CertC-FIO38ΒΆ

Do not copy a FILE object

Required inputs: IR

According to the C Standard, 7.21.3, paragraph 6 [ ISO/IEC 9899:2011],

The address of the FILE object used to control a stream may be significant; a copy of a FILE object need not serve in place of the original.

Consequently, do not copy a FILE object.

Noncompliant Code Example

This noncompliant code example can fail because a by-value copy of stdout is being used in the call to fputs(): 

#include <stdio.h>
 
int main(void) {
  FILE my_stdout = *stdout;
  if (fputs("Hello, World!\n", &my_stdout) == EOF) {
    /* Handle error */
  }
  return 0;
}

When compiled under Microsoft Visual Studio 2013 and run on Windows, this noncompliant example results in an "access violation" at runtime.

Compliant Solution

In this compliant solution, a copy of the stdout pointer to the FILE object is used in the call to fputs(): 

#include <stdio.h>
 
int main(void) {
  FILE *my_stdout = stdout;
  if (fputs("Hello, World!\n", my_stdout) == EOF) {
    /* Handle error */
  }
  return 0;
}
Risk Assessment

Using a copy of a FILE object in place of the original may result in a crash, which can be used in a denial-of-service attack.

Rule Severity Likelihood Remediation Cost Priority Level
FIO38-C Low Probable Medium P4 L3
Related Guidelines
Taxonomy Taxonomy item Relationship
ISO/IEC TS 17961:2013 Copying a FILE object [filecpy] Prior to 2018-01-12: CERT: Unspecified Relationship
Bibliography
[ ISO/IEC 9899:2011] 7.21.3, "Files"
Excerpt from SEI CERT C Coding Standard: Rules for Developing Safe, Reliable, and Secure Systems (2016 Edition) and SEI CERT C Coding Standard [https://cmu-sei.github.io/secure-coding-standards/sei-cert-c-coding-standard/rules/input-output-fio/fio38-c], Copyright (C) 1995-2026 Carnegie Mellon University. See section 9.4. "3rd-Party Licenses" in the documentation for full details.

Possible Messages

Key

Text

Severity

Disabled

file_pointer_dereference

A pointer to a FILE object shall not be dereferenced

None

False

Options