► |  | The Racket Reference

1 | Language Model
2 | Notation for Documentation
3 | Syntactic Forms
4 | Datatypes
5 | Structures
6 | Classes and Objects
7 | Units
8 | Contracts
9 | Pattern Matching
10 | Control Flow
11 | Concurrency and Parallelism
12 | Macros
13 | Input and Output
14 | Reflection and Security
15 | Operating System
16 | Memory Management
17 | Unsafe Operations
18 | Running Racket
| Bibliography
| Index

▼ | 13 | Input and Output

13.1 | Ports
13.2 | Byte and String Input
13.3 | Byte and String Output
13.4 | Reading
13.5 | Writing
13.6 | Pretty Printing
13.7 | Reader Extension
13.8 | Printer Extension
13.9 | Serialization
13.10 | Fast-Load Serialization

On this page:

read-char
read-byte
read-line
read-bytes-line
read-string
read-bytes
read-string!
read-bytes!
read-bytes-avail!
read-bytes-avail!*
read-bytes-avail!/ enable-break
peek-string
peek-bytes
peek-string!
peek-bytes!
peek-bytes-avail!
peek-bytes-avail!*
peek-bytes-avail!/ enable-break
read-char-or-special
read-byte-or-special
peek-char
peek-byte
peek-char-or-special
peek-byte-or-special
port-progress-evt
port-provides-progress-evts?
port-commit-peeked
byte-ready?
char-ready?
progress-evt?

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#### 13.2`` Byte and String Input

> procedure (read-char [in]) → (or/c char? eof-object?)
> in : input-port? = (current-input-port)

Reads a single character from in—which may involve reading several bytes to UTF-8-decode them into a character (see Ports); a minimal number of bytes are read/peeked to perform the decoding. If no bytes are available before an end-of-file, then eof is returned.

Examples:
> > (let ([ip (open-input-string "S2")])
> (print (read-char ip))
> (newline)
> (print (read-char ip))
> (newline)
> (print (read-char ip)))
> #\S
> #\2
> #<eof>
> > (let ([ip (open-input-bytes #"\316\273")])
> ; The byte string contains UTF-8-encoded content:
> (print (read-char ip)))
> #\λ

> procedure (read-byte [in]) → (or/c byte? eof-object?)
> in : input-port? = (current-input-port)

Reads a single byte from in. If no bytes are available before an end-of-file, then eof is returned.

Examples:
> > (let ([ip (open-input-string "a")])
> ; The two values in the following list should be the same.
> (list (read-byte ip) (char->integer #\a)))
> '(97 97)
> > (let ([ip (open-input-string (string #\λ))])
> ; This string has a two byte-encoding.
> (list (read-byte ip) (read-byte ip) (read-byte ip)))
> '(206 187 #<eof>)

> procedure (read-line [in mode]) → (or/c string? eof-object?)
> in : input-port? = (current-input-port)
> | mode |  | : |  | (or/c 'linefeed 'return 'return-linefeed 'any 'any-one)
> |  |  | = |  | 'linefeed

Returns a string containing the next line of bytes from in.

Characters are read from in until a line separator or an end-of-file is read. The line separator is not included in the result string (but it is removed from the port’s stream). If no characters are read before an end-of-file is encountered, eof is returned.

The mode argument determines the line separator(s). It must be one of the following symbols:

- 'linefeed breaks lines on linefeed characters.

- 'return breaks lines on return characters.

- 'return-linefeed breaks lines on return-linefeed combinations. If a return character is not followed by a linefeed character, it is included in the result string; similarly, a linefeed that is not preceded by a return is included in the result string.

- 'any breaks lines on any of a return character, linefeed character, or return-linefeed combination. If a return character is followed by a linefeed character, the two are treated as a combination.

- 'any-one breaks lines on either a return or linefeed character, without recognizing return-linefeed combinations.

Return and linefeed characters are detected after the conversions that are automatically performed when reading a file in text mode. For example, reading a file in text mode on Windows automatically changes return-linefeed combinations to a linefeed. Thus, when a file is opened in text mode, 'linefeed is usually the appropriate read-line mode.

Examples:
> > (let ([ip (open-input-string "x\ny\n")])
> (read-line ip))
> "x"
> > (let ([ip (open-input-string "x\ny\n")])
> (read-line ip 'return))
> "x\ny\n"
> > (let ([ip (open-input-string "x\ry\r")])
> (read-line ip 'return))
> "x"
> > (let ([ip (open-input-string "x\r\ny\r\n")])
> (read-line ip 'return-linefeed))
> "x"
> > (let ([ip (open-input-string "x\r\ny\nz")])
> (list (read-line ip 'any) (read-line ip 'any)))
> '("x" "y")
> > (let ([ip (open-input-string "x\r\ny\nz")])
> (list (read-line ip 'any-one) (read-line ip 'any-one)))
> '("x" "")

> procedure (read-bytes-line [in mode]) → (or/c bytes? eof-object?)
> in : input-port? = (current-input-port)
> | mode |  | : |  | (or/c 'linefeed 'return 'return-linefeed 'any 'any-one)
> |  |  | = |  | 'linefeed

Like read-line, but reads bytes and produces a byte string.

> procedure (read-string amt [in]) → (or/c string? eof-object?)
> amt : exact-nonnegative-integer?
> in : input-port? = (current-input-port)

> > > To read an entire port as a string, use port->string.

Returns a string containing the next amt characters from in.

If amt is 0, then the empty string is returned. Otherwise, if fewer than amt characters are available before an end-of-file is encountered, then the returned string will contain only those characters before the end-of-file; that is, the returned string’s length will be less than amt. (A temporary string of size amt is allocated while reading the input, even if the size of the result is less than amt characters.) If no characters are available before an end-of-file, then eof is returned.

If an error occurs during reading, some characters may be lost; that is, if read-string successfully reads some characters before encountering an error, the characters are dropped.

Example:
> > (let ([ip (open-input-string "supercalifragilisticexpialidocious")])
> (read-string 5 ip))
> "super"

> procedure (read-bytes amt [in]) → (or/c bytes? eof-object?)
> amt : exact-nonnegative-integer?
> in : input-port? = (current-input-port)

> > > To read an entire port as bytes, use port->bytes.

Like read-string, but reads bytes and produces a byte string.

Example:
> > (let ([ip (open-input-bytes
> (bytes 6
> 115 101 99 114 101
> 116))])
> (define length (read-byte ip))
> (bytes->string/utf-8 (read-bytes length ip)))
> "secret"

> procedure (read-string! str [in start-pos end-pos])
> → (or/c exact-positive-integer? eof-object?)
> str : (and/c string? (not/c immutable?))
> in : input-port? = (current-input-port)
> start-pos : exact-nonnegative-integer? = 0
> end-pos : exact-nonnegative-integer? = (string-length str)

Reads characters from in like read-string, but puts them into str starting from index start-pos (inclusive) up to end-pos (exclusive). Like substring, the exn:fail:contract exception is raised if start-pos or end-pos is out-of-range for str.

If the difference between start-pos and end-pos is 0, then 0 is returned and str is not modified. If no bytes are available before an end-of-file, then eof is returned. Otherwise, the return value is the number of characters read. If m characters are read and m<end-pos-start-pos, then str is not modified at indices start-pos+m through end-pos.

Example:
> > (let ([buffer (make-string 10 #\_)]
> [ip (open-input-string "cketRa")])
> (printf "~s\n" buffer)
> (read-string! buffer ip 2 6)
> (printf "~s\n" buffer)
> (read-string! buffer ip 0 2)
> (printf "~s\n" buffer))
> "__________"
> "__cket____"
> "Racket____"

> procedure (read-bytes! bstr [in start-pos end-pos])
> → (or/c exact-positive-integer? eof-object?)
> bstr : bytes?
> in : input-port? = (current-input-port)
> start-pos : exact-nonnegative-integer? = 0
> end-pos : exact-nonnegative-integer? = (bytes-length bstr)

Like read-string!, but reads bytes, puts them into a byte string, and returns the number of bytes read.

Example:
> > (let ([buffer (make-bytes 10 (char->integer #\_))]
> [ip (open-input-string "cketRa")])
> (printf "~s\n" buffer)
> (read-bytes! buffer ip 2 6)
> (printf "~s\n" buffer)
> (read-bytes! buffer ip 0 2)
> (printf "~s\n" buffer))
> #"__________"
> #"__cket____"
> #"Racket____"

> procedure (read-bytes-avail! bstr [in start-pos end-pos])
> → (or/c exact-positive-integer? eof-object? procedure?)
> bstr : bytes?
> in : input-port? = (current-input-port)
> start-pos : exact-nonnegative-integer? = 0
> end-pos : exact-nonnegative-integer? = (bytes-length bstr)

Like read-bytes!, but returns without blocking after having read the immediately available bytes, and it may return a procedure for a “special” result. The read-bytes-avail! procedure blocks only if no bytes (or specials) are yet available. Also unlike read-bytes!, read-bytes-avail! never drops bytes; if read-bytes-avail! successfully reads some bytes and then encounters an error, it suppresses the error (treating it roughly like an end-of-file) and returns the read bytes. (The error will be triggered by future reads.) If an error is encountered before any bytes have been read, an exception is raised.

When in produces a special value, as described in Custom Ports, the result is a procedure of four arguments. The four arguments correspond to the location of the special value within the port, as described in Custom Ports. If the procedure is called more than once with valid arguments, the exn:fail:contract exception is raised. If read-bytes-avail returns a special-producing procedure, then it does not place characters in bstr. Similarly, read-bytes-avail places only as many bytes into bstr as are available before a special value in the port’s stream.

> > procedure
> > (read-bytes-avail!* |  | bstr |
> > | [ | in |
> > |  | start-pos |
> > |  | end-pos]) |
> → (or/c exact-nonnegative-integer? eof-object? procedure?)
> bstr : bytes?
> in : input-port? = (current-input-port)
> start-pos : exact-nonnegative-integer? = 0
> end-pos : exact-nonnegative-integer? = (bytes-length bstr)

Like read-bytes-avail!, but returns 0 immediately if no bytes (or specials) are available for reading and the end-of-file is not reached.

> > procedure
> > (read-bytes-avail!/enable-break |  | bstr |
> > | [ | in |
> > |  | start-pos |
> > |  | end-pos]) |
> → (or/c exact-positive-integer? eof-object? procedure?)
> bstr : bytes?
> in : input-port? = (current-input-port)
> start-pos : exact-nonnegative-integer? = 0
> end-pos : exact-nonnegative-integer? = (bytes-length bstr)

Like read-bytes-avail!, but breaks are enabled during the read (see also Breaks). If breaking is disabled when read-bytes-avail!/enable-break is called, and if the exn:break exception is raised as a result of the call, then no bytes will have been read from in.

> procedure (peek-string amt skip-bytes-amt [in]) → (or/c string? eof-object?)
> amt : exact-nonnegative-integer?
> skip-bytes-amt : exact-nonnegative-integer?
> in : input-port? = (current-input-port)

Similar to read-string, except that the returned characters are peeked: preserved in the port for future reads and peeks. (More precisely, undecoded bytes are left for future reads and peeks.) The skip-bytes-amt argument indicates a number of bytes (not characters) in the input stream to skip before collecting characters to return; thus, in total, the next skip-bytes-amt bytes plus amt characters are inspected.

For most kinds of ports, inspecting skip-bytes-amt bytes and amt characters requires at least skip-bytes-amt+amt bytes of memory overhead associated with the port, at least until the bytes/characters are read. No such overhead is required when peeking into a string port (see String Ports), a pipe port (see Pipes), or a custom port with a specific peek procedure (depending on how the peek procedure is implemented; see Custom Ports).

If a port produces eof mid-stream, attempts to skip beyond the eof for a peek always produce eof until the eof is read.

> procedure (peek-bytes amt skip-bytes-amt [in]) → (or/c bytes? eof-object?)
> amt : exact-nonnegative-integer?
> skip-bytes-amt : exact-nonnegative-integer?
> in : input-port? = (current-input-port)

Like peek-string, but peeks bytes and produces a byte string.

> > procedure
> > (peek-string! |  | str |
> > |  | skip-bytes-amt |
> > | [ | in |
> > |  | start-pos |
> > |  | end-pos]) |
> → (or/c exact-positive-integer? eof-object?)
> str : (and/c string? (not/c immutable?))
> skip-bytes-amt : exact-nonnegative-integer?
> in : input-port? = (current-input-port)
> start-pos : exact-nonnegative-integer? = 0
> end-pos : exact-nonnegative-integer? = (string-length str)

Like read-string!, but for peeking, and with a skip-bytes-amt argument like peek-string.

> > procedure
> > (peek-bytes! |  | bstr |
> > |  | skip-bytes-amt |
> > | [ | in |
> > |  | start-pos |
> > |  | end-pos]) |
> → (or/c exact-positive-integer? eof-object?)
> bstr : (and/c bytes? (not/c immutable?))
> skip-bytes-amt : exact-nonnegative-integer?
> in : input-port? = (current-input-port)
> start-pos : exact-nonnegative-integer? = 0
> end-pos : exact-nonnegative-integer? = (bytes-length bstr)

Like peek-string!, but peeks bytes, puts them into a byte string, and returns the number of bytes read.

> > procedure
> > (peek-bytes-avail! |  | bstr |
> > |  | skip-bytes-amt |
> > | [ | progress |
> > |  | in |
> > |  | start-pos |
> > |  | end-pos]) |
> → (or/c exact-nonnegative-integer? eof-object? procedure?)
> bstr : (and/c bytes? (not/c immutable?))
> skip-bytes-amt : exact-nonnegative-integer?
> progress : (or/c progress-evt? #f) = #f
> in : input-port? = (current-input-port)
> start-pos : exact-nonnegative-integer? = 0
> end-pos : exact-nonnegative-integer? = (bytes-length bstr)

Like read-bytes-avail!, but for peeking, and with two extra arguments. The skip-bytes-amt argument is as in peek-bytes. The progress argument must be either #f or an event produced by port-progress-evt for in.

To peek, peek-bytes-avail! blocks until finding an end-of-file, at least one byte (or special) past the skipped bytes, or until a non-#f progress becomes ready. Furthermore, if progress is ready before bytes are peeked, no bytes are peeked or skipped, and progress may cut short the skipping process if it becomes available during the peek attempt. Furthermore, progress is checked even before determining whether the port is still open.

The result of peek-bytes-avail! is 0 only in the case that progress becomes ready before bytes are peeked.

> > procedure
> > (peek-bytes-avail!* |  | bstr |
> > |  | skip-bytes-amt |
> > | [ | progress |
> > |  | in |
> > |  | start-pos |
> > |  | end-pos]) |
> → (or/c exact-nonnegative-integer? eof-object? procedure?)
> bstr : (and/c bytes? (not/c immutable?))
> skip-bytes-amt : exact-nonnegative-integer?
> progress : (or/c progress-evt? #f) = #f
> in : input-port? = (current-input-port)
> start-pos : exact-nonnegative-integer? = 0
> end-pos : exact-nonnegative-integer? = (bytes-length bstr)

Like read-bytes-avail!*, but for peeking, and with skip-bytes-amt and progress arguments like peek-bytes-avail!. Since this procedure never blocks, it may return before even skip-amt bytes are available from the port.

> > procedure
> > (peek-bytes-avail!/enable-break |  | bstr |
> > |  | skip-bytes-amt |
> > | [ | progress |
> > |  | in |
> > |  | start-pos |
> > |  | end-pos]) |
> → (or/c exact-nonnegative-integer? eof-object? procedure?)
> bstr : (and/c bytes? (not/c immutable?))
> skip-bytes-amt : exact-nonnegative-integer?
> progress : (or/c progress-evt? #f) = #f
> in : input-port? = (current-input-port)
> start-pos : exact-nonnegative-integer? = 0
> end-pos : exact-nonnegative-integer? = (bytes-length bstr)

Like read-bytes-avail!/enable-break, but for peeking, and with skip-bytes-amt and progress arguments like peek-bytes-avail!.

> procedure (read-char-or-special [in]) → (or/c char? eof-object? any/c)
> in : input-port? = (current-input-port)

Like read-char, but if the input port returns a special value (through a value-generating procedure in a custom port; see Custom Ports and Special Comments for details), then the special value is returned.

> procedure (read-byte-or-special [in]) → (or/c byte? eof-object? any/c)
> in : input-port? = (current-input-port)

Like read-char-or-special, but reads and returns a byte instead of a character.

> procedure (peek-char [in skip-bytes-amt]) → (or/c char? eof-object?)
> in : input-port? = (current-input-port)
> skip-bytes-amt : exact-nonnegative-integer? = 0

Like read-char, but peeks instead of reading, and skips skip-bytes-amt bytes (not characters) at the start of the port.

> procedure (peek-byte [in skip-bytes-amt]) → (or/c byte? eof-object?)
> in : input-port? = (current-input-port)
> skip-bytes-amt : exact-nonnegative-integer? = 0

Like peek-char, but peeks and returns a byte instead of a character.

> procedure (peek-char-or-special [in skip-bytes-amt])
> → (or/c char? eof-object? any/c)
> in : input-port? = (current-input-port)
> skip-bytes-amt : exact-nonnegative-integer? = 0

Like peek-char, but if the input port returns a non-byte value after skip-bytes-amt byte positions, then it is returned.

> > procedure
> > (peek-byte-or-special | [ | in |
> > |  | skip-bytes-amt |
> > |  | progress]) |
> → (or/c byte? eof-object? any/c)
> in : input-port? = (current-input-port)
> skip-bytes-amt : exact-nonnegative-integer? = 0
> progress : (or/c progress-evt? #f) = #f

Like peek-char-or-special, but peeks and returns a byte instead of a character, and it supports a progress argument like peek-bytes-avail!.

> procedure (port-progress-evt [in]) → progress-evt?
> | in |  | : |  | (and/c input-port? port-provides-progress-evts?)
> |  |  | = |  | (current-input-port)

Returns a synchronizable event (see Events) that becomes ready for synchronization after any subsequent read from in or after in is closed. After the event becomes ready, it remains ready. The synchronization result of a progress event is the progress event itself.

> procedure (port-provides-progress-evts? in) → boolean
> in : input-port?

Returns #t if port-progress-evt can return an event for in. All built-in kinds of ports support progress events, but ports created with make-input-port (see Custom Ports) may not.

> procedure (port-commit-peeked amt progress evt [in]) → boolean?
> amt : exact-nonnegative-integer?
> progress : progress-evt?
> evt : evt?
> in : input-port? = (current-input-port)

Attempts to commit as read the first amt previously peeked bytes, non-byte specials, and eofs from in, or the first eof or special value peeked from in. Mid-stream eofs can be committed, but an eof when the port is exhausted does not necessarily commit, since it does not correspond to data in the stream.

The read commits only if progress does not become ready first (i.e., if no other process reads from in first), and only if evt is chosen by a sync within port-commit-peeked (in which case the event result is ignored); the evt must be either a channel-put event, channel, semaphore, semaphore-peek event, always event, or never event. Suspending the thread that calls port-commit-peeked may or may not prevent the commit from proceeding.

The result from port-commit-peeked is #t if data has been committed, and #f otherwise.

If no data has been peeked from in and progress is not ready, then exn:fail:contract exception is raised. If fewer than amt items have been peeked at the current start of in’s stream, then only the peeked items are committed as read. If in’s stream currently starts at an eof or a non-byte special value, then only the eof or special value is committed as read.

If progress is not a result of port-progress-evt applied to in, then exn:fail:contract exception is raised.

> procedure (byte-ready? [in]) → boolean?
> in : input-port? = (current-input-port)

Returns #t if (read-byte in) would not block (at the time that byte-ready? was called, at least). Equivalent to (and (sync/timeout 0 in) #t).

> procedure (char-ready? [in]) → boolean?
> in : input-port? = (current-input-port)

Returns #t if (read-char in) would not block (at the time that char-ready? was called, at least). Depending on the initial bytes of the stream, multiple bytes may be needed to form a UTF-8 encoding.

> procedure (progress-evt? v) → boolean?
> v : any/c
> (progress-evt? evt in) → boolean?
> evt : progress-evt?
> in : input-port?

With one argument, returns #t is v is a progress evt for some input port, #f otherwise.

With two arguments, returns #t if evt is a progress event for in, #f otherwise.

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