CERT C: Rec. FIO10-C

File renamed without checking existence of destination file occurs when you use the rename() function to rename a file without checking if a file with the new name already exists.

The rename() function has implementation-defined behavior if a file with the new name already exists. Unless you explicitly handle this situation, your code is not portable across implementations.

Before renaming a file using the rename() function, check for the existence of a file with the new name. If the file already exists, you can write specific code to either remove the file explicitly or preserve the file.

You can use implementation-specific functions to check for the existence of a file or remove an existing file. Since the functions are implementation-specific, there is less risk of silent errors when you port your code across implementations.

#include <stdio.h>

void renameFile(const char * srcFile, const char *destFile) {
    int res = rename(srcFile, destFile); // Noncompliant
    if (res != 0) {
        // Handle error
    }
}

In this example, the function renameFile() renames a file to the name destFile without checking if a file with that name already exists. This file renaming has implementation-defined behavior and can cause silent failures, thus violating the rule.

In POSIX, you can use the function access() to check for the existence of a file with the name destFile. You can rename to destFile only if a file with this name does not already exist.

#include <stdio.h>
#include <unistd.h>

void renameFile(const char * srcFile, const char *destFile) {
    int fileExists = access(destFile, F_OK);
    if (!fileExists) {
        int res = rename(srcFile, destFile); // Compliant
        if (res != 0) {
            // Handle error
        }
    }
}

This solution has the possibility of a race condition between the call to the acces() function and the rename() function. To avoid the race condition, you have to perform additional steps. For instance, you can use critical sections to ensure that the section of code in the renameFile() function is executed in an atomic manner.