/* fbpan — set /dev/fb0 yoffset (pan) to argv[1] and print the result. */
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <sys/ioctl.h>
#include <errno.h>
#include <stdint.h>

#define GETV    0x4600
#define PAN    0x4606

int main(int argc, char **argv) {
    int fd = open("/dev/fb0", O_RDWR);
    if (fd < 0) { perror("open"); return 1; }
    unsigned char v[160]; memset(v,0,sizeof v);
    if (ioctl(fd, GETV, v) < 0) { perror("getvar"); return 1; }
    uint32_t orig = *(uint32_t*)(v+20);
    uint32_t yres = *(uint32_t*)(v+4);
    uint32_t target = argc > 1 ? (uint32_t)atoi(argv[1]) : 0;
    if (yres) target = (target / yres) * yres;   /* snap to whole-screen offset */
    *(uint32_t*)(v+16) = 0;
    *(uint32_t*)(v+20) = target;
    errno = 0;
    int r = ioctl(fd, PAN, v);
    int e = errno;
    unsigned char w[160]; memset(w,0,sizeof w);
    ioctl(fd, GETV, w);
    printf("pan(%u): r=%d errno=%d(%s) yoffset %u -> %u  (yres=%u)\n",
           target, r, e, strerror(e), orig, *(uint32_t*)(w+20), yres);
    close(fd);
    return 0;
}