May request readahead on the entire file.

const IOHINT_PARALLEL = QIO_HINT_PARALLEL

IOHINT_PARALLEL means that we expect to have many channels working with this file in parallel. It might change the reading/writing implementation to something more efficient in that scenario.

record iostyle

The iostyle type represents I/O styles defining how Chapel’s basic types should be read or written.

var binary: uint(8) = 0

Perform binary I/O? 1 - yes, 0 - no. This field is ignored for iokind values other than dynamic.

var byteorder: uint(8) = iokind.native: uint(8)

What byte order should we use when performing binary I/O? This field is ignored for iokind values other than dynamic. It should be set to a value in iokind.

var str_style: int(64) = iostringstyle.data_toeof: int(64)

What string format should we use when writing strings in binary mode? See iostringstyle for more information on what the values of str_style mean.

var min_width_columns: uint(32) = 0

When performing text I/O, pad out to this many columns

var max_width_columns: uint(32) = max(uint(32))

When performing text I/O, do not use more than this many columns

var max_width_characters: uint(32) = max(uint(32))

When performing text I/O, do not use more than this many characters

var max_width_bytes: uint(32) = max(uint(32))

When performing text I/O, do not use more than this many bytes

var string_start: style_char_t = 34

What character do we start strings with, when appropriate? Default is “

var string_end: style_char_t = 34

What character do we end strings with, when appropriate? Default is “

var string_format: uint(8) = iostringformat.word: uint(8)

How should we format strings when performing text I/O? See iostringstyle for more information on what the values of str_style mean.

var bytes_prefix: style_char_t = 98

What character do we start bytes with, when appropriate? Default is “

var base: uint(8) = 0

When reading or writing a numeric value in a text mode channel, what base should be used for the number? Default of 0 means decimal. Bases 2, 8, 10, 16 are supported for integers. Bases 10 and 16 are supported for real values.

var point_char: style_char_t = 46

When reading or writing a numeric value in a text mode channel, how is the integer portion separated from the fractional portion? Default is ‘.’

var exponent_char: style_char_t = 101

When reading or writing a numeric value in a text mode channel, how is the exponent written? Default is ‘e’

var other_exponent_char: style_char_t = 112

When reading or writing a numeric value in a text mode channel, when base is > 10, how is the exponent written? Default is ‘e’

var positive_char: style_char_t = 43

What character denotes a positive number? Default is ‘+’

var negative_char: style_char_t = 45

What character denotes a negative number? Default is ‘-‘

var i_char: style_char_t = 105

What character follows an the imaginary number? Default is ‘i’

var prefix_base: uint(8) = 1

When writing in a base other than 10, should the prefix be used? (e.g. hexadecimal numbers are prefixed with 0x)

var pad_char: style_char_t = 32

When padding with spaces, which pad character to use? Default is ‘ ‘

var showplus: uint(8) = 0

When printing a positive numeric value, should the + be shown?

var uppercase: uint(8) = 0

When printing a numeric value in hexadecimal, should it be uppercase?

var leftjustify: uint(8) = 0

When printing a numeric value in a field of specified width, should the number be on the left (that is padded on the right?). The default is to right-justify the number.

var showpoint: uint(8) = 0

When printing an integral value using a real format, should a trailing decimal point be included? If so, the value 0 will be written as ‘0.’

var showpointzero: uint(8) = 1

When printing an integral value using a real format, should a trailing decimal point and zero be included? If so, the value 0 will be written as ‘0.0’

var precision: int(32) = -1

Specifies the precision for real format conversions. See the description of realfmt below.

var realfmt: uint(8) = 0

Formatting of real numbers:

• 0 means print out ‘precision’ number of significant digits (%g in printf)

• 1 means print out ‘precision’ number of digits after the decimal point (%f)

• 2 means always use exponential and ‘precision’ number of digits (%e)

var complex_style: uint(8) = 0
var array_style: uint(8) = 0
var aggregate_style: uint(8) = 0
var tuple_style: uint(8) = 0
proc defaultIOStyle(): iostyle
Returns

the default I/O style. See iostyle and I/O Styles

proc iostyle.native(str_style: int(64) = stringStyleWithVariableLength()): iostyle

Get an I/O style indicating binary I/O in native byte order.

Arguments

str_style – see iostringstyle - which format to use when reading or writing strings. Defaults to variable-byte length.

Returns

the requested iostyle

proc iostyle.big(str_style: int(64) = stringStyleWithVariableLength()): iostyle

Get an I/O style indicating binary I/O in big-endian byte order.

Arguments

str_style – see iostringstyle - which format to use when reading or writing strings. Defaults to variable-byte length.

Returns

the requested iostyle

proc iostyle.little(str_style: int(64) = stringStyleWithVariableLength()): iostyle

Get an I/O style indicating binary I/O in little-endian byte order.

Arguments

str_style – see iostringstyle - which format to use when reading or writing strings. Defaults to variable-byte length.

Returns

the requested iostyle

proc iostyle.text(): iostyle

Get an I/O style indicating text I/O.

Returns

the requested iostyle

type iohints = c_int

A value of the iohints type defines a set of hints about the I/O that the file or channel will perform. These hints may be used by the implementation to select optimized versions of the I/O operations.

The iohints type is implementation-defined. The following iohints constants are provided:

Other hints might be added in the future.

The following binary operators are defined on iohints:

• | for set union

• & for set intersection

• == for set equality

• 1= for set inequality

When an iohints formal has default intent, the actual is copied to the formal upon a function call and the formal cannot be assigned within the function.

The default value of the iohints type is undefined.

record file

The file type is implementation-defined. A value of the file type refers to the state that is used by the implementation to identify and interact with the OS file.

When a file formal argument has default intent, the actual is copied to the formal upon a function call and the formal cannot be assigned within the function.

The default value of the file type does not represent any OS file. It is illegal to perform any I/O operations on the default value.

proc init()
proc file.init=(x: file)
proc file.check() throws

Throw an error if this is not a valid representation of an OS file.

Throws

SystemError – Indicates that this does not represent an OS file.

proc file.close() throws

Close a file.

In order to free the resources allocated for a file, it must be closed using this method.

Closing a file does not guarantee immediate persistence of the performed updates, if any. In cases where immediate persistence is important, file.fsync should be used for that purpose prior to closing the file. In particular, even though closing the file might complete without errors, the data written might not persist in the event of a severe error like running out of storage space or power loss. See also Ensuring Successful I/O.

Files are automatically closed when the file variable goes out of scope and all channels using that file are closed. Programs may also explicitly close a file using this method.

It is an error to perform any I/O operations on a file that has been closed. It is an error to close a file when it has channels that have not been closed.

Throws

SystemError – Thrown if the file could not be closed.

proc file.fsync() throws

Sync a file to disk.

Commits file data to the device associated with this file. Data written to the file by a channel will be committed only if the channel has been closed or flushed.

This function will typically call the fsync system call.

Throws

SystemError – Thrown if the file could not be synced.

proc file.path: string throws

Get the path to an open file.

Note that not all files have a path (e.g. files opened with openmem), and that this function may not work on all operating systems.

The function Path.file.realPath is an alternative way to get the path to a file.

Throws

SystemError – Thrown if the path could not be retrieved.

proc file.tryGetPath(): string

Get the path to an open file, or return “unknown” if there was a problem getting the path to the open file.

proc file.size: int(64) throws

Get the current size of an open file. Note that the size can always change if other channels, tasks or programs are writing to the file.

Returns

the current file size

Throws

SystemError – Thrown if the size could not be retrieved.

proc open(path: string, mode: iomode, hints: iohints = IOHINT_NONE, style: iostyle = defaultIOStyle()): file throws

Open a file on a filesystem. Note that once the file is open, you will need to use a file.reader or file.writer to create a channel to actually perform I/O operations

Arguments
• path – which file to open (for example, “some/file.txt”).

• mode – specify whether to open the file for reading or writing and whether or not to create the file if it doesn’t exist. See iomode.

• hints – optional argument to specify any hints to the I/O system about this file. See iohints.

• style – optional argument to specify I/O style associated with this file. The provided style will be the default for any channels created for on this file, and that in turn will be the default for all I/O operations performed with those channels.

Returns

an open file to the requested resource.

Throws

SystemError – Thrown if the file could not be opened.

proc openfd(fd: fd_t, hints: iohints = IOHINT_NONE, style: iostyle = defaultIOStyle()): file throws

Create a Chapel file that works with a system file descriptor Note that once the file is open, you will need to use a file.reader or file.writer to create a channel to actually perform I/O operations

The system file descriptor will be closed when the Chapel file is closed.

Note

This function can be used to create Chapel files that refer to system file descriptors that do not support the seek functionality. For example, file descriptors that represent pipes or open socket connections have this property. In that case, the resulting file value should only be used with one channel at a time. The I/O system will ignore the channel offsets when reading or writing to files backed by non-seekable file descriptors.

Arguments
• fd – a system file descriptor (obtained with Sys.sys_open or Sys.sys_connect for example).

• hints – optional argument to specify any hints to the I/O system about this file. See iohints.

• style – optional argument to specify I/O style associated with this file. The provided style will be the default for any channels created for on this file, and that in turn will be the default for all I/O operations performed with those channels.

Returns

an open file using the specified file descriptor.

Throws

SystemError – Thrown if the file descriptor could not be retrieved.

proc openfp(fp: _file, hints: iohints = IOHINT_NONE, style: iostyle = defaultIOStyle()): file throws

Create a Chapel file that works with an open C file (ie a FILE*). Note that once the file is open, you will need to use a file.reader or file.writer to create a channel to actually perform I/O operations

Note

The resulting file value should only be used with one channel at a time. The I/O system will ignore the channel offsets when reading or writing to a file opened with openfp.

Arguments
• fp – a C FILE* to work with

• hints – optional argument to specify any hints to the I/O system about this file. See iohints.

• style – optional argument to specify I/O style associated with this file. The provided style will be the default for any channels created for on this file, and that in turn will be the default for all I/O operations performed with those channels.

Returns

an open file that uses the underlying FILE* argument.

Throws

SystemError – Thrown if the C file could not be retrieved.

proc opentmp(hints: iohints = IOHINT_NONE, style: iostyle = defaultIOStyle()): file throws

Open a temporary file. Note that once the file is open, you will need to use a file.reader or file.writer to create a channel to actually perform I/O operations.

The temporary file will be created in an OS-dependent temporary directory, for example “/tmp” is the typical location. The temporary file will be deleted upon closing.

Temporary files are always opened with iomode iomode.cwr; that is, a new file is created that supports both writing and reading.

Arguments
• hints – optional argument to specify any hints to the I/O system about this file. See iohints.

• style – optional argument to specify I/O style associated with this file. The provided style will be the default for any channels created for on this file, and that in turn will be the default for all I/O operations performed with those channels.

Returns

an open temporary file.

Throws

SystemError – Thrown if the temporary file could not be opened.

proc openmem(style: iostyle = defaultIOStyle()): file throws

Open a file that is backed by a buffer in memory that will not persist when the file is closed. Note that once the file is open, you will need to use a file.reader or file.writer to create a channel to actually perform I/O operations.

The resulting file supports both reading and writing.

Arguments

style – optional argument to specify I/O style associated with this file. The provided style will be the default for any channels created for on this file, and that in turn will be the default for all I/O operations performed with those channels.

Returns

an open memory file.

Throws

SystemError – Thrown if the memory buffered file could not be opened.

record channel

A channel supports either sequential reading or sequential writing to an underlying file object. A channel can buffer data. Read operations on the channel might return old data. Write operations might not have an immediate effect. Use channel.flush to control this buffering.

The channel type is implementation-defined. A value of the channel type refers to the state that is used to implement the channel operations.

When a channel formal has default intent, the actual is copied to the formal upon a function call and the formal cannot be assigned within the function.

The default value of the channel type is not associated with any file and so cannot be used to perform I/O.

The channel type is generic.

param writing: bool

writing is a boolean indicating whether the channels of this type support writing (when true) or reading (when false).

param kind: iokind

kind is an enum iokind that allows narrowing this channel’s I/O style for more efficient binary I/O.

param locking: bool

locking is a boolean indicating whether it is safe to use this channel concurrently (when true).

proc channel.init=(x: channel)
record ioChar

Represents a Unicode codepoint. I/O routines (such as channel.read and channel.write) can use arguments of this type in order to read or write a single Unicode codepoint.

var ch: int(32)

The codepoint value

record ioNewline

Represents a newline character or character sequence (ie \n). I/O routines (such as channel.read and channel.write) can use arguments of this type in order to read or write a newline. This is different from ‘n’ because an ioNewline always produces an actual newline, but in some cases writing \n will produce an escaped string (such as "\n").

When reading an ioNewline, read routines will skip any character sequence (including e.g. letters and numbers) to get to the newline character unless skipWhitespaceOnly is set to true.

var skipWhitespaceOnly: bool = false

Normally, we will skip anything at all to get to a n, but if skipWhitespaceOnly is set, it will be an error if we run into non-space characters other than n.

record ioLiteral

Used to represent a constant string we want to read or write.

When writing, the ioLiteral is output without any quoting or escaping.

When reading, the ioLiteral must be matched exactly - or else the read call will return an error with code SysBasic.EFORMAT.

var val: string

The value of the literal

var ignoreWhiteSpace: bool = true

Should read operations using this literal ignore and consume whitespace before the literal?

proc writeThis(f) throws
record ioBits

Represents a value with a particular bit length that we want to read or write. The I/O will always be done in binary mode.

var v: uint(64)

The bottom nbits of v will be read or written

var nbits: int(8)

How many of the low-order bits of v should we read or write?

proc channel.lock() throws

Acquire a channel’s lock.

Throws

SystemError – Thrown if the lock could not be acquired.

proc channel.unlock()

Release a channel’s lock.

proc channel.offset(): int(64)

Return the current offset of a channel.

Warning

If the channel can be used by multiple tasks, take care when doing operations that rely on the channel’s current offset. To prevent race conditions, first lock the channel with channel.lock, do the operations, then unlock it with channel.unlock. While holding the lock, use channel._offset instead.

Returns

the current offset of the channel

proc channel.advance(amount: int(64)) throws

Move a channel offset forward.

For a reading channel, this function will consume the next amount bytes. If EOF is reached, the channel position may be left at the EOF.

For a writing channel, this function will write amount zeros - or some other data if it is stored in the channel’s buffer, for example with channel._mark and channel._revert.

Throws

SystemError – Throws if the channel offset was not moved.

proc channel.advancePastByte(byte: uint(8)) throws

Reads until byte is found and then leave the channel offset just after it.

Throws
• EOFError – if the requested byte could not be found.

• SystemError – if another error occurred.

proc channel.mark() throws

mark a channel - that is, save the current offset of the channel on its mark stack. This function can only be called on a channel with locking==false.

The mark stack stores several channel offsets. For any channel offset that is between the minimum and maximum value in the mark stack, I/O operations on the channel will keep that region of the file buffered in memory so that those operations can be un-done. As a result, it is possible to perform I/O transactions on a channel. The basic steps for an I/O transaction are:

• mark the current position with channel.mark

• do something speculative (e.g. try to read 200 bytes of anything followed by a ‘B’)

• if the speculative operation was successful, commit the changes by calling channel.commit

• if the speculative operation was not successful, go back to the mark by calling channel.revert. Subsequent I/O operations will work as though nothing happened.

Note

Note that it is possible to request an entire file be buffered in memory using this feature, for example by marking at offset=0 and then advancing to the end of the file. It is important to be aware of these memory space requirements.

Returns

The offset that was marked

Throws

SystemError: if marking the channel failed

proc channel.revert()

Abort an I/O transaction. See channel.mark. This function will pop the last element from the mark stack and then leave the previous channel offset unchanged. This function can only be called on a channel with locking==false.

proc channel.commit()

Commit an I/O transaction. See channel.mark. This function will pop the last element from the mark stack and then set the channel offset to the popped offset. This function can only be called on a channel with locking==false.

proc channel.seek(start: int, end: int = max(int)) throws

Reset a channel to point to a new part of a file. This function allows one to jump to a different part of a file without creating a new channel. It can only be called on a channel with locking==false.

Besides setting a new start position, this function allows one to specify a new end position. Specifying the end position is usually not necessary for correct behavior but it might be an important performance optimization since the channel will not try to read data outside of the start..end region.

This function will, in most cases, discard the channel’s buffer. When writing, the data will be saved to the file before discarding.

Arguments
• start – the new start offset, measured in bytes and counting from 0

• end – optionally, a new end offset, measured in bytes and counting from 0

Throws

SystemError: if seeking failed. Possible reasons include that the file is not seekable, or that the channel is marked.

proc channel._offset(): int(64)

For a channel locked with channel.lock, return the offset of that channel.

proc channel._mark() throws

This routine is identical to channel.mark except that it can be called on channels with locking==true and should be called only once the channel has been locked with channel.lock. The channel should not be unlocked with channel.unlock until after the mark has been committed with channel._commit or reverted with channel._revert.

See channel.mark for details other than the locking discipline.

Returns

The offset that was marked

Throws

SystemError: if marking the channel failed

proc channel._revert()

Abort an I/O transaction. See channel._mark. This function will pop the last element from the mark stack and then leave the previous channel offset unchanged. This function should only be called on a channel that has already been locked and marked.

proc channel._commit()

Commit an I/O transaction. See channel._mark. This function will pop the last element from the mark stack and then set the channel offset to the popped offset. This function should only be called on a channel that has already been locked and marked.

proc channel._style(): iostyle

Return the current style used by a channel. This function should only be called on a locked channel.

proc channel._set_style(style: iostyle)

Set the style associated with a channel. This function should only be called on a locked channel.

proc channel.readWriteThisFromLocale()

Return the locale on which an ongoing I/O was started with a channel. This method will return nil unless it is called on a channel that is the formal argument to a readThis, writeThis, or readWriteThis method.

proc openreader(path: string, param kind = iokind.dynamic, param locking = true, start: int(64) = 0, end: int(64) = max(int(64)), hints: iohints = IOHINT_NONE, style: iostyle = defaultIOStyle()): channel(false, kind, locking) throws

Open a file at a particular path and return a reading channel for it. This function is equivalent to calling open and then file.reader on the resulting file.

Arguments
• path – which file to open (for example, “some/file.txt”).

• kindiokind compile-time argument to determine the corresponding parameter of the channel type. Defaults to iokind.dynamic, meaning that the associated iostyle controls the formatting choices.

• locking – compile-time argument to determine whether or not the channel should use locking; sets the corresponding parameter of the channel type. Defaults to true, but when safe, setting it to false can improve performance.

• start – zero-based byte offset indicating where in the file the channel should start reading. Defaults to 0.

• end – zero-based byte offset indicating where in the file the channel should no longer be allowed to read. Defaults to a max(int) - meaning no end point.

• hints – optional argument to specify any hints to the I/O system about this file. See iohints.

Returns

an open reading channel to the requested resource.

Throws

SystemError – Thrown if a reading channel could not be returned.

proc openwriter(path: string, param kind = iokind.dynamic, param locking = true, start: int(64) = 0, end: int(64) = max(int(64)), hints: iohints = IOHINT_NONE, style: iostyle = defaultIOStyle()): channel(true, kind, locking) throws

Open a file at a particular path and return a writing channel for it. This function is equivalent to calling open with iomode.cwr and then file.writer on the resulting file.

Arguments
• path – which file to open (for example, “some/file.txt”).

• kindiokind compile-time argument to determine the corresponding parameter of the channel type. Defaults to iokind.dynamic, meaning that the associated iostyle controls the formatting choices.

• locking – compile-time argument to determine whether or not the channel should use locking; sets the corresponding parameter of the channel type. Defaults to true, but when safe, setting it to false can improve performance.

• start – zero-based byte offset indicating where in the file the channel should start writing. Defaults to 0.

• end – zero-based byte offset indicating where in the file the channel should no longer be allowed to write. Defaults to a max(int) - meaning no end point.

• hints – optional argument to specify any hints to the I/O system about this file. See iohints.

Returns

an open writing channel to the requested resource.

Throws

SystemError – Thrown if a writing channel could not be returned.

proc file.reader(param kind = iokind.dynamic, param locking = true, start: int(64) = 0, end: int(64) = max(int(64)), hints: iohints = IOHINT_NONE, style: iostyle = this._style): channel(false, kind, locking) throws

Create a channel that supports reading from a file. See I/O Overview.

The start= and end= arguments define the region of the file that the channel will read from. These are byte offsets; the beginning of the file is at the offset 0. The defaults for these arguments enable the channel to access the entire file.

A channel will never read beyond its maximum end position. In addition, reading from a channel beyond the end of the underlying file will not extend that file. Reading beyond the end of the file or beyond the end offset of the channel will produce the error EEOF (and return false in many cases such as channel.read) to indicate that the end was reached.

Arguments
• kindiokind compile-time argument to determine the corresponding parameter of the channel type. Defaults to iokind.dynamic, meaning that the associated iostyle controls the formatting choices.

• locking – compile-time argument to determine whether or not the channel should use locking; sets the corresponding parameter of the channel type. Defaults to true, but when safe, setting it to false can improve performance.

• start – zero-based byte offset indicating where in the file the channel should start reading. Defaults to 0.

• end – zero-based byte offset indicating where in the file the channel should no longer be allowed to read. Defaults to a max(int) - meaning no end point.

• hints – provide hints about the I/O that this channel will perform. See iohints. The default value of IOHINT_NONE will cause the channel to use the hints provided when opening the file.

• style – provide a iostyle to use with this channel. The default value will be the iostyle associated with this file.

Throws

SystemError – Thrown if a file reader channel could not be returned.

proc file.lines(param locking: bool = true, start: int(64) = 0, end: int(64) = max(int(64)), hints: iohints = IOHINT_NONE, in local_style: iostyle = this._style) throws

Iterate over all of the lines in a file.

Returns

an object which yields strings read from the file

Throws

SystemError – Thrown if an ItemReader could not be returned.

proc file.writer(param kind = iokind.dynamic, param locking = true, start: int(64) = 0, end: int(64) = max(int(64)), hints: c_int = 0, style: iostyle = this._style): channel(true, kind, locking) throws

Create a channel that supports writing to a file. See I/O Overview.

The start= and end= arguments define the region of the file that the channel will write to. These are byte offsets; the beginning of the file is at the offset 0. The defaults for these arguments enable the channel to access the entire file.

When a channel writes to a file, it will replace file data that was previously stored at the relevant offset. If the offset is beyond the end of the file, the file will be extended.

A channel will never write beyond its maximum end position. It will extend the file only as necessary to store data written to the channel. In other words, specifying end here does not impact the file size directly; it impacts only the section of the file that this channel can write to. After all channels to a file are closed, that file will have a size equal to the last position written to by any channel.

Arguments
• kindiokind compile-time argument to determine the corresponding parameter of the channel type. Defaults to iokind.dynamic, meaning that the associated iostyle controls the formatting choices.

• locking – compile-time argument to determine whether or not the channel should use locking; sets the corresponding parameter of the channel type. Defaults to true, but when safe, setting it to false can improve performance.

• start – zero-based byte offset indicating where in the file the channel should start writing. Defaults to 0.

• end – zero-based byte offset indicating where in the file the channel should no longer be allowed to write. Defaults to a max(int) - meaning no end point.

• hints – provide hints about the I/O that this channel will perform. See iohints. The default value of IOHINT_NONE will cause the channel to use the hints provided when opening the file.

• style – provide a iostyle to use with this channel. The default value will be the iostyle associated with this file.

Throws

SystemError – Thrown if a file writer channel could not be returned.

proc channel.readwrite(const x) throws

For a writing channel, writes as with channel.write. For a reading channel, reads as with channel.read. Stores any error encountered in the channel. Does not return anything.

Throws

SystemError – When an IO error has occurred.

proc <~>(const ref ch: channel, const x) const ref throws

The <~> operator

This <~> operator is the same as calling channel.readwrite, except that it returns the channel so that multiple operator calls can be chained together.

Returns

ch

Throws

SystemError – When an IO error has occurred.

proc <~>(const ref r: channel, lit: ioLiteral) const ref throws

Overload to support reading an IO.ioLiteral without passing ioLiterals by reference, so that

reader <~> new ioLiteral("=")


works without requiring an explicit temporary value to store the ioLiteral.

proc <~>(const ref r: channel, nl: ioNewline) const ref throws

Overload to support reading an IO.ioNewline without passing ioNewline by reference, so that

reader <~> new ioNewline()


works without requiring an explicit temporary value to store the ioNewline.

proc channel.readWriteLiteral(lit: string, ignoreWhiteSpace = true) throws

Explicit call for reading or writing a literal as an alternative to using IO.ioLiteral.

proc channel.readWriteNewline() throws

Explicit call for reading or writing a newline as an alternative to using IO.ioNewline.

proc channel.binary(): bool

Returns true if this channel is configured for binary I/O.

proc channel.error(): syserr

Return any saved error code.

proc channel.setError(e: syserr)

Save an error code.

proc channel.clearError()

Clear any saved error code.

proc channel.writeBytes(x, len: ssize_t): bool throws

Write a sequence of bytes.

Throws

SystemError – Thrown if the byte sequence could not be written.

iter channel.lines()

Iterate over all of the lines ending in \n in a channel - the channel lock will be held while iterating over the lines.

Only serial iteration is supported.

Warning

This iterator executes on the current locale. This may impact multilocale performance if the current locale is not the same locale on which the channel was created.

Yields

lines ending in \n in channel

proc stringify(const args ...?k): string

Creates a string representing the result of writing the arguments.

Writes each argument, possibly using a writeThis method, to a string and returns the result.

proc channel.read(ref args ...?k): bool throws

returns true if read successfully, false if we encountered EOF

proc channel.read(ref args ...?k, style: iostyle): bool throws

Read values from a channel. The input will be consumed atomically - the channel lock will be held while reading all of the passed values.

Arguments
• args – a list of arguments to read. Basic types are handled internally, but for other types this function will call value.readThis() with a Reader argument as described in The readThis(), writeThis(), and readWriteThis() Methods.

• style – optional argument to provide an iostyle for this read. If this argument is not provided, use the current style associated with this channel.

Returns

true if the read succeeded, and false on end of file.

Throws

SystemError – Thrown if the channel could not be read.

proc channel.readline(arg: [] uint(8), out numRead: int, start = arg.domain.low, amount = arg.domain.high-start+1): bool throws

Read a line into a Chapel array of bytes. Reads until a \n is reached. The \n is returned in the array.

Throws a SystemError if a line could not be read from the channel.

Arguments
• arg – A 1D DefaultRectangular array which must have at least 1 element.

• start – Index to begin reading into.

• amount – The maximum amount of bytes to read.

Returns

true if the bytes were read without error.

proc channel.readline(ref arg: ?t): bool throws

Read a line into a Chapel string or bytes. Reads until a \n is reached. The \n is included in the resulting value.

Arguments

arg – a string or bytes to receive the line

Returns

true if a line was read without error, false upon EOF

Throws

SystemError – Thrown if data could not be read from the channel.

proc channel.readstring(ref str_out: string, len: int(64) = -1): bool throws

read a given number of bytes from a channel

Arguments
• str_out – The string to be read into

• len – Read up to len bytes from the channel, up until EOF (or some kind of I/O error). If the default value of -1 is provided, read until EOF starting from the channel’s current offset.

Returns

true if we read something, false upon EOF

Throws

SystemError – Thrown if the bytes could not be read from the channel.

proc channel.readbytes(ref bytes_out: bytes, len: int(64) = -1): bool throws

read a given number of bytes from a channel

Arguments
• bytes_out – The bytes to be read into

• len – Read up to len bytes from the channel, up until EOF (or some kind of I/O error). If the default value of -1 is provided, read until EOF starting from the channel’s current offset.

Returns

true if we read something, false upon EOF

Throws

SystemError – Thrown if the bytes could not be read from the channel.

proc channel.readbits(ref v: integral, nbits: integral): bool throws

Arguments
• v – where to store the read bits. This value will have its nbits least-significant bits set.

• nbits – how many bits to read

Returns

true if the bits were read without error, false upon EOF

Throws

SystemError – Thrown if the bits could not be read from the channel.

proc channel.writebits(v: integral, nbits: integral): bool throws

Write bits with binary I/O

Arguments
• v – a value containing nbits bits to write the least-significant bits

• nbits – how many bits to write

Returns

true if the bits were written without error, false on error

Throws
• IllegalArgumentError – Thrown if writing more bits than fit into v.

• SystemError – Thrown if the bits could not be written to the channel.

proc channel.readln(ref args ...?k, style: iostyle): bool throws

Read values from a channel and then consume any bytes until newline is reached. The input will be consumed atomically - the channel lock will be held while reading all of the passed values.

Arguments
• args – a list of arguments to read. This routine can be called with zero or more such arguments. Basic types are handled internally, but for other types this function will call value.readThis() with a Reader argument as described in The readThis(), writeThis(), and readWriteThis() Methods.

• style – optional argument to provide an iostyle for this read. If this argument is not provided, use the current style associated with this channel.

Returns

true if the read succeeded, and false upon end of file.

Throws

SystemError – Thrown if a line could not be read from the channel.

proc channel.read(type t) throws

Read a value of passed type.

Note

It is difficult to handle errors or to handle reaching the end of the file with this function. If such cases are important please use the channel.read returning the values read in arguments instead.

For example, the following line of code reads a value of type int from stdin and uses it to initialize a variable x:

var x = stdin.read(int)

Arguments

t – the type to read

Returns

Throws

SystemError – Thrown if the type could not be read from the channel.

proc channel.readln(type t) throws

Read a value of passed type followed by a newline.

Note

It is difficult to handle errors or to handle reaching the end of the file with this function. If such cases are important please use channel.readln instead.

Arguments

t – the type to read

Returns

Throws

SystemError – Thrown if the type could not be read from the channel.

proc channel.readln(type t ...?numTypes) throws

Read values of passed types followed by a newline and return a tuple containing the read values.

Arguments

t – more than one type to read

Returns

a tuple of the read values

Throws

SystemError – Thrown if the types could not be read from the channel.

proc channel.read(type t ...?numTypes) throws

Read values of passed types and return a tuple containing the read values.

Arguments

t – more than one type to read

Returns

a tuple of the read values

Throws

SystemError – Thrown if the types could not be read from the channel.

proc channel.write(const args ...?k, style: iostyle): bool throws

Write values to a channel. The output will be produced atomically - the channel lock will be held while writing all of the passed values.

Arguments
• args – a list of arguments to write. Basic types are handled internally, but for other types this function will call value.writeThis() with the channel as an argument.

• style – optional argument to provide an iostyle for this write. If this argument is not provided, use the current style associated with this channel.

Returns

true if the write succeeded

Throws

SystemError – Thrown if the values could not be written to the channel.

proc channel.writeln(const args ...?k, style: iostyle): bool throws

Write values to a channel followed by a newline. The output will be produced atomically - the channel lock will be held while writing all of the passed values.

Arguments
• args – a variable number of arguments to write. This method can be called with zero or more arguments. Basic types are handled internally, but for other types this function will call value.writeThis() with the channel as an argument.

• style – optional argument to provide an iostyle for this write. If this argument is not provided, use the current style associated with this channel.

Returns

true if the write succeeded

Throws

SystemError – Thrown if the values could not be written to the channel.

proc channel.flush() throws

Makes all writes to the channel, if any, available to concurrent viewers of its associated file, such as other channels or other applications accessing this file concurrently. Unlike file.fsync, this does not commit the written data to the file’s device.

Throws

SystemError – Thrown if the flush fails.

proc channel.assertEOF(errStr: string = "- Not at EOF")

Assert that a channel has reached end-of-file and that there was no error doing the read.

proc channel.close() throws

Close a channel. Implicitly performs the channel.flush operation (see Synchronization of Channel Data and Avoiding Data Races).

Throws

SystemError – Thrown if the channel is not successfully closed.

proc channel.isclosed()

Return true if a channel is currently closed.

record ItemReader

Wrapper class on a channel to make it only read values of a single type. Also supports an iterator yielding the read values.

type ItemType

What type do we read and yield?

param kind: iokind

the kind field for our channel

param locking: bool

the locking field for our channel

var ch: channel(false, kind, locking)

our channel

proc read(out arg: ItemType): bool throws

read a single item, throwing on error

iter these()

iterate through all items of that type read from the channel

proc channel.itemReader(type ItemType, param kind: iokind = iokind.dynamic)

Create and return an ItemReader that can yield read values of a single type.

record ItemWriter
type ItemType

What type do we write?

param kind: iokind

the kind field for our channel

param locking: bool

the locking field for our channel

var ch: channel(true, kind, locking)

our channel

proc write(arg: ItemType): bool throws

write a single item, throwing on error

proc channel.itemWriter(type ItemType, param kind: iokind = iokind.dynamic)

Create and return an ItemWriter that can write values of a single type.

const stdin: channel(false, iokind.dynamic, true)

standard input, otherwise known as file descriptor 0

const stdout: channel(true, iokind.dynamic, true)

standard output, otherwise known as file descriptor 1

const stderr: channel(true, iokind.dynamic, true)

standard error, otherwise known as file descriptor 2

proc read(ref args ...?n): bool throws

Equivalent to stdin.read. See channel.read

proc readln(ref args ...?n): bool throws

Equivalent to stdin.readln. See channel.readln

proc readln(type t ...?numTypes) throws

Equivalent to stdin.readln. See channel.readln for types

proc read(type t ...?numTypes) throws

Equivalent to stdin.read. See channel.read for types

Delete a file. This function is likely to be replaced by FileSystem.remove.

Arguments

path – the path to the file to remove

Throws

SystemError – Thrown if the file is not successfully deleted.

proc unicodeSupported(): bool
Returns

true if this version of the Chapel runtime supports UTF-8 output.

proc file.getchunk(start: int(64) = 0, end: int(64) = max(int(64))): (int(64), int(64)) throws

Returns (chunk start, chunk end) for the first chunk in the file containing data in the region start..end-1. Note that the returned chunk might not cover all of the region in question.

Returns (0,0) if no such value exists.

Arguments
• start – the file offset (starting from 0) where the region begins

• end – the file offset just after the region

Returns

a tuple of (chunkStart, chunkEnd) so that the bytes in chunkStart..chunkEnd-1 are stored in a manner that makes reading that chunk at a time most efficient

Throws

SystemError – Thrown if the chunk is not attained.

proc file.localesForRegion(start: int(64), end: int(64))

Returns the ‘best’ locale to run something working with the region of the file in start..end-1.

This must return the same result when called from different locales. Returns a domain of locales that are “best” for the given region. If no locales are “best” we return a domain containing all locales.

Arguments
• start – the file offset (starting from 0) where the region begins

• end – the file offset just after the region

Returns

a set of locales that are best for working with this region

Return type

domain(locale)