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According to the C Standard, Section 7.21.3, para. 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 copy of stdout is being used in the call to fputs().

int main(void) {
  FILE my_stdout = *(stdout);  /* violation */
  if (fputs("Hello, World!\n", &my_stdout) == EOF) {
    /* Handle error */
  }
  return 0;
}

For example, this noncompliant example fails with an "access violation" when compiled under Microsoft Visual Studio 2005 and run under Windows.

Compliant Solution

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

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

Related Vulnerabilities

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

Related Guidelines

CERT C++ Secure Coding Standard: FIO38-CPP. Do not use a copy of a FILE object for input and output

ISO/IEC 9899:2011 Section 7.21.3, "Files"

ISO/IEC TR 17961 (Draft) Copying a FILE object [filecpy]

Bibliography