cowboy_req:read_part - Read the next multipart headers


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 :: #{binary() => binary()}

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.

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.



The Req object.


A map of body reading options. Please refer to 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 map.

A done tuple is returned if there are no more parts to read.

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.


  • 2.0: Function introduced. Replaces part/1,2.


Read all parts
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}

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}
Read all part headers, skipping bodies
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}
Read a part header in larger chunks
{ok, Headers, Req} = cowboy_req:read_part(Req0, #{length => 1000000}).

See also

cowboy_req(3), cowboy_req:has_body(3), cowboy_req:body_length(3), cowboy_req:read_body(3), cowboy_req:read_urlencoded_body(3), cowboy_req:read_and_match_urlencoded_body(3), cowboy_req:read_part_body(3)

Cowboy 2.7 Function Reference


Version select

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