Question

I’m taking a networking class at school and am using C/GDB for the first time. Our assignment is to make a webserver that communicates with a client browser. I am well underway and can open files and send them to the client. Everything goes great till I open a very large file and then I seg fault. I’m not a pro at C/GDB so I’m sorry if that is causing me to ask silly questions and not be able to see the solution myself but when I looked at the dumped core I see my seg fault comes here:

if (-1 == (openfd = open(path, O_RDONLY)))

Specifically we are tasked with opening the file and the sending it to the client browser. My Algorithm goes:

1. Open/Error catch
2. Read the file into a buffer/Error catch
3. Send the file

We were also tasked with making sure that the server doesn’t crash when SENDING very large files. But my problem seems to be with opening them. I can send all my smaller files just fine. The file in question is 29.5MB.

The whole algorithm is:

ssize_t send_file(int conn, char *path, int len, int blksize, char *mime) {
  int openfd; // File descriptor for file we open at path
  int temp; // Counter for the size of the file that we send
  char buffer[len]; // Buffer to read the file we are opening that is len big

  // Open the file
  if (-1 == (openfd = open(path, O_RDONLY))) {
    send_head(conn, "", 400, strlen(ERROR_400));
    (void) send(conn, ERROR_400, strlen(ERROR_400), 0);
    logwrite(stdout, CANT_OPEN);
    return -1;
  }

  // Read from file
  if (-1 == read(openfd, buffer, len)) {
    send_head(conn, "", 400, strlen(ERROR_400));
    (void) send(conn, ERROR_400, strlen(ERROR_400), 0);
    logwrite(stdout, CANT_OPEN);
    return -1;
  }
  (void) close(openfd);

  // Send the buffer now
  logwrite(stdout, SUC_REQ);
  send_head(conn, mime, 200, len);      
  send(conn, &buffer[0], len, 0);
  return len;
}

I dunno if it is just a fact that a I am Unix/C novice. Sorry if it is. =( But you’re help is much appreciated.

Answer 1

Rather than using a variable length array, perhaps try allocated the memory using malloc.

char *buffer = malloc (len);

...

free (buffer);

I just did some simple tests on my system, and when I use variable length arrays of a big size (like the size you’re having trouble with), I also get a SEGFAULT.