foo.c

#ifdef _WIN32
#include <windows.h>
#endif

#define MAX_PATH_LENGTH 260

void transform_path(char *path) {
#ifdef _WIN32
    // Check if path is already a long path
    if (strlen(path) >= 7 && strncmp(path, "\\\\?\\", 4) == 0) {
        // Path is already a long path, so do nothing
        return;
    }
    if (strlen(path) >= MAX_PATH_LENGTH) {
        char long_path[MAX_PATH_LENGTH + 7];
        // Windows long path prefix is \\?\, so we add 4 extra characters
        // to account for this and a terminating null character
        sprintf(long_path, "\\\\?\\%s", path);
        strcpy(path, long_path);
    }
#else
    // On non-Windows systems, do nothing
    (void)path;
#endif
}
This updated implementation first checks whether the given path is already a Windows long path by verifying that the string starts with \\?\. If it is already a long path, the function returns immediately without making any modifications to the path. Otherwise, it proceeds with checking the length of the path and adding the long path prefix if necessary.