= cowboy_req:read_and_match_urlencoded_body(3)

== Name

cowboy_req:read_and_match_urlencoded_body - Read, parse
and match a urlencoded request body against constraints

== Description

[source,erlang]
----
read_and_match_urlencoded_body(Fields, Req)
    -> read_and_match_urlencoded_body(Fields, Req, #{})

read_and_match_urlencoded_body(Fields, Req, Opts)
    -> {ok, Body, Req}

Fields :: cowboy:fields()
Req    :: cowboy_req:req()
Opts   :: cowboy_req:read_body_opts()
Body   :: #{atom() => any()}
----

Read, parse and match a urlencoded request body against
constraints.

This function reads the request body and parses it as
`application/x-www-form-urlencoded`. It then applies
the given field constraints to the urlencoded data
and returns the result as a map.

The urlencoded media type is used by Web browsers when
submitting HTML forms using the POST method.

Cowboy will only return the values specified
in the fields list, and ignore all others. Fields can be
either the key requested; the key along with a list of
constraints; or the key, a list of constraints and a
default value in case the key is missing.

This function will crash if the key is missing and no
default value is provided. This function will also crash
if a constraint fails.

The key must be provided as an atom. The key of the
returned map will be that atom. The value may be converted
through the use of constraints, making this function able
to extract, validate and convert values all in one step.

Cowboy needs to read the full body before parsing. By default
it will read bodies of size up to 64KB. It is possible to
provide options to read larger bodies if required.

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

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

This function can only be called once. Calling it again will
result in undefined behavior.

== Arguments

Fields::

Fields to retrieve from the urlencoded body.
+
See link:man:cowboy(3)[cowboy(3)] for a complete description.

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, the `period`
to 5 seconds and `max_keys` to 100.

== Return value

An `ok` tuple is returned.

Desired values are returned as a map. The key is the atom
that was given in the list of fields, and the value is the
optionally converted value after applying constraints.

The map contains the same keys that were given in the fields.

An exception is triggered when the match fails.

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

* *2.17*: Option `max_keys` added.
* *2.5*: Function introduced.

== Examples

.Match fields
[source,erlang]
----
%% ID and Lang are binaries.
#{id := ID, lang := Lang}
    = cowboy_req:read_and_match_urlencoded_body(
        [id, lang], Req).
----

.Match fields and apply constraints
[source,erlang]
----
%% ID is an integer and Lang a non-empty binary.
#{id := ID, lang := Lang}
    = cowboy_req:read_and_match_urlencoded_body(
        [{id, int}, {lang, nonempty}], Req).
----

.Match fields with default values
[source,erlang]
----
#{lang := Lang}
    = cowboy_req:read_and_match_urlencoded_body(
        [{lang, [], <<"en-US">>}], Req).
----

.Allow large urlencoded bodies
[source,erlang]
----
{ok, Body, Req} = cowboy_req:read_and_match_urlencoded_body(
    Fields, 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(3)[cowboy_req:read_part(3)],
link:man:cowboy_req:read_part_body(3)[cowboy_req:read_part_body(3)]
