12.1.10 More Port Constructors, Procedures, and Events

► |  | Reference: Racket

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

► | 12 | Input and Output

12.1 | Ports
12.2 | Byte and String Input
12.3 | Byte and String Output
12.4 | Reading
12.5 | Writing
12.6 | The Reader
12.7 | The Printer
12.8 | Pretty Printing
12.9 | Reader Extension
12.10 | Printer Extension
12.11 | Serialization

▼ | 12.1 | Ports

12.1.1 | Encodings and Locales
12.1.2 | Managing Ports
12.1.3 | Port Buffers and Positions
12.1.4 | Counting Positions, Lines, and Columns
12.1.5 | File Ports
12.1.6 | String Ports
12.1.7 | Pipes
12.1.8 | Structures as Ports
12.1.9 | Custom Ports
12.1.10 | More Port Constructors, Procedures, and Events

► | 12.1.10 | More Port Constructors, Procedures, and Events

12.1.10.1 | Port String and List Conversions
12.1.10.2 | Creating Ports
12.1.10.3 | Port Events
12.1.10.4 | Copying Streams

On this page:

12.1.10.1`` Port String and List Conversions
port->list
port->string
port->bytes
port->lines
port->bytes-lines
display-lines
call-with-output-string
call-with-output-bytes
with-output-to-string
with-output-to-bytes
call-with-input-string
call-with-input-bytes
with-input-from-string
with-input-from-bytes
12.1.10.2`` Creating Ports
input-port-append
make-input-port/ read-to-peek
make-limited-input-port
make-pipe-with-specials
merge-input
open-output-nowhere
peeking-input-port
reencode-input-port
reencode-output-port
dup-input-port
dup-output-port
relocate-input-port
relocate-output-port
transplant-input-port
transplant-output-port
special-filter-input-port
12.1.10.3`` Port Events
eof-evt
read-bytes-evt
read-bytes!-evt
read-bytes-avail!-evt
read-string-evt
read-string!-evt
read-line-evt
read-bytes-line-evt
peek-bytes-evt
peek-bytes!-evt
peek-bytes-avail!-evt
peek-string-evt
peek-string!-evt
regexp-match-evt
12.1.10.4`` Copying Streams
convert-stream
copy-port

top← prev up next →

##### 12.1.10`` More Port Constructors, Procedures, and Events

( require racket/port)

The bindings documented in this section are provided by the racket/port and racket libraries, but not racket/base.

##### 12.1.10.1`` Port String and List Conversions

( port->list [r in]) → ( listof any/c)
r : ( input-port? . -> . any/c) = read
in : input-port? = ( current-input-port)

Returns a list whose elements are produced by calling r on in until it produces eof.

( port->string [in]) → string?
in : input-port? = ( current-input-port)

Reads all characters from in and returns them as a string.

( port->bytes [in]) → bytes?
in : input-port? = ( current-input-port)

Reads all bytes from in and returns them as a byte string.

( port->lines [in #:line-mode line-mode]) → ( listof string?)
in : input-port? = ( current-input-port)

| line-mode |  | : |  | ( or/c 'linefeed 'return 'return-linefeed 'any 'any-one)
|  |  | = |  | 'any

Read all characters from in, breaking them into lines. The line-mode argument is the same as the second argument to read-line, but the default is 'any instead of 'linefeed.

( port->bytes-lines | [ | in |  |  |  |
|  | #:line-mode line-mode]) |  | → |  | ( listof bytes?)

in : input-port? = ( current-input-port)

| line-mode |  | : |  | ( or/c 'linefeed 'return 'return-linefeed 'any 'any-one)
|  |  | = |  | 'any

Like port->lines, but reading bytes and collecting them into lines like read-bytes-line.

( display-lines |  | lst |  |  |  |
| [ | out |  |  |  |
|  | #:separator separator]) |  | → |  | void?

lst : list?
out : output-port? = ( current-output-port)
separator : any/c = #"\n"

Use display to each each element of lst to out, adding separator after each element.

( call-with-output-string proc) → string?
proc : ( output-port? . -> . any)

Calls proc with an output port that accumulates all output into a string, and returns the string.

The port passed to proc is like the one created by open-output-string, except that it is wrapped via dup-output-port, so that proc cannot access the port’s content using get-output-string. If control jumps back into proc, the port continues to accumulate new data, and call-with-output-string returns both the old data and newly accumulated data.

( call-with-output-bytes proc) → bytes?
proc : ( output-port? . -> . any)

Like call-with-output-string, but returns accumulated results in a byte string instead of a string. Furthermore, the port’s content is emptied when call-with-output-bytes returns, so that if control jumps back into proc and returns a second time, only the newly accumulated bytes are returned.

( with-output-to-string proc) → string?
proc : (-> any)

Equivalent to

( call-with-output-string
( lambda (p) ( parameterize ([ current-output-port p])
(proc))))

( with-output-to-bytes proc) → bytes?
proc : (-> any)

Equivalent to

( call-with-output-bytes
( lambda (p) ( parameterize ([ current-output-port p])
(proc))))

( call-with-input-string str proc) → any
str : string?
proc : ( input-port? . -> . any)

Equivalent to (proc ( open-input-string str)).

( call-with-input-bytes bstr proc) → any
bstr : bytes?
proc : ( input-port? . -> . any)

Equivalent to (proc ( open-input-bytes bstr)).

( with-input-from-string str proc) → any
str : string?
proc : (-> any)

Equivalent to

( parameterize ([ current-input-port ( open-input-string str)])
(proc))

( with-input-from-bytes bstr proc) → any
bstr : bytes?
proc : (-> any)

Equivalent to

( parameterize ([ current-input-port ( open-input-bytes str)])
(proc))

##### 12.1.10.2`` Creating Ports

( input-port-append close-at-eof? in ...) → input-port?
close-at-eof? : any/c
in : input-port?

Takes any number of input ports and returns an input port. Reading from the input port draws bytes (and special non-byte values) from the given input ports in order. If close-at-eof? is true, then each port is closed when an end-of-file is encountered from the port, or when the result input port is closed. Otherwise, data not read from the returned input port remains available for reading in its original input port.

See also merge-input, which interleaves data from multiple input ports as it becomes available.

( make-input-port/read-to-peek |  | name |  |  |  |
|  | read-in |  |  |  |
|  | fast-peek |  |  |  |
|  | close |  |  |  |
| [ | get-location |  |  |  |
|  | count-lines! |  |  |  |
|  | init-position |  |  |  |
|  | buffer-mode |  |  |  |
|  | buffering? |  |  |  |
|  | on-consume]) |  | → |  | input-port?

name : any/c

read-in

:

( bytes?
. -> . ( or/c exact-nonnegative-integer?
eof-object?
procedure?
evt?))

fast-peek

:

( or/c #f
( bytes? exact-nonnegative-integer?
( bytes? exact-nonnegative-integer?
. -> . ( or/c exact-nonnegative-integer?
eof-object?
procedure?
evt?
#f))
. -> . ( or/c exact-nonnegative-integer?
eof-object?
procedure?
evt?
#f)))

close : (-> any)

get-location

:

( or/c
(->
( values
( or/c exact-positive-integer? #f)
( or/c exact-nonnegative-integer? #f)
( or/c exact-positive-integer? #f)))
#f)

=

#f

count-lines! : (-> any) = void
init-position : exact-positive-integer? = 1

buffer-mode

:

( or/c ( case-> (( or/c 'block 'none) . -> . any)
(-> ( or/c 'block 'none #f)))
#f)

|  |  | = |  | #f

buffering? : any/c = #f

on-consume

:

( or/c (( or/c exact-nonnegative-integer? eof-object?
procedure? evt?)
. -> . any)
#f)

|  |  | = |  | #f

Similar to make-input-port, but if the given read-in returns an event, the event’s value must be 0. The resulting port’s peek operation is implemented automatically (in terms of read-in) in a way that can handle special non-byte values. The progress-event and commit operations are also implemented automatically. The resulting port is thread-safe, but not kill-safe (i.e., if a thread is terminated or suspended while using the port, the port may become damaged).

The read-in, close, get-lcoation, count-lines!, init-position, and buffer-mode procedures are the same as for make-input-port.

The fast-peek argument can be either #f or a procedure of three arguments: a byte string to receive a peek, a skip count, and a procedure of two arguments. The fast-peek procedure can either implement the requested peek, or it can dispatch to its third argument to implement the peek. The fast-peek is not used when a peek request has an associated progress event.

The buffering? argument determines whether read-in can be called to read more characters than are immediately demanded by the user of the new port. If buffer mode is not #f, then buffering? determines the initial buffer mode, and buffering? is enabled after a buffering change only if the new mode is 'block.

If on-consumed is not #f, it is called when data is read from the port, as opposed to merely peeked. The argument to on-consume is the result value of the port’s reading procedure, so it can be an integer or any result from read-in.

( make-limited-input-port |  | in |  |  |  |
|  | limit |  |  |  |
| [ | close-orig?]) |  | → |  | input-port?

in : input-port?
limit : exact-nonnegative-integer?
close-orig? : any/c = #t

Returns a port whose content is drawn from in, but where an end-of-file is reported after limit bytes (and non-byte special values) are read. If close-orig? is true, then the original port is closed if the returned port is closed.

Bytes are consumed from in only when they are consumed from the returned port. In particular, peeking into the returned port peeks into the original port.

If in is used directly while the resulting port is also used, then the limit bytes provided by the port need not be contiguous parts of the original port’s stream.

( make-pipe-with-specials | [ | limit |  |  |  |
|  | in-name |  |  |  |

out-name])

→

input-port? |  | output-port?

limit : exact-nonnegative-integer? = #f
in-name : any/c = 'pipe
out-name : any/c = 'pipe

Returns two ports: an input port and an output port. The ports behave like those returned by make-pipe, except that the ports support non-byte values written with procedures such as write-special and read with procedures such as get-byte-or-special.

The limit argument determines the maximum capacity of the pipe in bytes, but this limit is disabled if special values are written to the pipe before limit is reached. The limit is re-enabled after the special value is read from the pipe.

The optional in-name and out-name arguments determine the names of the result ports.

( merge-input a-in b-in [buffer-limit]) → input-port?
a-in : input-port?
b-in : input-port?
buffer-limit : ( or/c exact-nonnegative-integer? #f) = 4096

Accepts two input ports and returns a new input port. The new port merges the data from two original ports, so data can be read from the new port whenever it is available from either original port. The data from the original ports are interleaved. When an end-of-file has been read from an original port, it no longer contributes characters to the new port. After an end-of-file has been read from both original ports, the new port returns end-of-file. Closing the merged port does not close the original ports.

The optional buffer-limit argument limits the number of bytes to be buffered from a-in and b-in, so that the merge process does not advance arbitrarily beyond the rate of consumption of the merged data. A #f value disables the limit. As for make-pipe-with-specials, buffer-limit does not apply when a special value is produced by one of the input ports before the limit is reached.

See also input-port-append, which concatenates input streams instead of interleaving them.

( open-output-nowhere [name special-ok?]) → output-port?
name : any/c = 'nowhere
special-ok? : any/c = #t

Creates and returns an output port that discards all output sent to it (without blocking). The name argument is used as the port’s name. If the special-ok? argument is true, then the resulting port supports write-special, otherwise it does not.

( peeking-input-port in [name skip]) → input-port
in : input-port?
name : any/c = ( object-name in)
skip : exact-nonnegative-integer? = 0

Returns an input port whose content is determined by peeking into in. In other words, the resulting port contains an internal skip count, and each read of the port peeks into in with the internal skip count, and then increments the skip count according to the amount of data successfully peeked.

The optional name argument is the name of the resulting port. The skip argument is the port initial skip count, and it defaults to 0.

( reencode-input-port |  | in |  |  |  |
|  | encoding |  |  |  |
|  | error-bytes |  |  |  |
| [ | close? |  |  |  |
|  | name |  |  |  |
|  | convert-newlines? |  |  |  |
|  | enc-error]) |  | → |  | input-port?

in : input-port?
encoding : string?
error-bytes : ( or/c #f bytes?)
close? : any/c = #t
name : any/c = ( object-name in)
convert-newlines? : any/c = #f

| enc-error |  | : |  | ( string? input-port? . -> . any)
|  |  | = |  | ( lambda (msg port) ( error ...))

Produces an input port that draws bytes from in, but converts the byte stream using ( bytes-open-converter encoding-str "UTF-8"). In addition, if convert-newlines? is true, then decoded sequences that correspond to UTF-8 encodings of "\r\n", "\r\u0085", "\r", "\u0085", and "\u2028" are all converted to the UTF-8 encoding of "\n".

If error-bytes is provided and not #f, then the given byte sequence is used in place of bytes from in that trigger conversion errors. Otherwise, if a conversion is encountered, enc-error is called, which must raise an exception.

If close? is true, then closing the result input port also closes in. The name argument is used as the name of the result input port.

In non-buffered mode, the resulting input port attempts to draw bytes from in only as needed to satisfy requests. Toward that end, the input port assumes that at least n bytes must be read to satisfy a request for n bytes. (This is true even if the port has already drawn some bytes, as long as those bytes form an incomplete encoding sequence.)

( reencode-output-port |  | out |  |  |  |
|  | encoding |  |  |  |
|  | error-bytes |  |  |  |
| [ | close? |  |  |  |
|  | name |  |  |  |
|  | newline-bytes |  |  |  |
|  | enc-error]) |  | → |  | output-port?

out : output-port?
encoding : string?
error-bytes : ( or/c #f bytes?)
close? : any/c = #t
name : any/c = ( object-name out)
newline-bytes : ( or/c #f bytes?) = #f

| enc-error |  | : |  | ( string? output-port? . -> . any)
|  |  | = |  | ( lambda (msg port) ( error ...))

Produces an output port that directs bytes to out, but converts its byte stream using ( bytes-open-converter "UTF-8" encoding-str). In addition, if newline-bytes is not #f, then byets written to the port that are the UTF-8 encoding of "\n" are first converted to newline-bytes (before applying the convert from UTF-8 to encoding-str).

If error-bytes is provided and not #f, then the given byte sequence is used in place of bytes send to the output port that trigger conversion errors. Otherwise, enc-error is called, which must raise an exception.

If close? is true, then closing the result output port also closes out. The name argument is used as the name of the result output port.

The resulting port supports buffering, and the initial buffer mode is ( or ( file-stream-buffer-mode out) 'block). In 'block mode, the port’s buffer is flushed only when it is full or a flush is requested explicitly. In 'line mode, the buffer is flushed whenever a newline or carriage-return byte is written to the port. In 'none mode, the port’s buffer is flushed after every write. Implicit flushes for 'line or 'none leave bytes in the buffer when they are part of an incomplete encoding sequence.

The resulting output port does not support atomic writes. An explicit flush or special-write to the output port can hang if the most recently written bytes form an incomplete encoding sequence.

( dup-input-port in [close?]) → input-port?
in : input-port?
close? : any/c = #f

Returns an input port that draws directly from in. Closing the resulting port closes in only if close? is #t.

The new port is initialized with the port read handler of in, but setting the handler on the result port does not affect reading directly from in.

( dup-output-port out [close?]) → output-port?
out : output-port?
close? : any/c = #f

Returns an output port that propagates data directly to out. Closing the resulting port closes out only if close? is #t.

The new port is initialized with the port display handler and port write handler of out, but setting the handlers on the result port does not affect writing directly to out.

( relocate-input-port |  | in |  |  |  |
|  | line |  |  |  |
|  | column |  |  |  |
|  | position |  |  |  |
| [ | close?]) |  | → |  | input-port?

in : input-port?
line : ( or/c exact-positive-integer? #f)
column : ( or/c exact-nonnegative-integer? #f)
position : exact-positive-integer?
close? : any/c = #t

Produces an input port that is equivalent to in except in how it reports location information. The resulting port’s content starts with the remaining content of in, and it starts at the given line, column, and position. A #f for the line or column means that the line and column will always be reported as #f.

The line and column values are used only if line counting is enabled for in and for the resulting port, typically through port-count-lines!. The column value determines the column for the first line (i.e., the one numbered line), and later lines start at column 0. The given position is used even if line counting is not enabled.

When line counting is on for the resulting port, reading from in instead of the resulting port increments location reports from the resulting port. Otherwise, the resulting port’s position does not increment when data is read from in.

If close? is true, then closing the resulting port also closes in. If close? is #f, then closing the resulting port does not close in.

( relocate-output-port |  | out |  |  |  |
|  | line |  |  |  |
|  | column |  |  |  |
|  | position |  |  |  |
| [ | close?]) |  | → |  | output-port?

out : output-port?
line : ( or/c exact-positive-integer? #f)
column : ( or/c exact-nonnegative-integer? #f)
position : exact-positive-integer?
close? : any/c = #t

Like relocate-input-port, but for output ports.

( transplant-input-port |  | in |  |  |  |
|  | get-location |  |  |  |
|  | init-pos |  |  |  |
| [ | close? |  |  |  |
|  | count-lines!]) |  | → |  | input-port?

in : input-port?

get-location

:

( or/c
(->
( values
( or/c exact-positive-integer? #f)
( or/c exact-nonnegative-integer? #f)
( or/c exact-positive-integer? #f)))
#f)

init-pos : exact-positive-integer?
close? : any/c = #t
count-lines! : (-> any) = void

Like relocate-input-port, except that arbitrary position information can be produced (when line counting is enabled) via get-location, which used as for make-input-port. If get-location is #f, then the port counts lines in the usual way starting from init-pos, independent of locations reported by in.

If count-lines! is supplied, it is called when line counting is enabled for the resulting port. The default is void.

( transplant-output-port |  | in |  |  |  |
|  | get-location |  |  |  |
|  | init-pos |  |  |  |
| [ | close? |  |  |  |
|  | count-lines!]) |  | → |  | output-port?

in : input-port?

get-location

:

( or/c
(->
( values
( or/c exact-positive-integer? #f)
( or/c exact-nonnegative-integer? #f)
( or/c exact-positive-integer? #f)))
#f)

init-pos : exact-positive-integer?
close? : any/c = #t
count-lines! : (-> any) = void

Like transplant-input-port, but for output ports.

( special-filter-input-port in proc [close?]) → input-port?
in : input-port?

proc

:

( procedure? bytes? . -> . ( or/c exact-nonnegative-integer?
eof-object?
procedure?
evt?))

close? : any/c = #t

Produces an input port that that is equivalent to in, except that when in produces a procedure to access a special value, proc is applied to the procedure to allow the special value to be replaced with an alternative. The proc is called with the special-value procedure and the byte string that was given to the port’s read or peek function (see make-input-port), and the result is used as te read or peek function’s result. The proc can modify the byte string to substitute a byte for the special value, but the byte string is guaranteed only to hold at least one byte.

If close? is true, then closing the resulting input port also closes in.

##### 12.1.10.3`` Port Events

( eof-evt in) → evt?
in : input-port?

Returns a synchronizable event is that is ready when in produces an eof. If in produces a mid-stream eof, the eof is consumed by the event only if the event is chosen in a synchronization.

( read-bytes-evt k in) → evt?
k : exact-nonnegative-integer?
in : input-port?

Returns a synchronizable event is that is ready when k bytes can be read from in, or when an end-of-file is encountered in in. If k is 0, then the event is ready immediately with "". For non-zero k, if no bytes are available before an end-of-file, the event’s result is eof. Otherwise the event’s result is a byte string of up to k bytes, which contains as many bytes as are available (up to k) before an available end-of-file. (The result is a byte string on less than k bytes only when an end-of-file is encountered.)

Bytes are read from the port if and only if the event is chosen in a synchronization, and the returned bytes always represent contiguous bytes in the port’s stream.

The event can be synchronized multiple times – event concurrently – and each synchronization corresponds to a distinct read request.

The in must support progress events, and it must not produce a special non-byte value during the read attempt.

( read-bytes!-evt bstr in) → evt?
bstr : ( and/c bytes? ( not/c immutable?))
in : input-port?

Like read-bytes-evt, except that the read bytes are placed into bstr, and the number of bytes to read corresponds to ( bytes-length bstr). The event’s result is either eof or the number of read bytes.

The bstr may be mutated any time after the first synchronization attempt on the event. If the event is not synchronized multiple times concurrently, bstr-bytes is never mutated by the event after it is chosen in a synchronization (no matter how many synchronization attempts preceded the choice). Thus, the event may be sensibly used multiple times until a successful choice, but should not be used in multiple concurrent synchronizations.

( read-bytes-avail!-evt bstr in) → evt?
bstr : ( and/c bytes? ( not/c immutable?))
in : input-port?

Like read-bytes!-evt, except that the event reads only as many bytes as are immediately available, after at least one byte or one eof becomes available.

( read-string-evt k in) → evt?
k : exact-nonnegative-integer?
in : input-port?

Like read-bytes-evt, but for character strings instead of byte strings.

( read-string!-evt str in) → evt?
str : ( and/c string? ( not/c immutable?))
in : input-port?

Like read-bytes!-evt, but for a character string instead of a byte string.

( read-line-evt in mode) → evt?
in : input-port?
mode : ( or/c 'linefeed 'return 'return-linefeed 'any 'any-one)

Returns a synchronizable event that is ready when a line of characters or end-of-file can be read from inport. The meaning of mode is the same as for read-line. The event result is the read line of characters (not including the line separator).

A line is read from the port if and only if the event is chosen in a synchronization, and the returned line always represents contiguous bytes in the port’s stream.

( read-bytes-line-evt in mode) → evt?
in : input-port?
mode : ( or/c 'linefeed 'return 'return-linefeed 'any 'any-one)

Like read-line-evt, but returns a byte string instead of a string.

( peek-bytes-evt k skip progress in) → evt?
k : exact-nonnegative-integer?
skip : exact-nonnegative-integer?
progress : evt?
in : input-port?
( peek-bytes!-evt bstr skip progress in) → evt?
bstr : ( and/c bytes? ( not/c immutable?))
skip : exact-nonnegative-integer?
progress : ( or/c evt? #f)
in : input-port?
( peek-bytes-avail!-evt bstr skip progress in) → evt?
bstr : ( and/c bytes? ( not/c immutable?))
skip : exact-nonnegative-integer?
progress : ( or/c evt? #f)
in : input-port?
( peek-string-evt k in) → evt?
k : exact-nonnegative-integer?
in : input-port?
( peek-string!-evt str in) → evt?
str : ( and/c string? ( not/c immutable?))
in : input-port?

Like the read-...-evt functions, but for peeking. The skip argument indicates the number of bytes to skip, and progress indicates an event that effectively cancels the peek (so that the event never becomes ready). The progress argument can be #f, in which case the event is never cancelled.

( regexp-match-evt pattern in) → any
pattern : ( or/c string? bytes? regexp? byte-regexp?)
in : input-port?

Returns a synchronizable event that is ready when pattern matches the stream of bytes/characters from in; see also regexp-match. The event’s value is the result of the match, in the same form as the result of regexp-match.

If pattern does not require a start-of-stream match, then bytes skipped to complete the match are read and discarded when the event is chosen in a synchronization.

Bytes are read from the port if and only if the event is chosen in a synchronization, and the returned match always represents contiguous bytes in the port’s stream. If not-yet-available bytes from the port might contribute to the match, the event is not ready. Similarly, if pattern begins with a start-of-stream ^ and the pattern does not initially match, then the event cannot become ready until bytes have been read from the port.

The event can be synchronized multiple times – even concurrently – and each synchronization corresponds to a distinct match request.

The in port must support progress events. If in returns a special non-byte value during the match attempt, it is treated like eof.

##### 12.1.10.4`` Copying Streams

( convert-stream |  | from-encoding |  |  |  |
|  | in |  |  |  |
|  | from-encoding |  |  |  |
|  | out) |  | → |  | void?

from-encoding : string?
in : input-port?
from-encoding : string?
out : output-port?

Reads data from in, converts it using ( bytes-open-converter from-encoding-string to-encoding-string) and writes the converted bytes to out. The convert-stream procedure returns after reaching eof in in.

If opening the converter fails, the exn:fail exception is raised. Similarly, if a conversion error occurs at any point while reading in, then exn:fail exception is raised.

( copy-port in out ...+) → void?
in : input-port?
out : output-port?

Reads data from in and writes it back out to out, returning when in produces eof. The copy is efficient, and it is without significant buffer delays (i.e., a byte that becomes available on in is immediately transferred to out, even if future reads on in must block). If in produces a special non-byte value, it is transferred to out using write-special.

This function is often called from a “background” thread to continuously pump data from one stream to another.

If multiple outs are provided, case data from in is written to every out. The different outs block output to each other, because each block of data read from in is written completely to one out before moving to the next out. The outs are written in the provided order, so non-blocking ports (e.g., to a file) should be placed first in the argument list.

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