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in reply to Decode perl cgi response in C

cURL and libcurl are C-based projects. curl will happily fetch plain text given a URL. Firefox is written mostly in C as well. Perl is written in C, and obviously is up to the task. These are examples of C projects that can send requests to a webserver, listen on a port for a response, and handle it when it comes in. Plain text is probably the easiest response to handle.

You don't have to look very far to find C programs interacting with webservers. Is that really what you were asking?


Dave

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Re^2: Decode perl cgi response in C
by stenasc (Novice) on Jul 22, 2013 at 08:25 UTC

    Yes, I have write some C TCP/UDP web clients and servers in the past and I don't have an issue there. What I have is an embedded system, that needs to process the test output from a cgi file. I hust need a little info on the way to decode this data. I send a post command to the cgi file on the server and wait for a response. Instead of the desired test response, which should be "Hello", I receive the text responses "Download" and "OK". I will read up on cURL to find out the details on the ports so I can decode the text.

[reply]
      I get the feeling that you want help on the HTTP protocol. If you don't have (or can't use) any libraries to help, a minimal method would be to open a TCP connection and send a request message, which consists of the following:
      • A request line, for example GET /images/logo.png HTTP/1.1, which requests a resource called /images/logo.png from the server.
      • Request Headers, such as Accept-Language: en
      • An empty line.
      • An optional message body.
      The request line and headers must all end with <CR><LF> (that is, a carriage return character followed by a line feed character). The empty line must consist of only <CR><LF> and no other whitespace. In the HTTP/1.1 protocol, all headers except Host are optional (but in practice, it's also optional if there's only one name associated with the IP address). A request line containing only the path name is accepted by servers to maintain compatibility with HTTP clients before the HTTP/1.0 specification.

      You will then be able to read the response message, which consists of the following:

      • A Status-Line (for example HTTP/1.1 200 OK, which indicates that the client's request succeeded)
      • Zero of more Response Headers, such as Content-Type
      • An empty line
      • An optional message body
      The Status-Line and headers must all end with <CR><LF> (a carriage return followed by a line feed). The empty line must consist of only <CR><LF> and no other whitespace.

      On my computer, entering the command "telnet www.perlmonks.org 80" returns the message "Connecting To www.perlmonks.org...". If I then enter the command "GET / HTTP/1.1" and hit the <Enter> key twice, I get the following: 

      HTTP/1.1 200 OK
Via: 1.1 ETC-USRPXY-02
Connection: Keep-Alive
Proxy-Connection: Keep-Alive
Content-Length: 47589
Date: Wed, 24 Jul 2013 14:08:33 GMT
Age: 74
Content-Type: text/html
ETag: "b9e5-4e24268fd6b00"
Server: Apache/2.2.24
Accept-Ranges: bytes
Last-Modified: Wed, 24 Jul 2013 14:06:04 GMT

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<html xmlns="http://www.w3.org/1999/xhtml" xml:lang="en">
<head>

      [download]
      ... followed by roughly 47 KB of other data. You can experiment by using telnet to connect to your server and see what you get back.
[reply]
[d/l]

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