CERT C: Rule FIO46-C

Do not access a closed file

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Description

Rule Definition

Do not access a closed file.¹

Polyspace Implementation

The rule checker checks for Use of previously closed resource.

Examples

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Use of previously closed resource

Issue

Use of previously closed resource occurs when a function operates on a stream that you closed earlier in your code.

Risk

The standard states that the value of a FILE* pointer is indeterminate after you close the stream associated with it. Operations using the FILE* pointer can produce unintended results.

Fix

One possible fix is to close the stream only at the end of operations. Another fix is to reopen the stream before using it again.

Example - Use of FILE* Pointer After Closing Stream

#include <stdio.h>

void func(void) {
    FILE *fp;
    void *ptr;

    fp = fopen("tmp","w");
    if(fp != NULL) {
        fclose(fp);
        fprintf(fp,"text"); //Noncompliant
    }
}

In this example, fclose closes the stream associated with fp. When you use fprintf on fp after fclose, the Use of previously closed resource defect appears.

Correction — Close Stream After All Operations

One possible correction is to reverse the order of the fprintf and fclose operations.

#include <stdio.h>

void func(void) {
    FILE *fp;
    void *ptr;

    fp = fopen("tmp","w");
    if(fp != NULL) {
        fprintf(fp,"text");
        fclose(fp);
    }
}

Check Information

Group: Rule 09. Input Output (FIO)

Version History

Introduced in R2019a

¹ This software has been created by MathWorks incorporating portions of: the “SEI CERT-C Website,” © 2017 Carnegie Mellon University, the SEI CERT-C++ Web site © 2017 Carnegie Mellon University, ”SEI CERT C Coding Standard – Rules for Developing safe, Reliable and Secure systems – 2016 Edition,” © 2016 Carnegie Mellon University, and “SEI CERT C++ Coding Standard – Rules for Developing safe, Reliable and Secure systems in C++ – 2016 Edition” © 2016 Carnegie Mellon University, with special permission from its Software Engineering Institute.

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