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

Non-Compliant Coding Example

The following non-compliant code example logs the 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, the file log is opened twice simultaneously. The result is implementation-defined and potentially 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(void) {
  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.

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

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

      09. Input Output (FIO)       FIO32-C. Reserved