read

while ((n = read(sockfd, recvline, MAXLINE)) > 0)
{
    recvline[n] = 0;    /* null terminate */

    if (fputs(recvline, stdout) == EOF)
        err_sys("fputs error");
}

if (n < 0)
    err_sys("read error");

We read the server's reply and display the result using the standard I/O fputs function. We must be careful when using TCP because it is a byte-stream protocol with no record boundaries.

The daytime server's reply is normally a 26-byte string of the form

Mon May 26 20 : 58 : 40 2003\r\n

where \r is the ASCII carriage return and \n is the ASCII linefeed. With a byte-stream protocol, these 26 bytes can be returned in numerous ways: a single TCP segment containing all 26 bytes of data, in 26 TCP segments each containing 1 byte of data, or any other combination that totals to 26 bytes. Normally, a single segment containing all 26 bytes of data is returned, but with larger data sizes, we cannot assume that the server's reply will be returned by a single read. Therefore, when reading from a TCP socket, we always need to code the read in a loop and terminate the loop when either read returns 0 (i.e., the other end closed the connection) or a value less than 0 (an error).

In this example, the end of the record is being denoted by the server closing the connection. This technique is also used by version 1.0 of the Hypertext Transfer Protocol (HTTP). Other techniques are available. For example, the Simple Mail Transfer Protocol (SMTP) marks the end of a record with the two-byte sequence of an ASCII carriage return followed by an ASCII linefeed. Sun Remote Procedure Call (RPC) and the Domain Name System (DNS) place a binary count containing the record length in front of each record that is sent when using TCP. The important concept here is that TCP itself provides no record markers: If an application wants to delineate the ends of records, it must do so itself and there are a few common ways to accomplish this.