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Opening a file that is already open has implementation-defined behavior. While some platforms may forbid a file simultaneously being opened multiple times, platforms that allow it may facilitate dangerous race conditions.

Noncompliant Code Example

This noncompliant code example logs the program's state at runtime.

void do_stuff(void) {
  FILE *logfile = fopen("log", "a");
  if (logfile == NULL) {
    /* Handle error */
  }

  /* Write logs pertaining to do_stuff() */
  fprintf(logfile, "do_stuff\n");

  /* ... */
}

int main(void) {
  FILE *logfile = fopen("log", "a");
  if (logfile == NULL) {
    /* Handle error */
  }

  /* Write logs pertaining to main() */
  fprintf(logfile, "main\n");

  do_stuff();

  /* ... */
  return 0;
}

However, the file log is opened twice simultaneously. The result is implementation-defined and potentially dangerous.

Implementation Details

On a Linux machine running gcc 4.3.2, this program produces

do_stuff
main

which does not indicate the order in which data was logged.

Compliant Solution

In this compliant solution, a reference to the file pointer is passed as an argument to functions that need to perform operations on that file. This eliminates the need to open the same file multiple times.

void do_stuff(FILE *logfile) {
  /* Write logs pertaining to do_stuff() */
  fprintf(logfile, "do_stuff\n");

  /* ... */
}

int main(void) {
  FILE *logfile = fopen("log", "a");
  if (logfile == NULL) {
    /* Handle error */
  }

  /* Write logs pertaining to main() */
  fprintf(logfile, "main\n");

  do_stuff(logfile);

  /* ... */
  return 0;
}