According to the C Standard, 7.23.3, paragraph 6 [ISO/IEC 9899:2024],

The address of the FILE object used to control a stream may be significant; a copy of a FILE object is not required to 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.

Automated Detection

Tool	Version	Checker	Description
Astrée		file-dereference	Partially checked
Axivion Bauhaus Suite		CertC-FIO38	Fully implemented
Clang		misc-non-copyable-objects	Checked with clang-tidy
Compass/ROSE			Can detect simple violations of this rule
Coverity		MISRA C 2012 Rule 22.5	Partially implemented
Cppcheck Premium		premium-cert-fio38-c
Helix QAC		C1485, C5028 C++3113, C++3114
Klocwork		MISRA.FILE_PTR.DEREF.2012 MISRA.FILE_PTR.DEREF.CAST.2012 MISRA.FILE_PTR.DEREF.INDIRECT.2012 MISRA.FILE_PTR.DEREF.RETURN.2012
LDRA tool suite		591 S	Fully implemented
Parasoft C/C++test		CERT_C-FIO38-a	A pointer to a FILE object shall not be dereferenced
PC-lint Plus		9047	Partially supported: reports when a FILE pointer is dereferenced
Polyspace Bug Finder		CERT C: Rule FIO38-C	Checks for misuse of a FILE object (rule fully covered)
RuleChecker		file-dereference	Partially checked

Related Vulnerabilities

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

Related Guidelines

Key here (explains table format and definitions)

Bibliography