Before learning about how to write an I/O procedure, it's helpful to understand how to use one. To open a file using a custom I/O procedure, use mmioOpen as you would to open any other file. Use a plus sign (+) in the filename to separate the name of the physical file from the name of the element of the file you want to open. For example, the following statement opens a file element named “element” from a file named “filename.arc”:
mmioOpen("filename.arc+element", NULL, MMIO_READ);
When the file I/O manager encounters an plus sign in a filename, it looks at the preceding filename extension to determine which I/O procedure to associate with the file. In the previous example, the file I/O manager will attempt to use the I/O procedure associated with the .ARC filename extension. If no I/O procedure is installed, the mmioOpen function returns an error.