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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

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is not required to serve in place of the original.

Consequently, do not use a copy of a FILE object in any input/output operations.

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;
}

For example, this noncompliant example fails with When compiled under Microsoft Visual Studio 2013 and run on Windows, this noncompliant example results in an "access violation" when compiled under Microsoft Visual Studio 2005 and run under Windowsat 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

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Related Vulnerabilities

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

Other Languages

This rule appears in the C++ Secure Coding Standard as FIO38-CPP. Do not use a copy of a FILE object for input and output.

References

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

Related Guidelines

Key here (explains table format and definitions)

Bibliography

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Image Added Image Added FIO37-C. Do not assume character data has been read      09. Input Output (FIO)      Image Modified