Demonstrate how iCloud Drive is surprisingly unreliable.

a.c

#include <stdio.h>
#include <time.h>
#include <unistd.h>

/*
  This super simple C code demonstrates how iCloud Drive is unreliable.
  Obviously, this code is creating a temporary file first, then creating a backup then rename it to original one.
  This is actually used in [OBS Studio](https://obsproject.com/) settings file writing.

  Build this command with `clang`,

  ```
  clang a.c
  ```

  then run `a.out` in the iCloud Drive directory (`~/Library/Mobile Documents/com~apple~CloudDocs`).
  You will see the following strange iCloud Drive behavior.

  After 1st execution:
  ```
  test.txt
  ```

  OK, expected.

  After 2nd execution:
  ```
  test.txt
  test.txt.bak
  ```

  OK, expected.

  After 3rd execution:
  ```
  test.txt
  test.txt.txt
  ```

  Huh, where is `test.txt.bak` and what is `test.txt.txt` ?!

  After 4th execution:
  ```
  test.txt
  test.txt.bak
  test.txt.txt
  ```

  Okay, keep going...

  After 5th execution:
  ```
  test.txt
  test.txt 2.txt
  test.txt.txt
  ```

  Come on! what is `test.txt 2.txt` ?!

  iCloud Drive seems like confused if `rename(3)` is executed in a sequence,
  which is a totally valid operation.
 */

int main() {
  const char *file = "test.txt";
  const char *tmp = "test.txt.tmp";
  const char *bak = "test.txt.bak";

  FILE * const f = fopen(tmp, "w");
  if (f == NULL) {
    return -1;
  }
  fprintf(f, "%ld\n", time(NULL));
  fclose(f);

  if (access(file, F_OK) == 0) {
    if (rename(file, bak) != 0) {
      fprintf(stderr, "Failed to rename %s to %s.\n", file, bak);
      return -1;
    }
  }

  if (rename(tmp, file) != 0) {
    fprintf(stderr, "Failed to rename %s to %s.\n", tmp, file);
    return -1;
  }

  return 0;
}