calculer la taille d'une image png

#include <stdio.h>

#define HEADER_LG 8

char header[HEADER_LG] = {
             0x89 ,0x50 ,0x4E ,0x47 ,0xD ,0xA ,0x1A ,0xA };

int main (int argc, char *argv[])
{
char buf[HEADER_LG];
FILE *in;
char devnull[8];
unsigned char w=0,w1=0,h=0,h1=0,w2=0,w3=0,h2=0,h3=0;

        if ( argc < 2 ) { printf("png_info <filename>\n"); exit(0);}

        if((in = fopen(argv[1],"rb"))== NULL){
                printf("impossible a ouvrir: \n",argv[1]);
                exit(0);
        }
        fread(buf,1,HEADER_LG,in);
        if ( memcmp(buf, header, HEADER_LG) ) {
          printf("mauvais entete \n");
          exit(1);
        }

        fread(devnull,1,8,in);

        fread(&w,1,1,in);
        fread(&w1,1,1,in);
        fread(&w2,1,1,in);
        fread(&w3,1,1,in);
        fread(&h,1,1,in);
        fread(&h1,1,1,in);
        fread(&h2,1,1,in);
        fread(&h3,1,1,in);

        printf("%s width=\"%d\" height=\"%d\"\n",argv[1],