Utility to switch displays in winXP

displayswitch.c

#include <windows.h>
#include <stdio.h>
#include <stdint.h>

#define DEVNAME_LEN 32
#define A2I(x) ((uint8_t)(x-48))

static DEVMODEA dmActive;
static DEVMODEA dmWanted;
static char nameActive[DEVNAME_LEN];
static char nameWanted[DEVNAME_LEN];

BOOL changeDisplay(int wantedNr) {
    #define CHNGDISPTEST(...) if (ChangeDisplaySettingsExA(__VA_ARGS__) != DISP_CHANGE_SUCCESSFUL) return FALSE;
    int devCnt = 0;
    static DEVMODEA dm;
    static DISPLAY_DEVICEA dd;
    
    memset(&dm, 0, sizeof(DEVMODEA));
    memset(&dd, 0, sizeof(DISPLAY_DEVICEA));
    dd.cb = sizeof(DISPLAY_DEVICEA);
    dm.dmSize = sizeof(DEVMODEA);
    
    while (EnumDisplayDevicesA(NULL, devCnt, &dd, 0)) {
        if (EnumDisplaySettingsA(dd.DeviceName, ENUM_REGISTRY_SETTINGS, &dm)) {
            if (dm.dmPelsHeight > 0 && dm.dmPelsWidth > 0) {
                if (dd.StateFlags & DISPLAY_DEVICE_PRIMARY_DEVICE) {
                    if (devCnt == wantedNr) return TRUE;
                    
                    memcpy(&dmActive, &dm, sizeof(DEVMODEA));
                    memcpy(nameActive, dd.DeviceName, DEVNAME_LEN);
                }
                if (devCnt == wantedNr) {
                    memcpy(&dmWanted, &dm, sizeof(DEVMODEA));
                    memcpy(nameWanted, dd.DeviceName, DEVNAME_LEN);
                }
            }
            //printf("%s\n", dd.DeviceName);
        }
        devCnt++;
    }
    if (devCnt < wantedNr) return FALSE;
    
    dmActive.dmPosition.x = dmWanted.dmPosition.x = 0;
    dmActive.dmPosition.y = dmWanted.dmPosition.y = 0;
    dmActive.dmPelsHeight = dmWanted.dmPelsHeight = 0;
    dmActive.dmPelsWidth  = dmWanted.dmPelsWidth  = 0;
    dmWanted.dmFields     = DM_PELSHEIGHT|DM_PELSWIDTH|DM_POSITION;
    CHNGDISPTEST(nameWanted, &dmWanted, NULL, CDS_RESET|CDS_UPDATEREGISTRY, NULL);
    dmWanted.dmFields     = DM_POSITION;
    CHNGDISPTEST(nameWanted, &dmWanted, NULL, CDS_RESET|CDS_UPDATEREGISTRY|CDS_SET_PRIMARY, NULL);
    dmActive.dmFields     = DM_PELSHEIGHT|DM_PELSWIDTH|DM_POSITION;
    CHNGDISPTEST(nameActive, &dmActive, NULL, CDS_RESET|CDS_UPDATEREGISTRY, NULL);
    
    return TRUE;
    #undef CHNGDISPTEST
}

int main(int argc, char *argv[]) {
    uint8_t wantedNr;
    
    if (argc < 2 ) {
        printf("Utility to switch between multiple displays in single-monitor mode.\n");
        printf("For windows 2000 and XP.\n\n");
        printf("Usage: displayswitch.exe NUMBER\n");
        return 1;
    }
    if ((wantedNr = A2I(argv[1][0])-1) > 9) return 1;
    return changeDisplay(wantedNr) ? 0 : 1;
}