TCP Programming Gotchas

Listing 1: Parsing AL messages from a TCP byte stream.

#define MAX_MSG 100            /* maximum length of a message */
#define TERM_CHAR '@'          /* message termination character */
#define NULL_CHAR 0x00

int errno;

int read_msg (fd, bp)
int fd;
char *bp;

{
static char stream_buffer [MAX_MSG + ];
static char * sbp = stream_buffer;
static int  rcnt = 0;
int rc, i;
char *cp;

    errno = 0;
    while (1)
          {
          if (rcnt > 0)
             {
             cp = strchr (stream_buffer, TERM_CHAR);
             if (cp != NULL_CHAR)
                {
                i = (cp - stream_buffer);
                strncpy (bp, stream_buffer, i);
                rcnt = rcnt - i - 1;
                if (rcnt > 0)
                   {
                   strncpy (stream_buffer, &stream_buffer [i+1], rcnt);
                   stream_buffer [rcnt] = NULL_CHAR;
                   }
                return (i);
                }
             }

          if (rcnt >= MAX_MSG)
             break;

          rc = recv (fd, &stream_buffer [rcnt], MAX_MSG - rcnt, 0);
          stream_buffer [rcnt + rc] = 0x0;
          if (rc < 0)
             {
             *bp = NULL_CHAR;
             rcnt = 0;
             return (-1);
             }
          else if (rc == 0)
             {
             *bp = NULL_CHAR;
             rcnt = 0;
             errno = 0;
             return (0);
             }
          else
             rcnt += rc;
          }

          *bp = NULL_CHAR; 
          rcnt = 0; 
          errno = 0; 
          return (-1);
}