The trouble with symbolic links

The trouble with symbolic links

As Chris points out, the problem is not so much symlinks as Time Of Check to Time of Use (TOCTOU). Take, for example, a problem I've faced writing a simple C program:

1. I want my program to read and store user data/configuration.
2. The traditional place for that data (over the thirty years of the program so far) has been ~/.PROGRAM.
3. Modern XDG systems use ~/.local/share for such purposes (actually, possibly using XDG_DATA_HOME from the environment).
4. I'd like my program to continue to use ~/.PROGRAM if it already exists, otherwise use ${XDG_DATA_HOME}/PROGRAM if $XDG_DATA_HOME is set (creating the directory, and any parents as required, if it doesn't exist), otherwise use ~/.local/share/PROGRAM (creating the directory, and any parents as required, if it doesn't exist).

The naïve way would be to code up something like the following pseudo-C for determining which directory to use:

    const char *traditional_path = "/home/john/.PROGRAM";    /* Just for testing */
    const char *xdg_default_path = "/home/john/.local/share/PROGRAM";
    const char *xdg_data_home = getenv("XDG_DATA_HOME");
    char *xdg_path;

    if (stat(traditional_path, &statbuf) == 0 && S_ISDIR(statbuf.st_mode)) {
        return traditional_path;
    } else if (xdg_data_home != NULL && *xdg_data_home != '\0' && *xdg_data_home == '/') {
        /* Use xdg_data_home + "/PROGRAM"; */
        xdg_path = strcat_malloc(xdg_data_home, "/PROGRAM");
        return xdg_path;
    } else {
        return xdg_default_path;
    }

And there you have it: a TOCTOU error (check for /home/john/.trader before creating the final target path using mkdir() later). No symlinks need be involved.

Now you can merrily post your solution to the above problem! :-)