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],
                  (w1+256*w)*256+w3+256*w2,
                  (h1+256*h)*256+h3+256*h2);

        fclose(in);     
}