c - How is `dup2` actually working? -

i try figure out how dup2 works. goal duplicate standard input , display on standard output (like parrot :) )

i made basic test file:

#include <stdio.h> #include <stdlib.h> #include <unistd.h>  int main (void){   file* fp= fopen("test.txt", "w");   int fildes = fileno(fp);    dup2(fildes, 1);   printf("test\n");    close(fildes);   fclose(fp);   return exit_success; } 

edit the text doesn't appear in shell, file stays empty.

this working, made mistake.

my idea realize entry duplication this:

#include <stdio.h> #include <unistd.h>     typedef struct {   int read;   int write; } pipe_t;  int main (void){   pipe_t my_pipe;   int pid;    if (( pipe((int *)&my_pipe)== -1) || ( (pid = fork()) == -1))     return 1;    if(pid > 0){     close(my_pipe.read);     dup2(my_pipe.write, 0);     int c;     do{       read(my_pipe.write, &c, 1);       write(my_pipe.write, &c, 1);       fflush(stdin);     }while(c != '.');     close(my_pipe.write);   }    else{     close(my_pipe.write);     dup2(my_pipe.read, 1);     int c;     do{       read(my_pipe.read ,&c, 1);       write(my_pipe.read, &c, 1);       fflush(stdout);     }while(c != '.');     close(my_pipe.read);   }   return exit_success; } 

the same code remove dup2 , replace read(my_pipe.write, &c, 1); read(0, &c, 1); , write(my_pipe.read, &c, 1); write(1, &c, 1); works. 1 take inputs , nothing.

i noticed opening file reading, may want open writing write it.

fp = fopen("test.txt", "w") 

also should not use close() , fclose() close file. fclose() more appropriate.