C String Help

Scrag · 04-13-2004, 05:20 PM

Im studying C and unix network programming. Trying to make a cheap http server as i go. Anyway, i have a buffer setup to recieve the http client connect info, and I wanna be able to run a routine that responds to the (GET)'s that the server recieves. Basically I need the routine to search through variable buffer (which contains info below) and be able to extract the string like "nak_skull.JPG" from the GET below, to then send that file.

Contents of buffer variable -------------------------------------------------------------------

GET / HTTP/1.1
Host: 127.0.0.1:8080
User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.4) Gecko/20030630
Accept: text/xml,application/xml,application/xhtml+xml,text/html;q=0.9,text/plain;q=0.
GET /nak_skull.JPG HTTP/1.1
Host: 127.0.0.1:8080
User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.4) Gecko/20030630
Accept: video/x-mng,image/png,image/jpeg,image/gif;q=0.2,*/*;q=0.1
etc.......

-------------------------------------------------------------------------------------------------
THANKS!!!!!!!!!!!!!!!!

PS....If there is a better way to do this altogether...please let me know.

itsme86 · 04-13-2004, 06:33 PM

You could try something like this:

Code:

void parse_requests(char *buffer)
{
  char str[4096], *p = buffer, *q;

  while((p = strstr(p, "GET ")))
  {
    p += 4;  // 4 comes from strlen("GET ");

    if(!(q = strchr(p, ' ')))
      q = p + strlen(p);
    strncpy(str, p, q-p); // grab everything after space following GET to next space or end of buffer
    str[q-p] = '\0';

    handle_get(str);
  }
}

The parse_requests() function should be passed that buffer that you mentioned. The handle_get() function should handle the security and sending of the individual request.

Scrag · 04-13-2004, 08:27 PM

THANKS!!

That works just like I hoped it would, but it displays /filename. I tried changing the ' ' to '/', but then it displays nothing. Anyway we can alter this or add something that removed the / from the beginning of the filename.

Thanks again,

itsme86 · 04-13-2004, 11:27 PM

Sure, just change the "GET " strings to "GET /" in the code above and change the p += 4; to p += 5;

That should do the trick.

Just a note: the code I presented can be exploited by buffer overruns. If you mean for the server to be available to the public you might want to add something like 'if(q-p >= 4095) continue;' so there's no chance the strncpy() can overflow the str array. With that if() statement, any requests that are too long will just be ignored. Another approach would be to dynamically allocate memory for str instead of declaring it with a fixed length of 4096 (which was just some arbitrary number I pulled out of a hat anyway).

Scrag · 04-14-2004, 11:23 AM

Hey thanks again. Im not actaually planning on running the server, I just wanna get the basics working now, then improve on it later. But ill note what u said.

Thanks again.

Scrag · 04-14-2004, 11:55 AM

Me again,

I tried modifying the code as u said, changing "GET" to "GET /" and +=5. However, program still returns /filename. I tried modifying a few other things, like changing the ' ' to '/'. Tried a few other things also, but always still returns /filename.

Thanks!

itsme86 · 04-14-2004, 12:47 PM

Hmm, it works fine for me.

Code:

#include <stdio.h>
#include <string.h>

void handle_get(char *str)
{
  printf("Handling request for file: %s\n", str);
}

void parse_requests(char *buffer)
{
  char str[4096], *p = buffer, *q;

  while((p = strstr(p, "GET /")))
  {
    p += 5;  // 5 comes from strlen("GET /");

    if(!(q = strchr(p, ' ')))
      q = p + strlen(p);
    strncpy(str, p, q-p); // grab everything after space following GET to next
    str[q-p] = '\0';

    handle_get(str);
  }
}

int main(void)
{
  char buffer[] = "GET /nak_skull.JPG HTTP/1.1";

  parse_requests(buffer);

  return 0;
}

and then:

itsme@dreams:~/C$ ./http
Handling request for file: nak_skull.JPG
itsme@dreams:~/C$