File I/O in Memory

A memory file is a block of memory that is perceived as a file by an application. This can be useful if you already have a file image in memory. Memory files let you reduce the number of special-case conditions in your code because, for I/O purposes, you can treat file memory images as if they were disk-based files.

Like I/O buffers, memory files can use memory allocated by the application or by the MMIO Manager. In addition, memory files can be expandable or non-expandable.

Memory is expandable when the system allocates an internal buffer using the MMIO_ALLOCBUF flag of the mmioOpen function. When the MMIO Manager reaches the end of an expandable memory file, it expands the memory file by a predefined increment.

Use the mmioOpen function to open a memory file. Specify NULL for the szFileName parameter and the MMIO_READWRITE flag, as shown:

hmmio = mmioOpen(NULL, &mmioinfo, MMIO_READWRITE);

In addition, set the pmmioinfo parameter to point to an MMIOINFO structure set up as follows:

Set the pIOProc field to NULL.
Set the fccIOProc field to FOURCC_MEM.
Set the pchBuffer field to point to the memory block. To request that the MMIO Manager allocate the memory block, set pchBuffer to NULL.
Set the cchBuffer field to the initial size of the memory block.
Set the aulInfo[0] field to the minimum expansion size of the memory block. For a non-expandable memory file, set aulInfo[0] to NULL.
Set all other fields to 0.

The following code fragment shows how to open a memory file using a buffer named achMyBuffer.

        /* set mmioinfo structure to 0 */        mmioinfo.fccIOProc= FOURCC_MEM
        mmioinfo.pchBuffer= achMyBuffer
        mmioinfo.cchBuffer= cchMyBuffer
hmmio = mmioOpen("NULL", &mmioinfo, 0);

The following code fragment shows how to open a memory file with 1 byte initially and expand up to 1KB as required.

        /* set mmioinfo structure to 0 */
        mmioinfo.fccIOProc= FOURCC_MEM
        mmioinfo.pchBuffer= NULL
        mmioinfo.cchBuffer= 1
        mmioinfo.aulInfo[0] = 1024;
hmmio = mmioOpen("NULL", &mmioinfo, MMIO_CREATE)

Allocating Memory for Memory Files

There are no restrictions on allocating memory for use as a non-expandable memory file. You can use static memory or stack memory, or you can use locally allocated or globally allocated memory.

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