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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. Do not use a copy of a FILE object in any input/output operations.

Non-Compliant Code Example

This non-compliant code example can fail because a copy of stdout is being used in the call to fputs().

#include <stdio.h>

int main(void) {
    FILE my_stdout = *(stdout);
    fputs("Hello, World!\n", &my_stdout);

    return 0;
}

Platform Specific Details

This non-compliant example does fails with an "access violation" when compiled under Microsoft Visual Studio 2005 and run on an IA-32 platform.

Compliant Solution

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

#include <stdio.h>

int main(void) {
    FILE *my_stdout = stdout;
    fputs("Hello, World!\n", my_stdout);
    return 0;
}

Risk Assessment

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

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Examples of vulnerabilities resulting from the violation of this rule can be found on the CERT website.

References

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