Unix Redirection in C

Resources

First example

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>

int main() {
  int fd1, fd2;
  char c;

  fd1 = open("foobar.txt", O_RDONLY);
  fd2 = open("foobar.txt", O_RDONLY);
  read(fd2, &c, 1);
  read(fd2, &c, 1);
  read(fd2, &c, 1);
  read(fd2, &c, 1);
  printf("c = %c\n", c);

  //read(fd2, &c, 1);
  // dup2(fd2, fd1);
  read(fd1, &c, 1);
  dup2(fd1, fd2);
  read(fd2, &c, 1);
  printf("c = %c\n", c);
  exit(0);
}
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> gcc one.c && ./a.out
c = b
c = o
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Second example

#include <unistd.h>
#include <stdio.h>
#include <stdlib.h>
#include <fcntl.h>

#define STR_LEN 6

int main() {
  int fd1;


  fd1 = open("foobar.txt", O_RDONLY);
  if (dup2(fd1, STDIN_FILENO) < 0) {
    printf("Unable to duplicate file descriptor.");
    exit(EXIT_FAILURE);
  }

  char *c1 = (char*)malloc(STR_LEN);
  char *c2 = (char*)malloc(STR_LEN);

  scanf("%s %s", c1, c2);
  printf("c1 = %s\nc2 = %s\n", c1, c2);

  // SAVE THE WHALES, FREE THE MALLOCS
  free(c1);
  free(c2);

  exit(0);
}
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> gcc three.c&& ./a.out
c1 = foobar
c2 = test
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