This function writes to a file that was opened using mmioOpen.

#define INCL_MMIOOS2
#include <os2.h>

HMMIO    hmmio;      /*  Open file handle. */
PCHAR    pchBuffer;  /*  Buffer to write from. */
LONG     cBytes;     /*  Number of bytes. */
LONG     rc;         /*  Number of bytes actually written. */

rc = mmioWrite(hmmio, pchBuffer, cBytes);