FIO46-C. Do not access a closed file

The behavior of a program is undefined when it uses the value of a pointer to a FILE object after the associated file is closed (see undefined behavior 148.) Programs that close the standard streams (especially stdout but also stderr and stdin) must be careful not to use the stream objects in subsequent function calls, particularly those that implicitly operate on such objects (such as printf(), perror(), and getc()).

This rule can be generalized to other representations of files, such as an int representing a POSIX file descriptor that has been passed to close()..

Noncompliant Code Example

In this noncompliant code example the printf() function is called after the stdout stream is closed.

#include <stdio.h>
 
int close_stdout(void) {
  if (fclose(stdout) == EOF) {
    return -1;
  }
 
  printf("stdout successfully closed.\n");
  return 0;
}

Compliant Solution

In this compliant solution, stdout is not used again after it is closed. This must remain true for the remainder of the program. 

#include <stdio.h>
 
int close_stdout(void) {
  if (fclose(stdout) == EOF) {
    return -1;
  }

  fprintf(stderr, "stdout successfully closed.\n");
  return 0;
}

Risk Assessment

Using the value of a pointer to a FILE object after the associated file is closed is undefined behavior.

Automated Detection

Related Vulnerabilities

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

Bibliography