In Windows have a look at this...
In these situations, caching can be turned off. This is done at the time the file is opened by passing FILE_FLAG_NO_BUFFERING as a value for the dwFlagsAndAttributes parameter of CreateFile. When caching is disabled, all read and write operations directly access the physical disk. However, the file metadata may still be cached. To flush the metadata to disk, use the FlushFileBuffers function.
Some applications, such as virus-checking software, require that their write operations be flushed to disk immediately; Windows provides this ability through write-through caching. A process enables write-through caching for a specific I/O operation by passing the FILE_FLAG_WRITE_THROUGH flag into its call to CreateFile. With write-through caching enabled, data is still written into the cache, but the cache manager writes the data immediately to disk rather than incurring a delay by using the lazy writer. A process can also force a flush of a file it has opened by calling the FlushFileBuffers function.
But I don't know how that affects performance and whether it always works.
EDIT: If it is Windows only, you are probably better using the native file IO functions (CreateFile, WriteFile, etc.) than the C lib ones (fopen, fwrite, etc.) anyway. If you need to go cross-platform I'd look and see if there is a boost library that does what you want.