Linux dup and dup2 what was the real application scenario?-CodePudding

Recently in "Linux high-performance server programming", the book has a piece of code is written in the dup function with a simple CGI servers, the code is like this:

 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 

Int main (int arg c, char * argv []) 
{
If (argc
=2){
Usage: printf (" % s ip_address port_number \ n ", the basename (argv [0])); 
return 1; 
} 
IP=const char * argv [1]. 
Int port=atoi (argv [2]). 

Struct sockaddr_in address; 
Bzero (& amp; Address, sizeof (address)); 
Address. Sin_family=AF_INET; 
Inet_pton (AF_INET, IP, & amp; Address. Sin_addr); 
Address. Sin_port=htons (port); 

Int the sock=socket (PF_INET SOCK_STREAM, 0). 
Assert (sock>=0); 

Int ret=bind (sock, (struct sockaddr *) & amp; Address, sizeof (address)); 
Assert (ret!=1); 

Ret=listen (sock, 5); 
Assert (ret!=1); 

Struct sockaddr_in client; 
Socklen_t client_addrlength=sizeof (the client); 
Int connfd=accept (the sock, (struct sockaddr *) & amp; Client, & amp; Client_addrlength); 
If (connfd<0) 
{
Printf (" errno is: % d \ n ", errno); 
} 
The else 
{
Close (STDOUT_FILENO); 
Dup (connfd); 
Printf (" abcd \ n "); 
Close (connfd); 
} 

Close (sock); 
return 0; 
}

Read my own feeling and ordinary TCP communications each difference, so I doubt dup really is used in the function in the network programming? In other places use and use in port redirection symbols what's the difference?