The Perl Toolchain Summit needs more sponsors. If your company depends on Perl, please support this very important event.


IO::Async::Stream - event callbacks and write bufering for a stream filehandle


 use IO::Async::Stream;

 use IO::Async::Loop;
 my $loop = IO::Async::Loop->new;

 my $stream = IO::Async::Stream->new(
    read_handle  => \*STDIN,
    write_handle => \*STDOUT,

    on_read => sub {
       my ( $self, $buffref, $eof ) = @_;

       while( $$buffref =~ s/^(.*\n)// ) {
          print "Received a line $1";

       if( $eof ) {
          print "EOF; last partial line is $$buffref\n";

       return 0;

 $loop->add( $stream );

 $stream->write( "An initial line here\n" );


This subclass of IO::Async::Handle contains a filehandle that represents a byte-stream. It provides buffering for both incoming and outgoing data. It invokes the on_read handler when new data is read from the filehandle. Data may be written to the filehandle by calling the write method.

For implementing real network protocols that are based on messages sent over a byte-stream (such as a TCP socket), it may be more appropriate to use a subclass of IO::Async::Protocol::Stream.


The following events are invoked, either using subclass methods or CODE references in parameters:

$ret = on_read \$buffer, $eof

Invoked when more data is available in the internal receiving buffer.

The first argument is a reference to a plain perl string. The code should inspect and remove any data it likes, but is not required to remove all, or indeed any of the data. Any data remaining in the buffer will be preserved for the next call, the next time more data is received from the handle.

In this way, it is easy to implement code that reads records of some form when completed, but ignores partially-received records, until all the data is present. If the handler is confident no more useful data remains, it should return 0. If not, it should return 1, and the handler will be called again. This makes it easy to implement code that handles multiple incoming records at the same time. See the examples at the end of this documentation for more detail.

The second argument is a scalar indicating whether the stream has reported an end-of-file (EOF) condition. A reference to the buffer is passed to the handler in the usual way, so it may inspect data contained in it. Once the handler returns a false value, it will not be called again, as the handle is now at EOF and no more data can arrive.

The on_read code may also dynamically replace itself with a new callback by returning a CODE reference instead of 0 or 1. The original callback or method that the object first started with may be restored by returning undef. Whenever the callback is changed in this way, the new code is called again; even if the read buffer is currently empty. See the examples at the end of this documentation for more detail.

The push_on_read method can be used to insert new, temporary handlers that take precedence over the global on_read handler. This event is only used if there are no further pending handlers created by push_on_read.


Optional. Invoked when the read handle indicates an end-of-file (EOF) condition. If there is any data in the buffer still to be processed, the on_read event will be invoked first, before this one.


Optional. Invoked when the write handle indicates an end-of-file (EOF) condition. Note that this condition can only be detected after a write syscall returns the EPIPE error. If there is no data pending to be written then it will not be detected yet.

on_read_error $errno

Optional. Invoked when the sysread method on the read handle fails.

on_write_error $errno

Optional. Invoked when the syswrite method on the write handle fails.

The on_read_error and on_write_error handlers are passed the value of $! at the time the error occured. (The $! variable itself, by its nature, may have changed from the original error by the time this handler runs so it should always use the value passed in).

If an error occurs when the corresponding error callback is not supplied, and there is not a handler for it, then the close method is called instead.

on_read_high_watermark $length

on_read_low_watermark $length

Optional. Invoked when the read buffer grows larger than the high watermark or smaller than the low watermark respectively. These are edge-triggered events; they will only be triggered once per crossing, not continuously while the buffer remains above or below the given limit.

If these event handlers are not defined, the default behaviour is to disable read-ready notifications if the read buffer grows larger than the high watermark (so as to avoid it growing arbitrarily if nothing is consuming it), and re-enable notifications again once something has read enough to cause it to drop. If these events are overridden, the overriding code will have to perform this behaviour if required, by using



Optional. Invoked when the writing data buffer becomes empty.



Optional. These two events inform when the filehandle becomes writeable, and when it stops being writeable. on_writeable_start is invoked by the on_write_ready event if previously it was known to be not writeable. on_writeable_stop is invoked after a syswrite operation fails with EAGAIN or EWOULDBLOCK. These two events track the writeability state, and ensure that only state change cause events to be invoked. A stream starts off being presumed writeable, so the first of these events to be observed will be on_writeable_stop.


The following named parameters may be passed to new or configure:

read_handle => IO

The IO handle to read from. Must implement fileno and sysread methods.

write_handle => IO

The IO handle to write to. Must implement fileno and syswrite methods.

handle => IO

Shortcut to specifying the same IO handle for both of the above.

on_read => CODE
on_read_error => CODE
on_outgoing_empty => CODE
on_write_error => CODE
on_writeable_start => CODE
on_writeable_stop => CODE

CODE references for event handlers.

autoflush => BOOL

Optional. If true, the write method will attempt to write data to the operating system immediately, without waiting for the loop to indicate the filehandle is write-ready. This is useful, for example, on streams that should contain up-to-date logging or console information.

It currently defaults to false for any file handle, but future versions of IO::Async may enable this by default on STDOUT and STDERR.

read_len => INT

Optional. Sets the buffer size for read calls. Defaults to 8 KiBytes.

read_all => BOOL

Optional. If true, attempt to read as much data from the kernel as possible when the handle becomes readable. By default this is turned off, meaning at most one fixed-size buffer is read. If there is still more data in the kernel's buffer, the handle will still be readable, and will be read from again.

This behaviour allows multiple streams and sockets to be multiplexed simultaneously, meaning that a large bulk transfer on one cannot starve other filehandles of processing time. Turning this option on may improve bulk data transfer rate, at the risk of delaying or stalling processing on other filehandles.

write_len => INT

Optional. Sets the buffer size for write calls. Defaults to 8 KiBytes.

write_all => BOOL

Optional. Analogous to the read_all option, but for writing. When autoflush is enabled, this option only affects deferred writing if the initial attempt failed due to buffer space.

read_high_watermark => INT
read_low_watermark => INT

Optional. If defined, gives a way to implement flow control or other behaviours that depend on the size of Stream's read buffer.

If after more data is read from the underlying filehandle the read buffer is now larger than the high watermark, the on_read_high_watermark event is triggered (which, by default, will disable read-ready notifications and pause reading from the filehandle).

If after data is consumed by an on_read handler the read buffer is now smaller than the low watermark, the on_read_low_watermark event is triggered (which, by default, will re-enable read-ready notifications and resume reading from the filehandle). For to be possible, the read handler would have to be one added by the push_on_read method or one of the Future-returning read_* methods.

By default these options are not defined, so this behaviour will not happen. read_low_watermark may not be set to a larger value than read_high_watermark, but it may be set to a smaller value, creating a hysteresis region. If either option is defined then both must be.

If these options are used with the default event handlers, be careful not to cause deadlocks by having a high watermark sufficiently low that a single on_read invocation might not consider it finished yet.

reader => STRING|CODE
writer => STRING|CODE

Optional. If defined, gives the name of a method or a CODE reference to use to implement the actual reading from or writing to the filehandle. These will be invoked as

 $stream->reader( $read_handle, $buffer, $len )
 $stream->writer( $write_handle, $buffer, $len )

Each is expected to modify the passed buffer; reader by appending to it, writer by removing a prefix from it. Each is expected to return a true value on success, zero on EOF, or undef with $! set for errors. If not provided, they will be substituted by implenentations using sysread and syswrite on the underlying handle, respectively.

close_on_read_eof => BOOL

Optional. Usually true, but if set to a false value then the stream will not be closed when an EOF condition occurs on read. This is normally not useful as at that point the underlying stream filehandle is no longer useable, but it may be useful for reading regular files, or interacting with TTY devices.

encoding => STRING

If supplied, sets the name of encoding of the underlying stream. If an encoding is set, then the write method will expect to receive Unicode strings and encodes them into bytes, and incoming bytes will be decoded into Unicode strings for the on_read event.

If an encoding is not supplied then write and on_read will work in byte strings.

IMPORTANT NOTE: in order to handle reads of UTF-8 content or other multibyte encodings, the code implementing the on_read event uses a feature of Encode; the STOP_AT_PARTIAL flag. While this flag has existed for a while and is used by the :encoding PerlIO layer itself for similar purposes, the flag is not officially documented by the Encode module. In principle this undocumented feature could be subject to change, in practice I believe it to be reasonably stable.

This note applies only to the on_read event; data written using the write method does not rely on any undocumented features of Encode.

If a read handle is given, it is required that either an on_read callback reference is configured, or that the object provides an on_read method. It is optional whether either is true for on_outgoing_empty; if neither is supplied then no action will be taken when the writing buffer becomes empty.

An on_read handler may be supplied even if no read handle is yet given, to be used when a read handle is eventually provided by the set_handles method.

This condition is checked at the time the object is added to a Loop; it is allowed to create a IO::Async::Stream object with a read handle but without a on_read handler, provided that one is later given using configure before the stream is added to its containing Loop, either directly or by being a child of another Notifier already in a Loop, or added to one.


$stream->want_readready_for_read( $set )

$stream->want_readready_for_write( $set )

Mutators for the want_readready property on IO::Async::Handle, which control whether the read or write behaviour should be continued once the filehandle becomes ready for read.

Normally, want_readready_for_read is always true (though the read watermark behaviour can modify it), and want_readready_for_write is not used. However, if a custom writer function is provided, it may find this useful for being invoked again if it cannot proceed with a write operation until the filehandle becomes readable (such as during transport negotiation or SSL key management, for example).

$stream->want_writeready_for_write( $set )

$stream->want_writeready_for_read( $set )

Mutators for the want_writeready property on IO::Async::Handle, which control whether the write or read behaviour should be continued once the filehandle becomes ready for write.

Normally, want_writeready_for_write is managed by the write method and associated flushing, and want_writeready_for_read is not used. However, if a custom reader function is provided, it may find this useful for being invoked again if it cannot proceed with a read operation until the filehandle becomes writable (such as during transport negotiation or SSL key management, for example).


A synonym for close_when_empty. This should not be used when the deferred wait behaviour is required, as the behaviour of close may change in a future version of IO::Async. Instead, call close_when_empty directly.


If the write buffer is empty, this method calls close on the underlying IO handles, and removes the stream from its containing loop. If the write buffer still contains data, then this is deferred until the buffer is empty. This is intended for "write-then-close" one-shot streams.

 $stream->write( "Here is my final data\n" );

Because of this deferred nature, it may not be suitable for error handling. See instead the close_now method.


This method immediately closes the underlying IO handles and removes the stream from the containing loop. It will not wait to flush the remaining data in the write buffer.

$eof = $stream->is_read_eof

$eof = $stream->is_write_eof

Returns true after an EOF condition is reported on either the read or the write handle, respectively.

$stream->write( $data, %params )

This method adds data to the outgoing data queue, or writes it immediately, according to the autoflush parameter.

If the autoflush option is set, this method will try immediately to write the data to the underlying filehandle. If this completes successfully then it will have been written by the time this method returns. If it fails to write completely, then the data is queued as if autoflush were not set, and will be flushed as normal.

$data can either be a plain string, a Future, or a CODE reference. If it is a plain string it is written immediately. If it is not, its value will be used to generate more $data values, eventually leading to strings to be written.

If $data is a Future, the Stream will wait until it is ready, and take the single value it yields.

If $data is a CODE reference, it will be repeatedly invoked to generate new values. Each time the filehandle is ready to write more data to it, the function is invoked. Once the function has finished generating data it should return undef. The function is passed the Stream object as its first argument.

It is allowed that Futures yield CODE references, or CODE references return Futures, as well as plain strings.

For example, to stream the contents of an existing opened filehandle:

 open my $fileh, "<", $path or die "Cannot open $path - $!";

 $stream->write( sub {
    my ( $stream ) = @_;

    sysread $fileh, my $buffer, 8192 or return;
    return $buffer;
 } );

Takes the following optional named parameters in %params:

write_len => INT

Overrides the write_len parameter for the data written by this call.

on_write => CODE

A CODE reference which will be invoked after every successful syswrite operation on the underlying filehandle. It will be passed the number of bytes that were written by this call, which may not be the entire length of the buffer - if it takes more than one syscall operation to empty the buffer then this callback will be invoked multiple times.

 $on_write->( $stream, $len )
on_flush => CODE

A CODE reference which will be invoked once the data queued by this write call has been flushed. This will be invoked even if the buffer itself is not yet empty; if more data has been queued since the call.

 $on_flush->( $stream )

If the object is not yet a member of a loop and doesn't yet have a write_handle, then calls to the write method will simply queue the data and return. It will be flushed when the object is added to the loop.

If $data is a defined but empty string, the write is still queued, and the on_flush continuation will be invoked, if supplied. This can be used to obtain a marker, to invoke some code once the output queue has been flushed up to this point.

$f = $stream->write( ... )

If called in non-void context, this method returns a Future which will complete (with no value) when the write operation has been flushed. This may be used as an alternative to, or combined with, the on_flush callback.

$stream->push_on_read( $on_read )

Pushes a new temporary on_read handler to the end of the queue. This queue, if non-empty, is used to provide on_read event handling code in preference to using the object's main event handler or method. New handlers can be supplied at any time, and they will be used in first-in first-out (FIFO) order.

As with the main on_read event handler, each can return a (defined) boolean to indicate if they wish to be invoked again or not, another CODE reference to replace themself with, or undef to indicate it is now complete and should be removed. When a temporary handler returns undef it is shifted from the queue and the next one, if present, is invoked instead. If there are no more then the object's main handler is invoked instead.


The following methods all return a Future which will become ready when enough data has been read by the Stream into its buffer. At this point, the data is removed from the buffer and given to the Future object to complete it.

 my $f = $stream->read_...

 my ( $string ) = $f->get;

Unlike the on_read event handlers, these methods don't allow for access to "partial" results; they only provide the final result once it is ready.

If a Future is cancelled before it completes it is removed from the read queue without consuming any data; i.e. each Future atomically either completes or is cancelled.

Since it is possible to use a readable Stream entirely using these Future-returning methods instead of the on_read event, it may be useful to configure a trivial return-false event handler to keep it from consuming any input, and to allow it to be added to a Loop in the first place.

 my $stream = IO::Async::Stream->new( on_read => sub { 0 }, ... );
 $loop->add( $stream );

 my $f = $stream->read_...

If a read EOF or error condition happens while there are read Futures pending, they are all completed. In the case of a read EOF, they are done with undef; in the case of a read error they are failed using the $! error value as the failure.

 $f->fail( $message, sysread => $! )

If a read EOF condition happens to the currently-processing read Future, it will return a partial result. The calling code can detect this by the fact that the returned data is not complete according to the specification (too short in read_exactly's case, or lacking the ending pattern in read_until's case). Additionally, each Future will yield the $eof value in its results.

 my ( $string, $eof ) = $f->get;

$f = $stream->read_atmost( $len )

$f = $stream->read_exactly( $len )

Completes the Future when the read buffer contains $len or more characters of input. read_atmost will also complete after the first invocation of on_read, even if fewer characters are available, whereas read_exactly will wait until at least $len are available.

$f = $stream->read_until( $end )

Completes the Future when the read buffer contains a match for $end, which may either be a plain string or a compiled Regexp reference. Yields the prefix of the buffer before and including this match.

$f = $stream->read_until_eof

Completes the Future when the stream is eventually closed at EOF, and yields all of the data that was available.


$stream = IO::Async::Stream->new_for_stdin

$stream = IO::Async::Stream->new_for_stdout

$stream = IO::Async::Stream->new_for_stdio

Return a IO::Async::Stream object preconfigured with the correct read_handle, write_handle or both.

$future = $stream->connect( %args )

A convenient wrapper for calling the connect method on the underlying IO::Async::Loop object, passing the socktype hint as stream if not otherwise supplied.


A line-based on_read method

The following on_read method accepts incoming \n-terminated lines and prints them to the program's STDOUT stream.

 sub on_read
    my $self = shift;
    my ( $buffref, $eof ) = @_;

    while( $$buffref =~ s/^(.*\n)// ) {
       print "Received a line: $1";

    return 0;

Because a reference to the buffer itself is passed, it is simple to use a s/// regular expression on the scalar it points at, to both check if data is ready (i.e. a whole line), and to remove it from the buffer. If no data is available then 0 is returned, to indicate it should not be tried again. If a line was successfully extracted, then 1 is returned, to indicate it should try again in case more lines exist in the buffer.

For implementing real network protocols that are based on lines of text it may be more appropriate to use a subclass of IO::Async::Protocol::LineStream.

Reading binary data

This on_read method accepts incoming records in 16-byte chunks, printing each one.

 sub on_read
    my ( $self, $buffref, $eof ) = @_;

    if( length $$buffref >= 16 ) {
       my $record = substr( $$buffref, 0, 16, "" );
       print "Received a 16-byte record: $record\n";

       return 1;

    if( $eof and length $$buffref ) {
       print "EOF: a partial record still exists\n";

    return 0;

The 4-argument form of substr() extracts the 16-byte record from the buffer and assigns it to the $record variable, if there was enough data in the buffer to extract it.

A lot of protocols use a fixed-size header, followed by a variable-sized body of data, whose size is given by one of the fields of the header. The following on_read method extracts messages in such a protocol.

 sub on_read
    my ( $self, $buffref, $eof ) = @_;

    return 0 unless length $$buffref >= 8; # "N n n" consumes 8 bytes

    my ( $len, $x, $y ) = unpack $$buffref, "N n n";

    return 0 unless length $$buffref >= 8 + $len;

    substr( $$buffref, 0, 8, "" );
    my $data = substr( $$buffref, 0, $len, "" );

    print "A record with values x=$x y=$y\n";

    return 1;

In this example, the header is unpack()ed first, to extract the body length, and then the body is extracted. If the buffer does not have enough data yet for a complete message then 0 is returned, and the buffer is left unmodified for next time. Only when there are enough bytes in total does it use substr() to remove them.

Dynamic replacement of on_read

Consider the following protocol (inspired by IMAP), which consists of \n-terminated lines that may have an optional data block attached. The presence of such a data block, as well as its size, is indicated by the line prefix.

 sub on_read
    my $self = shift;
    my ( $buffref, $eof ) = @_;

    if( $$buffref =~ s/^DATA (\d+):(.*)\n// ) {
       my $length = $1;
       my $line   = $2;

       return sub {
          my $self = shift;
          my ( $buffref, $eof ) = @_;

          return 0 unless length $$buffref >= $length;

          # Take and remove the data from the buffer
          my $data = substr( $$buffref, 0, $length, "" );

          print "Received a line $line with some data ($data)\n";

          return undef; # Restore the original method
    elsif( $$buffref =~ s/^LINE:(.*)\n// ) {
       my $line = $1;

       print "Received a line $line with no data\n";

       return 1;
    else {
       print STDERR "Unrecognised input\n";
       # Handle it somehow

In the case where trailing data is supplied, a new temporary on_read callback is provided in a closure. This closure captures the $length variable so it knows how much data to expect. It also captures the $line variable so it can use it in the event report. When this method has finished reading the data, it reports the event, then restores the original method by returning undef.


  • IO::Handle - Supply object methods for I/O handles


Paul Evans <>