Atomic::Pipe - Send atomic messages from multiple writers across a POSIX pipe.


Normally if you write to a pipe from multiple processes/threads, the messages will come mixed together unpredictably. Some messages may be interrupted by parts of messages from other writers. This module takes advantage of some POSIX specifications to allow multiple writers to send arbitrary data down a pipe in atomic chunks to avoid the issue.

NOTE: This only works for POSIX compliant pipes on POSIX compliant systems. Also some features may not be available on older systems, or some platforms.


    POSIX.1 says that write(2)s of less than PIPE_BUF bytes must be
    atomic: the output data is written to the pipe as a contiguous
    sequence.  Writes of more than PIPE_BUF bytes may be nonatomic: the
    kernel may interleave the data with data written by other processes.
    POSIX.1 requires PIPE_BUF to be at least 512 bytes.  (On Linux,
    PIPE_BUF is 4096 bytes.) [...]

Under the hood this module will split your message into small sections of slightly smaller than the PIPE_BUF limit. Each message will be sent as 1 atomic chunk with a 4 byte prefix indicating what process id it came from, what thread id it came from, a chunk ID (in descending order, so if there are 3 chunks the first will have id 2, the second 1, and the final chunk is always 0 allowing a flush as it knows it is done) and then 1 byte with the length of the data section to follow.

On the receiving end this module will read chunks and re-assemble them based on the header data. So the reader will always get complete messages. Note that message order is not guarenteed when messages are sent from multiple processes or threads. Though all messages from any given thread/process should be in order.


    use Atomic::Pipe;

    my ($r, $w) = Atomic::Pipe->pair;

    # Chunks will be set to the number of atomic chunks the message was split
    # into. It is fine to ignore the value returned, it will always be an
    # integer 1 or larger.
    my $chunks = $w->send_message("Hello");

    # $msg now contains "Hello";
    my $msg = $r->read_message;

    # Note, you can set the reader to be non-blocking:

    # Writer too (but buffers unwritten items until your next write_burst(),
    # write_message(), or flush(), or will do a writing block when the pipe
    # instance is destroyed.

    # $msg2 will be undef as no messages were sent, and blocking is turned off.
    my $msg2 = $r->read_message;

Fork example from tests:

    use Test2::V0;
    use Test2::Require::RealFork;
    use Test2::IPC;
    use Atomic::Pipe;

    my ($r, $w) = Atomic::Pipe->pair;

    # For simplicty
    $SIG{CHLD} = 'IGNORE';

    # Forks and runs your coderef, then exits.
    sub worker(&) { ... }

    worker { is($w->write_message("aa" x $w->PIPE_BUF), 3, "$$ Wrote 3 chunks") };
    worker { is($w->write_message("bb" x $w->PIPE_BUF), 3, "$$ Wrote 3 chunks") };
    worker { is($w->write_message("cc" x $w->PIPE_BUF), 3, "$$ Wrote 3 chunks") };

    my @messages = ();
    push @messages => $r->read_message for 1 .. 3;

        [sort @messages],
        [sort(('aa' x PIPE_BUF), ('bb' x PIPE_BUF), ('cc' x PIPE_BUF))],
        "Got all 3 long messages, not mangled or mixed, order not guarenteed"



Mixed data mode is a special use-case for Atomic::Pipe. In this mode the assumption is that the writer end of the pipe uses the pipe as STDOUT or STDERR, and as such a lot of random non-atomic prints can happen on the writer end of the pipe. The special case is when you want to send atomic-chunks of data inline with the random prints, and in the end extract the data from the noise. The atomic nature of messages and bursts makes this possible.

Please note that mixed data mode makes use of 3 ASCII control characters:

SHIFT OUT (^N or \x0E)

Used to start a burst

SHIFT IN (^O or \x0F)

Used to terminate a burst

If this directly follows a SHIFT-OUT it marks the burst as being part of a data-message.

If the random prints include SHIFT OUT then they will confuse the read-side parser and it will not be possible to extract data, in fact reading from the pipe will become quite unpredictable. In practice this is unlikely to cause issues, but printing a binary file or random noise could do it.

A burst may not include SHIFT IN as the SHIFT IN control+character marks the end of a burst. A burst may also not start with the DATA LINK ESCAPE control character as that is used to mark the start of a data-message.

data-messages may contain any data/characters/bytes as they messages include a length so an embedded SHIFT IN will not terminate things early.

    # Create a pair in mixed-data mode
    my ($r, $w) = Atomic::Pipe->pair(mixed_data_mode => 1);

    # Open STDOUT to the write handle
    open(STDOUT, '>&', $w->{wh}) or die "Could not clone write handle: $!";

    # For sanity

    print "A line!\n";

    print "Start a line ..."; # Note no "\n"

    # Any number of newlines is fine the message will send/recieve as a whole.
    $w->write_burst("This is a burst message\n\n\n");

    # Data will be broken into atomic chunks and sent

    print "... Finish the line we started earlier\n";

    my ($type, $data) = $r->get_line_burst_or_data;
    # Type: 'line'
    # Data: "A line!\n"

    ($type, $data) = $r->get_line_burst_or_data;
    # Type: 'burst'
    # Data: "This is a burst message\n\n\n"

    ($type, $data) = $r->get_line_burst_or_data;
    # Type: 'message'
    # Data: $data

    ($type, $data) = $r->get_line_burst_or_data;
    # Type: 'line'
    # Data: "Start a line ...... Finish the line we started earlier\n"

    # mixed-data mode is always non-blocking
    ($type, $data) = $r->get_line_burst_or_data;
    # Type: undef
    # Data: undef

You can also turn mixed-data mode after construction, but you must do so on both ends:


Doing so will make the pipe non-blocking, and will make all bursts/messages include the necessary control characters.



$bytes = Atomic::Pipe->PIPE_BUF

Get the maximum number of bytes for an atomic write to a pipe.

($r, $w) = Atomic::Pipe->pair

Create a pipe, returns a list consisting of a reader and a writer.

$p = Atomic::Pipe->new

If you really must have a new() method it is here for you to abuse. The returned pipe has both handles, it is your job to then turn it into 2 clones one with the reader and one with the writer. It is also your job to make you do not have too many handles floating around preventing an EOF.

$r = Atomic::Pipe->read_fifo($FIFO_PATH)
$w = Atomic::Pipe->write_fifo($FIFO_PATH)

These 2 constructors let you connect to a FIFO by filesystem path.

The interface difference (read_fifo and write_fifo vs specifying a mode) is because the modes to use for fifo's are not obvious ('+<' for reading).

NOTE: THERE IS NO EOF for the read-end in the process that created the fifo. You need to figure out when the last message is received on your own somehow. If you use blocking reads in a loop with no loop exit condition then the loop will never end even after all writers are gone.

$p = Atomic::Pipe->from_fh($fh)
$p = Atomic::Pipe->from_fh($mode, $fh)

Create an instance around an existing filehandle (A clone of the handle will be made and kept internally).

This will fail if the handle is not a pipe.

If no mode is provided this constructor will determine the mode (reader or writer) for you from the given handle. Note: This works on linux, but not BSD or Solaris, on most platforms your must provide a mode.

Valid modes:




Write-only and reuse fileno.




Read-only and reuse fileno.

$p = Atomic::Pipe->from_fd($mode, $fd)

$fd must be a file descriptor number.

This will fail if the fd is not a pipe.

You must specify one of these modes (as a string):




Write-only and reuse fileno.




Read-only and reuse fileno.




Send a message in atomic chunks.

$msg = $p->read_message

Get the next message. This will block until a message is received unless you set $p->blocking(0). If blocking is turned off, and no message is ready, this will return undef. This will also return undef when the pipe is closed (EOF).

$bool = $p->blocking

Get/Set blocking status. This works on read and write handles. On writers this will write as many chunks/bursts as it can, then buffer any remaining until your next write_message(), write_burst(), or flush(), at which point it will write as much as it can again. If the instance is garbage collected with chunks/bursts in the buffer it will block until all can be written.

$bool = $p->pending_output

True if the pipe is a non-blocking writer and there is pending output waiting for a flush (and for the pipe to have room for the new data).


Write any buffered items. This is only useful on writers that are in non-blocking mode, it is a no-op everywhere else.

$bool = $r->eof()

True if all writers are closed, and the buffers do not contain any usable data.

Usable data means raw data that has yet to be processed, complete messages, or complete data bursts. Any of these can still be retreieved using read_message(), or get_line_burst_or_data().


Close this end of the pipe (or both ends if this is not yet split into reader/writer pairs).

$undef_or_bytes = $p->fits_in_burst($data)

This will return undef if the data DES NOT fit in a burst. This will return the size of the data in bytes if it will fit in a burst.

$undef_or_true = $p->write_burst($data)

Attempt to write $data in a single atomic burst. If the data is too big to write atomically this method will not write any data and will return undef. If the data does fit in an atomic write then a true value will be returned.

Note: YOU MUST NOT USE read_message() when writing bursts. This method sends the data as-is with no data-header or modification. This method should be used when the other side is reading the pipe directly without an Atomic::Pipe on the receiving end.

The primary use case of this is if you have multiple writers sending short plain-text messages that will not exceed the atomic pipe buffer limit (minimum of 512 bytes on systems that support atomic pipes accoring to POSIX).

$fh = $p->rh
$fh = $p->wh

Get the read or write handles.

$read_size = $p->read_size()

Get/set the read size. This is how much data to ATTEMPT to read each time fill_buffer() is called. The default is 65,536 which is the default pipe size on linux, though the value is hardcoded currently.

$bytes = $p->fill_buffer

Read a chunk of data from the pipe and store it in the internal buffer. Bytes read are returned. This is only useful if you want to pull data out of the pipe (maybe to unblock the writer?) but do not want to process any of the data yet.

This is automatically called as needed by other methods, usually you do not need to use it directly.


On some newer linux systems it is possible to get/set the pipe size. On supported systems these allow you to do that, on other systems they are no-ops, and any that return a value will return undef.

Note: This has nothing to do with the similarly named PIPE_BUF which cannot be changed. This simply effects how much data can sit in a pipe before the writers block, it does not effect the max size of atomic writes.

$bytes = $p->size

Current size of the pipe buffer.

$bytes = $p->max_size

Maximum size, or undef if that cannot be determined. (Linux only for now).


Attempt to set the pipe size in bytes. It may not work, so check $p->size.


Attempt to set the pipe to the specified size, but if the size is larger than the maximum fall back to the maximum size instead.


If you used Atomic::Pipe->new() you need to now split the one object into readers and writers. These help you do that.

$bool = $p->is_reader

This returns true if this instance is ONLY a reader.


This returns true if this instance is ONLY a writer.


This copies the object into a reader-only copy.


This copies the object into a writer-only copy.


This turnes the object into a reader-only. Note that if you have no writer-copies then effectively makes it impossible to write to the pipe as you cannot get a writer anymore.


This turnes the object into a writer-only. Note that if you have no reader-copies then effectively makes it impossible to read from the pipe as you cannot get a reader anymore.



Enable mixed-data mode. Also makes read-side non-blocking.

($type, $data) = $r->get_line_burst_or_data()

Get a line, a burst, or a message from the pipe. Always non-blocking, will return (undef, undef) if no complete line/burst/message is ready.

$type will be one of: undef, 'line', 'burst', or 'message'.

$data will either be undef, or a complete line, burst, or message.


The source code repository for Atomic-Pipe can be found at


Chad Granum <>


Chad Granum <>


Copyright 2020 Chad Granum <>.

This program is free software; you can redistribute it and/or modify it under the same terms as Perl itself.