= cowboy_req:read_body(3)

== Name

cowboy_req:read_body - Read the request body

== Description

[source,erlang]
----
read_body(Req :: cowboy_req:req())
    -> read_body(Req, #{})

read_body(Req :: cowboy_req:req(), Opts)
    -> {ok,   Data :: binary(), Req}
     | {more, Data :: binary(), Req}

Opts :: cowboy_req:read_body_opts()
----

Read the request body.

This function reads a chunk of the request body. A `more` tuple
is returned when more data remains to be read. Call the function
repeatedly until an `ok` tuple is returned to read the entire body.

An `ok` tuple with empty data is returned when the request has no body,
or when calling this function again after the body has already
been read. It is therefore safe to call this function directly.
Note that the body can only be read once.

This function reads the request body from the connection process.
The connection process is responsible for reading from the socket.
The exact behavior varies depending on the protocol.

The options therefore are only related to the communication
between the request process and the connection process.

Cowboy will automatically handle protocol details including
the expect header, chunked transfer-encoding and others.

Once the body has been read fully, Cowboy sets the content-length
header if it was not previously provided.

== Arguments

Req::

The Req object.

Opts::

A map of body reading options.
+
The `length` option can be used to request smaller or bigger
chunks of data to be sent. It is a best effort approach, Cowboy
may send more data than configured on occasions. It defaults
to 8MB.
+
The `period` indicates how long the connection process will wait
before it provides us with the data it received. It defaults
to 15 seconds.
+
The connection process sends data to the request process when
either the `length` of data or the `period` of time is reached.
+
The `timeout` option is a safeguard in case the connection
process becomes unresponsive. The function will crash if no
message was received in that interval. The timeout should be
larger than the period. It defaults to the period + 1 second.
+
Auto mode can be enabled by setting the `length` to `auto` and
the `period` to `infinity`. When auto mode is used, Cowboy will
send data to the handler as soon as it receives it, regardless
of its size. It will wait indefinitely until data is available.
Auto mode's main purpose is asynchronous body reading using
link:man:cowboy_req:cast(3)[cowboy_req:cast(3)].

== Return value

A `more` tuple is returned when there are more data to be read.

An `ok` tuple is returned when there are no more data to be read,
either because this is the last chunk of data, the body has already
been read, or there was no body to begin with.

The data is always returned as a binary.

The Req object returned in the tuple must be used from that point
onward. It contains a more up to date representation of the request.
For example it may have an added content-length header once the
body has been read.

== Changelog

* *2.11*: The `length` option now accepts `auto` and the
  period now accepts `infinity`. This adds support for
  reading the body in auto mode.
* *2.0*: Function introduced. Replaces `body/1,2`.

== Examples

.Read the entire body
[source,erlang]
----
read_body(Req0, Acc) ->
    case cowboy_req:read_body(Req0) of
        {ok, Data, Req} -> {ok, << Acc/binary, Data/binary >>, Req};
        {more, Data, Req} -> read_body(Req, << Acc/binary, Data/binary >>)
    end.
----

.Read the body in small chunks
[source,erlang]
----
cowboy_req:read_body(Req, #{length => 64000}).
----

== See also

link:man:cowboy_req(3)[cowboy_req(3)],
link:man:cowboy_req:has_body(3)[cowboy_req:has_body(3)],
link:man:cowboy_req:body_length(3)[cowboy_req:body_length(3)],
link:man:cowboy_req:read_urlencoded_body(3)[cowboy_req:read_urlencoded_body(3)],
link:man:cowboy_req:read_and_match_urlencoded_body(3)[cowboy_req:read_and_match_urlencoded_body(3)],
link:man:cowboy_req:read_part(3)[cowboy_req:read_part(3)],
link:man:cowboy_req:read_part_body(3)[cowboy_req:read_part_body(3)]
