CERT C: Rec. FIO03-C

File not opened in exclusive mode occurs when you open a file for writing without adequate check for whether the file already exists.

For instance, you might be opening a file using the fopen() or fopen_s() function using only the w access specifier:

FILE * fp = fopen(file_name, "w");

The checker reports calls to the following functions if they open a file for writing without additional specifications to check if the file already exists. Most functions support opening a file in exclusive mode. In this mode, the file opening fails if the file already exists and returns an error code to indicate the failure. If a file is opened in exclusive mode, the checker assumes that the developer has taken into account the possibility of pre-existing files.

The checker also does not report calls to file opening functions if they open a file in append or truncate mode.

Without adequate checks, if the file opened for writing already exists, its contents are destroyed.

When opening a file for writing, allow for the possibility that the file already exists. Depending on the function that you use for file opening, you can perform appropriate checks for existing files:

You can follow similar fixes for other file-opening functions.

#include <stdio.h>

void readFile(char *fileName) {
    FILE *fptr;

    // Open file
    int errorCode = fopen_s(&fptr, fileName, "w"); // Noncompliant
    if (errorCode != 0) {
        // Handle error
    }
}

In this example, a file is opened using fopen_s() with the w access specifier (write mode). If the file already exists, the file opening action destroys previous contents of the file.

One possible correction is to open the file with the wx access specifier (write mode with exclusive access). In this mode, if the file already exists, the file opening action fails and returns an error code that can be checked for appropriate error handling.

#include <stdio.h>
#include <errno.h>

void readFile(char *fileName) {
    FILE *fptr;

    // Open file
    int errorCode = fopen_s(&fptr, fileName, "wx"); // Compliant
    if (errorCode != 0) {
        // Handle error
    }
}