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HTTP Parser

This is a parser for HTTP messages written in C. It parses both requests and responses. The parser is designed to be used in performance HTTP applications. It does not make any syscalls nor allocations, it does not buffer data, it can be interrupted at anytime. Depending on your architecture, it only requires about 40 bytes of data per message stream (in a web server that is per connection).


  • No dependencies
  • Handles persistent streams (keep-alive).
  • Decodes chunked encoding.
  • Upgrade support
  • Defends against buffer overflow attacks.

The parser extracts the following information from HTTP messages:

  • Header fields and values
  • Content-Length
  • Request method
  • Response status code
  • Transfer-Encoding
  • HTTP version
  • Request path, query string, fragment
  • Message body


One http_parser object is used per TCP connection. Initialize the struct using http_parser_init() and set the callbacks. That might look something like this for a request parser:

http_parser_settings settings;
settings.on_path = my_path_callback;
settings.on_header_field = my_header_field_callback;
/* ... */

http_parser *parser = malloc(sizeof(http_parser));
http_parser_init(parser, HTTP_REQUEST);
parser->data = my_socket;

When data is received on the socket execute the parser and check for errors.

size_t len = 80*1024, nparsed;
char buf[len];
ssize_t recved;

recved = recv(fd, buf, len, 0);

if (recved < 0) {
  /* Handle error. */

/* Start up / continue the parser.
 * Note we pass recved==0 to signal that EOF has been recieved.
nparsed = http_parser_execute(parser, &settings, buf, recved);

if (parser->upgrade) {
  /* handle new protocol */
} else if (nparsed != recved) {
  /* Handle error. Usually just close the connection. */

HTTP needs to know where the end of the stream is. For example, sometimes servers send responses without Content-Length and expect the client to consume input (for the body) until EOF. To tell http_parser about EOF, give 0 as the forth parameter to http_parser_execute(). Callbacks and errors can still be encountered during an EOF, so one must still be prepared to receive them.

Scalar valued message information such as status_code, method, and the HTTP version are stored in the parser structure. This data is only temporally stored in http_parser and gets reset on each new message. If this information is needed later, copy it out of the structure during the headers_complete callback.

The parser decodes the transfer-encoding for both requests and responses transparently. That is, a chunked encoding is decoded before being sent to the on_body callback.

The Special Problem of Upgrade

HTTP supports upgrading the connection to a different protocol. An increasingly common example of this is the Web Socket protocol which sends a request like

    GET /demo HTTP/1.1
    Upgrade: WebSocket
    Connection: Upgrade
    WebSocket-Protocol: sample

followed by non-HTTP data.

(See for more information the Web Socket protocol.)

To support this, the parser will treat this as a normal HTTP message without a body. Issuing both on_headers_complete and on_message_complete callbacks. However http_parser_execute() will stop parsing at the end of the headers and return.

The user is expected to check if parser->upgrade has been set to 1 after http_parser_execute() returns. Non-HTTP data begins at the buffer supplied offset by the return value of http_parser_execute().


During the http_parser_execute() call, the callbacks set in http_parser_settings will be executed. The parser maintains state and never looks behind, so buffering the data is not necessary. If you need to save certain data for later usage, you can do that from the callbacks.

There are two types of callbacks:

  • notification typedef int (*http_cb) (http_parser*); Callbacks: on_message_begin, on_headers_complete, on_message_complete.
  • data typedef int (*http_data_cb) (http_parser*, const char *at, size_t length); Callbacks: (requests only) on_path, on_query_string, on_uri, on_fragment, (common) on_header_field, on_header_value, on_body;

Callbacks must return 0 on success. Returning a non-zero value indicates error to the parser, making it exit immediately.

In case you parse HTTP message in chunks (i.e. read() request line from socket, parse, read half headers, parse, etc) your data callbacks may be called more than once. Http-parser guarantees that data pointer is only valid for the lifetime of callback. You can also read() into a heap allocated buffer to avoid copying memory around if this fits your application.

Reading headers may be a tricky task if you read/parse headers partially. Basically, you need to remember whether last header callback was field or value and apply following logic:

(on_header_field and on_header_value shortened to on_h_*)
 ------------------------ ------------ --------------------------------------------
| State (prev. callback) | Callback   | Description/action                         |
 ------------------------ ------------ --------------------------------------------
| nothing (first call)   | on_h_field | Allocate new buffer and copy callback data |
|                        |            | into it                                    |
 ------------------------ ------------ --------------------------------------------
| value                  | on_h_field | New header started.                        |
|                        |            | Copy current name,value buffers to headers |
|                        |            | list and allocate new buffer for new name  |
 ------------------------ ------------ --------------------------------------------
| field                  | on_h_field | Previous name continues. Reallocate name   |
|                        |            | buffer and append callback data to it      |
 ------------------------ ------------ --------------------------------------------
| field                  | on_h_value | Value for current header started. Allocate |
|                        |            | new buffer and copy callback data to it    |
 ------------------------ ------------ --------------------------------------------
| value                  | on_h_value | Value continues. Reallocate value buffer   |
|                        |            | and append callback data to it             |
 ------------------------ ------------ --------------------------------------------

See examples of reading in headers: