Unix Redirection In C

This short post is a recount of an exploration into redirection in the C language.

As always, let's go to our friend Wikipedia to set the definition for us:

Basic redirection can use < to redirect input and > to redirect output.

For example, we can use the redirect output operator to redirect the output from echo "Hello!" into a file example.txt.

> echo "Hello!" > example.txt
> cat example.txt
Hello!
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As mentioned by our pal Wikipedia, we can use the dup2 system call in C to manage a similar thing!

A simple example

In our first example, we are going to write a simple example of two variables that open a foobar.txt that iterates character by character.

touch foobar.txt one.c
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Inside of foobar.txt, add th following:

foobar test
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As for the contents of one.c:

#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);
  // c becomes f
  read(fd2, &c, 1);
  // c becomes o
  read(fd2, &c, 1);
  // c becomes o
  read(fd2, &c, 1);
  // c becomes b
  read(fd2, &c, 1);
  printf("c = %c\n", c); // c = b

  // now reading in fd1, so c becomes f again
  read(fd1, &c, 1);
  printf("c = %c\n", c); // c = f

  // redirect and now fd2 is now back at f
  dup2(fd1, fd2);
  // reading back fd2 which has been redirected,
  // so c actually becomes o!
  read(fd2, &c, 1);
  printf("c = %c\n", c); // c = 0
  exit(0);
}
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The comments in the code explain what is happening in order, but we're just going to print out the result by running gcc one.c && ./a.out. The output binary a.out is the name given since we do not provide output to GCC.

> gcc one.c && ./a.out
c = b
c = f
c = o
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To explain further what is going on:

A more readable example

The above can seem hard to comprehend - it is better playing around with this stuff in C. This example, I decided to use scanf to read in from stdin in the second example, because I feel like the example was a little clearer for me.

Create a file two.c.

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

#define STR_LEN 6

int main() {
  int fd1;
  char *c1 = (char*)malloc(STR_LEN);
  char *c2 = (char*)malloc(STR_LEN);
  // notice no values
  printf("c1 = %s\nc2 = %s\n", c1, c2);

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

  // scan c1 and c2 in from stdin
  scanf("%s %s", c1, c2);
  // values now becomes "foobar" and "test" respectively
  printf("c1 = %s\nc2 = %s\n", c1, c2);

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

  exit(0);
}
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Now if we run gcc two.c && ./a.out, we get the following:

> gcc two.c && ./a.out
c1 =
c2 =
c1 = foobar
c2 = test
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In this case, we do the following:

Hooray! Redirection to stdin is a success (and no segmentation faults).

I will likely redo this exercise in Rust and Golang this week to show the how-to.

Resources

Image credit: Michael Kubler