CERT C: Rec. FIO24-C

Do not open a file that is already open

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Description

Rule Definition

Do not open a file that is already open.¹

Polyspace Implementation

The rule checker checks for Opening previously opened resource.

Examples

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Opening previously opened resource

Issue

Opening previously opened resource checks for file opening functions that are opening an already opened file.

Risk

If you open a resource multiple times, you can encounter:

A race condition when accessing the file.
Undefined or unexpected behavior for that file.
Portability issues when you run your program on different targets.

Fix

Once a resource is open, close the resource before reopening.

Example - File Reopened With New Permissions

#include <stdio.h>
const char* logfile = "my_file.log";

void doubleresourceopen()
{
    FILE* fpa = fopen(logfile, "w");
    if (fpa == NULL) {
        return;
    }
    (void)fprintf(fpa, "Writing");
    FILE* fpb = fopen(logfile, "r"); //Noncompliant
    (void)fclose(fpa);
    (void)fclose(fpb);
}

In this example, a logfile is opened in the first line of this function with write privileges. Halfway through the function, the logfile is opened again with read privileges.

Correction — Close Before Reopening

One possible correction is to close the file before reopening the file with different privileges.

#include <stdio.h>
const char* logfile = "my_file.log";

void doubleresourceopen()
{
    FILE* fpa = fopen(logfile, "w");
    if (fpa == NULL) {
        return;
    }
    (void)fprintf(fpa, "Writing");
    (void)fclose(fpa);
    FILE* fpb = fopen(logfile, "r");
    (void)fclose(fpb);
}

Check Information

Group: Rec. 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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