This macro converts four characters to a four-character code (FOURCC).

#define INCL_MMIOOS2
#include <os2.h>

CHAR      ch0;  /*  First character. */
CHAR      ch1;  /*  Second character. */
CHAR      ch2;  /*  Third character. */
CHAR      ch3;  /*  Fourth character. */
FOURCC    rc;   /*  Four-character code. */

rc = mmioFOURCC(ch0, ch1, ch2, ch3);