= cowboy_req:read_part(3)
== Name
cowboy_req:read_part - Read the next multipart headers
== Description
[source,erlang]
----
read_part(Req :: cowboy_req:req())
-> read_part(Req, #{})
read_part(Req :: cowboy_req:req(), Opts)
-> {ok, Headers, Req} | {done, Req}
Opts :: cowboy_req:read_body_opts()
Headers :: cow_multipart:headers()
----
Read the next part of a multipart body.
This function reads the request body and parses it as
multipart. Each parts of a multipart representation have
their own headers and body. This function parses and returns
headers. Examples of multipart media types are
`multipart/form-data` and `multipart/byteranges`.
Cowboy will skip any data remaining until the beginning of
the next part. This includes the preamble to the multipart
message but also the body of a previous part if it hasn't
been read. Both are skipped automatically when calling this
function.
Cowboy will read the body before parsing in chunks of size
up to 64KB, with a period of 5 seconds. This is tailored for
reading part headers and might not be the most efficient for
skipping the previous part's body.
The headers returned are MIME headers, *NOT* HTTP headers.
They can be parsed using the functions from the `cow_multipart`
module. In addition, the `cow_multipart:form_data/1` function
can be used to quickly extract information from `multipart/form-data`
representations.
// @todo Proper link to cow_multipart:form_data.
Once a part has been read, it can not be read again.
Once the body has been read, Cowboy sets the content-length
header if it was not previously provided.
// @todo Limit the maximum size of multipart headers.
== Arguments
Req::
The Req object.
Opts::
A map of body reading options. Please refer to
link:man:cowboy_req:read_body(3)[cowboy_req:read_body(3)]
for details about each option.
+
This function defaults the `length` to 64KB and the `period`
to 5 seconds.
== Return value
An `ok` tuple is returned containing the next part's headers
as a list of key/values.
A `done` tuple is returned if there are no more parts to read.
The Req object returned in the tuple must be used for 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
* *2.0*: Function introduced. Replaces `part/1,2`.
== Examples
.Read all parts
[source,erlang]
----
acc_multipart(Req0, Acc) ->
case cowboy_req:read_part(Req0) of
{ok, Headers, Req1} ->
{ok, Body, Req} = stream_body(Req1, <<>>),
acc_multipart(Req, [{Headers, Body}|Acc]);
{done, Req} ->
{lists:reverse(Acc), Req}
end.
stream_body(Req0, Acc) ->
case cowboy_req:read_part_body(Req0) of
{more, Data, Req} ->
stream_body(Req, << Acc/binary, Data/binary >>);
{ok, Data, Req} ->
{ok, << Acc/binary, Data/binary >>, Req}
end.
----
.Read all part headers, skipping bodies
[source,erlang]
----
skip_body_multipart(Req0, Acc) ->
case cowboy_req:read_part(Req0) of
{ok, Headers, Req} ->
skip_body_multipart(Req, [Headers|Acc]);
{done, Req} ->
{lists:reverse(Acc), Req}
end.
----
.Read a part header in larger chunks
[source,erlang]
----
{ok, Headers, Req} = cowboy_req:read_part(Req0, #{length => 1000000}).
----
== 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_body(3)[cowboy_req:read_body(3)],
link:man:cowboy_req:read_urlencoded_body(3)[cowboy_req:read_urlencoded_body(3)],
link:man:cowboy_req:read_part_body(3)[cowboy_req:read_part_body(3)]
