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#include <stdio.h>
 

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.

h2. Non-Compliant Coding Example

The following non-compliant code example logs the program's state at runtime.
{code:bgColor=#ffcccc
void do_stuff(void) {
  FILE *logfile = fopen("log", "a");
  if (logfile == NULL) {
    /* Check for errors, write Handle error */
  }

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

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

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

  do_stuff();
 
  if (fclose(logfile) == EOF) {
    /* ...Handle error */
  }
{null  return 0;
}
However, 

#include <stdio.h>
  twice simultaneously. The result is implementation-defined and potentially dangerous.

h2. 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.
{code:bgColor=#ccccff}
void do_stuff(FILE **filelogfile) {
  FILE *logfile = *file;

  /* Check for errors, writeWrite logs pertaining to do_stuff,() etc. */
  fprintf(logfile, "do_stuff\n");
}

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

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

  do_stuff(&logfile);
 
  if (fclose(logfile) == EOF) {
    /* ...Handle error */
  }
{code}

h2. Risk Assessment

Simultaneously opening a file multiple times could result in abnormal program termination or a data integrity violation.
|| Rule || Severity || Likelihood || Remediation Cost || Priority || Level ||
| FIO31-A | *2* (medium) | *2* (probable) | *2* (medium) | {color:#cc9900}{*}P8{*}{color} | {color:#cc9900}{*}L2{*}{color} |

h2. References

\[[ISO/IEC 9899-1999:TC2|AA. C References#ISO/IEC 9899-1999TC2]\] Section 7.19.3, "Files"
  return 0;
}