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

Non-Compliant Coding Example

The following non-compliant code write logs about the programs state at runtime.

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

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

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

  do_stuff()

  /* ... */
}

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

Compliant Solution

In this compliant solution, a reference to the file pointer is passed around so that the file does not have to be opened twice separately.

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

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

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

  do_stuff(&logfile)

  /* ... */
}

Risk Assessment

Simultaneously opening a file multiple times could result in abnormal program termination or a data integrity violation.

References

\[[ISO/IEC 9899-1999:TC2|AA. C References#ISO/IEC 9899-1999TC2]\] Section 7.19.3, "Files"