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Editorial Team
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Asked: 2026-06-12T00:17:44+00:00

I’m trying to write a basic HTTP file server using sockets in C and

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I’m trying to write a basic HTTP file server using sockets in C and have two main problems.

First, when I try to access “ip:port/” through a browser, I get the index.html file but it’s in complete gibberish. However, when I access “ip:port/not_in_directory” my little custom 404 message comes back just fine. If I set the default file to a existing pdf file, gibberish comes back as well (length of pdf gibberish).

Secondly, when I try to access “ip:port/file_that_exists”, I get my 404 instead of the correct file. While debugging, I do see that scratch_pad has the correct file name, with no excess or missing characters, but !access(scratch_pad, R_OK) still comes out to be 0. Is there some necessary cast even though access() takes a char* for the file name?

EDIT: Second issue has been solved (I was using a pointer to deallocated buffer as file name). However, everything that’s picked up by the browser is gibberish. I’m removing more non-essential code. 

#define FILE_NOT_FOUND "HTTP/1.0 404 FILE NOT FOUND\r\n"
#define FILE_FOUND "HTTP/1.0 200 OK\r\n"
#define SERVER_NAME "Server: Test Server\r\n"
#define CONTENT_TYPE "Content-Type: text/html\r\n"
#define DATE "Date: "
#define MESSAGE_BREAK "\r\n"
#define NOT_FOUND_HTML "<!DOCTYPE html>\n<html>\n<title>404 Not Found</title>\n<body>\n<p>Nope</p>\n</body>\n</html>\r\n"

  if(!access(pulled_name, R_OK | W_OK)) {
      printf("file found on server\n");
      server_file = fopen(pulled_name, "rb+");
      fseek(server_file, 0L, SEEK_END);
      file_size = ftell(server_file);
      fseek(server_file, 0l, SEEK_SET);
      printf("opened successfully and set file_size to %i\n", file_size);
      getting_time = time(NULL);
      c_time = localtime(&getting_time);
      current_time = asctime(c_time);
      printf("set time for message\n");

  message = malloc((int) strlen(FILE_FOUND)
                + (int) strlen(SERVER_NAME)
                + (int) strlen(CONTENT_TYPE)
                + (int) strlen(DATE)
                + (int) strlen(current_time)
                + (int) strlen(MESSAGE_BREAK));

  printf("malloc'ed message\n");

  message_size = (int)(strlen(message)*sizeof(char));     
  memset(message, 0, message_size);
  strcpy(message, FILE_FOUND);
  strcat(message, SERVER_NAME);
  strcat(message, CONTENT_TYPE);
  strcat(message, DATE);
  strcat(message, current_time);
  strcat(message, MESSAGE_BREAK);
  printf("built message\n");

  printf("set message_size to %i\n", message_size);
  while(total_sent < message_size) {
    n = send(new_socket_fd, message, message_size, 0);
    if (n==-1) {
      printf("error sending message\n");
      break;
    }
    total_sent += n;
    printf("sent %i\n", total_sent);
  }
  free(message);

  total_sent = 0;
  while(total_sent < file_size) {
    n = send(new_socket_fd, server_file, file_size, 0);
    if (n==-1) {
      printf("error sending file\n");
      break;
    }
    total_sent += n;
    printf("sent %i\n", total_sent);
  }
  fclose(server_file);
  close(new_socket_fd);
  printf("closed client connection!\n");
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1 Answer

  1. Editorial Team

    There several instances of this mistake:

    memset(buffer, '\0', sizeof(buffer));

    when the type of the first argument is a char*, which means the buffer will not be filled with null characters but only the first sizeof(char*) bytes (typically 4 or 8). In this case, it means that buffer may not be null terminated (as recv() does not append null characters). Both printf() and strtok() require string arguments to be null terminated. It is common to see garbage characters printed after a string with a call to printf("%s", p); with a non-null terminated string.

    Fix all memset() calls by explicitly specifying the size of the buffer pointed to by the first argument:

    const size_t BUFFER_SIZE      = 512;
const size_t SCRATCH_PAD_SIZE = 256;
buffer                        = malloc(BUFFER_SIZE);     /*Check return value*/
scratch_pad                   = malloc(SCRATCH_PAD_SIZE);/*to ensure success.*/
memset(buffer,      0, BUFFER_SIZE);
memset(scratch_pad, 0, SCRATCH_PAD_SIZE);

    The sizeof(char) is guaranteed to be 1 so it can be omitted from the malloc() argument.

    Other misuse of memset():

    memset(message, '0', sizeof(message));
    • '0' is a the character zero, not null
    • sizeof(message) is the sizeof(char*)

    change to:

    memset(message, 0, /* the actual size of message*/ );
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