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The behavior when simultaneously Simultaneously opening a file multiple times is has implementation-defined behavior. On some platforms, this is not allowed. On others, it might result in race conditions.

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The following non-compliant code write example logs about the programs program's state at runtime.

void do_stuff(void) {
  FILE * logfile = fopen("log", "a");
  
  /* Check for errors, write logs pertaining to do_stuff(), etc. */
}

int main(void) {
  FILE * logfile = fopen("log", "a");

  /* Check for errors, write logs pertaining to main(), etc. */

  do_stuff()

  /* ... */
}

However, it opens the file log is opened twice simultaneously. As stated above, the The result is implementation-defined and potentially dangerous.

Compliant Solution

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void do_stuff(FILE **file) {
  FILE * logfile = *file;
  
  /* Check for errors, write logs pertaining to do_stuff, etc. */
}

int main(void) {
  FILE * logfile = fopen("log", "a");

  /* Check for errors, write logs pertaining to main, etc. */

  do_stuff(&logfile)

  /* ... */
}

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