test.c

#include <stdio.h>
#include <stddef.h>
//#include "jpeglib.h"

#define MAX_COMPS_IN_SCAN 4
typedef struct {
  int comps_in_scan;		/* number of components encoded in this scan */
  int component_index[MAX_COMPS_IN_SCAN]; /* their SOF/comp_info[] indexes */
  int Ss, Se;			/* progressive JPEG spectral selection parms */
  int Ah, Al;			/* progressive JPEG successive approx. parms */
} jpeg_scan_info;

int main(int argc, char** argv)
{
    printf("jpeg_scan_info sizeof: %d\n", (int)sizeof(jpeg_scan_info));

    printf("  : %d\n", offsetof(jpeg_scan_info, comps_in_scan)); 
    printf("  : %d\n", offsetof(jpeg_scan_info, component_index)); 
    printf("  : %d\n", offsetof(jpeg_scan_info, Ss)); 
    printf("  : %d\n", offsetof(jpeg_scan_info, Se)); 
    printf("  : %d\n", offsetof(jpeg_scan_info, Ah)); 
    printf("  : %d\n", offsetof(jpeg_scan_info, Al)); 

    //int component_index[MAX_COMPS_IN_SCAN];
    //printf("sizeof(int component_index[MAX_COMPS_IN_SCAN]: %d\n", (int)sizeof(component_index));
    return 0;
}