practical examples use dup or dup2

Question

I know what dup / dup2 does, but I have no idea when it would be used.

Any practical examples?

Thanks.

Answer 1

I/O redirection in the shell would most likely be implemented using dup2/fcnlt system calls.

We can easily emulate the $program 2>&1 > logfile.log type of redirection using the dup2 function.

The program below redirects both stdout and stderr .i.e emulates behavior of $program 2>&1 > output using the dup2.

#include 
#include 
#include 
#include 

int
main(void){
    int close_this_fd;
    dup2(close_this_fd = open("output", O_WRONLY), 1);
    dup2(1,2);
    close(close_this_fd);
    fprintf(stdout, "standard output\n");
    fprintf(stderr, "standard error\n");
    fflush(stdout);
    sleep(100); //sleep to examine the filedes in /proc/pid/fd level.
    return;
}

vagrant@precise64:/vagrant/advC$ ./a.out
^Z
[2]+  Stopped                 ./a.out
vagrant@precise64:/vagrant/advC$ cat output
standard error
standard output
vagrant@precise64:/vagrant/advC$ ll /proc/2761/fd
total 0
dr-x------ 2 vagrant vagrant  0 Jun 20 22:07 ./
dr-xr-xr-x 8 vagrant vagrant  0 Jun 20 22:07 ../
lrwx------ 1 vagrant vagrant 64 Jun 20 22:07 0 -> /dev/pts/0
l-wx------ 1 vagrant vagrant 64 Jun 20 22:07 1 -> /vagrant/advC/output
l-wx------ 1 vagrant vagrant 64 Jun 20 22:07 2 -> /vagrant/advC/output