freopen example in C

Header file:

     stdio.h

Synopsis:

     FILE *freopen(const char *path, const char *mode, FILE *stream);

Description:

     Opens the given file in the specified mode and associates the stream with it.  Returns stream on success, or NULL on failure.

freopen stdin example:

  #include<stdio.h>

  #include<string.h>

  int main() {

        FILE *fp;

        char str[100];

        fp = freopen("file.txt", "r", stdin);

        while (1) {

                fgets(str, sizeof(str), stdin);

                str[strlen(str) - 1] = '\0';

                if (strncmp(str, "quit", 4) == 0)

                        break;

                else

                        puts(str);

        }

        return 0;

  }

  Output:

  jp@jp-VirtualBox:~/$ cat file.txt 
  hello world
  hello world
  hello world
  quit

  jp@jp-VirtualBox:~/$ ./a.out

  hello world
  hello world
  hello world

freopen stdout example:

  #include<stdio.h>

  #include<string.h>

  int main() {

        FILE *fp;

        char str[100];

        fp = freopen("file.txt", "w", stdout);

        printf("Enter your input:\n");

        fgets(str, sizeof(str), stdin);

        printf("%s", str);

        fclose(fp);

        return 0;

  }

  Output:

  jp@jp-VirtualBox:~/$ ls

  a.out  freopen.c  freopen_err.c  freopen_out.c

  jp@jp-VirtualBox:~/$ ./a.out

  hello world

  jp@jp-VirtualBox:~/$ ls

  a.out  freopen.c  freopen_err.c  freopen_out.c  file.txt

  jp@jp-VirtualBox:~/cpgms/exp$ cat file.txt

  Enter your input:

  hello world

freopen stderr example:

  #include<stdio.h>

  #include<string.h>

  int main() {

        FILE *fp;

        fp = freopen("file.txt", "w", stderr);

        fputs("hello world\n", stderr);

        fclose(fp);

        return 0;

  }

  Output:

  jp@jp-VirtualBox:~/cpgms/exp$ ls

  a.out  freopen.c  freopen_err.c  freopen_out.c

  jp@jp-VirtualBox:~/cpgms/exp$ ./a.out

  jp@jp-VirtualBox:~/cpgms/exp$ cat file.txt

  hello world