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Opening a file that is already open has implementation-defined behavior, according to the C Standard, subclause 7.21.3, paragraph 8 [ISO/IEC 9899:2011]:

Functions that open additional (nontemporary) files require a file name, which is a string. The rules for composing valid file names are implementation-defined. Whether the same file can be simultaneously open multiple times is also implementation-defined.

Some platforms may forbid a file simultaneously being opened multiple times, but platforms that allow it may facilitate dangerous race conditions.

Noncompliant Code Example

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

#include <stdio.h>
 
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();
 
  fclose(logfile);
  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 reference eliminates the need to open the same file multiple times.

#include <stdio.h>
 
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);
 
  fclose(logfile);
  return 0;
}

Implementation Details

On a Linux machine running GCC 4.3.2, this program produces

main
do_stuff

which matches the order in which logging occurred.

Automated Detection

Risk Assessment

Simultaneously opening a file multiple times can result in abnormal program termination or data integrity violations.

Related Vulnerabilities

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

Related Guidelines

 Bibliography