Unix Network Programming Episode 6

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Unix errno Value

When an error occurs in a Unix function (such as one of the socket functions), the global variable errno is set to a positive value indicating the type of error and the function normally returns –1. Our err_sys function looks at the value of errno and prints the corresponding error message string (e.g., “Connection timed out” if errno equals ETIMEDOUT).

The value of errno is set by a function only if an error occurs. Its value is undefined if the function does not return an error. All of the positive error values are constants with all-uppercase names beginning with “E,” and are normally defined in the <sys/errno.h> header. No error has a value of 0.

A Simple Daytime Server

We can write a simple version of a TCP daytime server, which will work with the client.

#include "unp.h"
#include <time.h>

int main(int argc, char **argv)
{
    int listenfd, connfd;
    struct sockaddr_in servaddr;
    char buff[MAXLINE];
    time_t ticks;

    listenfd=Sock(AF_INET, SOCK_STREAM, 0);

    bzeros(&servaddr, sizeof(servaddr));
    servaddr.sin_family=AF_INET;
    servaddr.sin_addr.s_addr=htonl(INADDR_ANY);
    servaddr.sin_port=htons(13);

    Bind(listenfd, (SA *)&servaddr, sizeof(servaddr));

    Listen(listenfd, LISTENQ);

    for(;;)
    {
        connfd=Accept(listenfd, (SA *)NULL, NULL);

        ticks=time(NULL);
        snprintf(buff, sizeof(buff), "%.24s\r\n", ctime(&ticks));
        Write(connfd, buff, strlen(buff));

        Close(connfd);
    }
}

Create a TCP socket

The style used throughout the book for an infinite loop is

for ( ; ; ) {
	. . .
}

The current time and date are returned by the library function time, which returns the number of seconds since the Unix Epoch: 00:00:00 January 1, 1970, Coordinated Universal Time (UTC). The next library function, ctime, converts this integer value into a human-readable string such as

Mon May 26 20:58:40 2003

A carriage return and linefeed are appended to the string by snprintf, and the result is written to the client by write.