// switch_stdout.c: demonstrates use of dup2() and dup() to redirect // standard output to a new location temporarily and then restore it. #include <stdio.h> #include <stdlib.h> #include <unistd.h> #include <fcntl.h> int main(){ int stdout_bak = dup(STDOUT_FILENO); // make a backup of stdout system("rm -f mystery.txt"); // remove file using a shell command int out_fd = open("mystery.txt", // open file mystery.txt O_WRONLY|O_CREAT, // for writing, create if needed S_IRUSR|S_IWUSR); // give user read/write permission if(out_fd == -1){ // check for errors opening file perror("Couldn't open file 'mystery.txt'"); exit(1); // bail out if open fails } printf("1. Now you see me.\n"); // print to the screen dup2(out_fd, STDOUT_FILENO); // change stdout to go to mystery.txt printf("2. Now you don't!\n"); // print goes to mystery.txt close(out_fd); // close mystery.txt dup2(stdout_bak, STDOUT_FILENO); // restore stdout to screen printf("3. How mysterious...\n"); // print goes to screen return 0; } Good old `printf()' is used in `switch_stdout.c' in several places but seems to change its behavior in some of these spots. Can you describe this variation in behavior?

Database System Concepts

7th Edition

ISBN:9780078022159

Author:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan

Publisher:Abraham Silberschatz Professor, Henry F. Korth, S. Sudarshan

Chapter1: Introduction

Section: Chapter Questions

Problem 1PE

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// switch_stdout.c: demonstrates use of dup2() and dup() to redirect

// standard output to a new location temporarily and then restore it.

#include <stdio.h>

#include <stdlib.h>

#include <unistd.h>

#include <fcntl.h>

int main(){

int stdout_bak = dup(STDOUT_FILENO); // make a backup of stdout

system("rm -f mystery.txt"); // remove file using a shell command

int out_fd = open("mystery.txt", // open file mystery.txt

O_WRONLY|O_CREAT, // for writing, create if needed

S_IRUSR|S_IWUSR); // give user read/write permission

if(out_fd == -1){ // check for errors opening file

perror("Couldn't open file 'mystery.txt'");

exit(1); // bail out if open fails

}

printf("1. Now you see me.\n"); // print to the screen

dup2(out_fd, STDOUT_FILENO); // change stdout to go to mystery.txt

printf("2. Now you don't!\n"); // print goes to mystery.txt

close(out_fd); // close mystery.txt

dup2(stdout_bak, STDOUT_FILENO); // restore stdout to screen

printf("3. How mysterious...\n"); // print goes to screen

return 0;

}

Good old `printf()' is used in `switch_stdout.c' in several places but
seems to change its behavior in some of these spots. Can you describe this variation in behavior?

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