libpng unread data - c

Question

It seems that when decoding PNG files with libpng it doesn't read the last 16 bytes, so I seek 16 bytes forward to get to the end. Can I assume that this is true for all PNG files?
#include<sys/stat.h>
#include<unistd.h>
#include<fcntl.h>
#include<stdlib.h>
#include<stdio.h>
#include<png.h>
int fd;
void png_read(png_struct *png,png_byte *data,png_size_t len){
read(fd,data,len);
}
int main(void){
fd=open("foo.png",O_RDONLY);
png_struct *png=png_create_read_struct(PNG_LIBPNG_VER_STRING,0,0,0);
png_info *png_info=png_create_info_struct(png);
png_set_read_fn(png,0,png_read);
struct stat s;
fstat(fd,&s);
printf("File Size: %d\n",s.st_size);
png_read_info(png,png_info);
int x=png_get_image_width(png,png_info);
int y=png_get_image_height(png,png_info);
int c=png_get_channels(png,png_info);
char *buf=malloc(x*y*c);
char **row=malloc(sizeof(*row)*y);
{
int i=0;
while(i<y){
row[i]=buf+x*i*c;
i++;
}
}
png_read_image(png,(png_byte**)row);
printf("Ending File Position: %d\n",lseek(fd,0,SEEK_CUR));
return(0);
}
.
File Size: 20279
Ending File Position: 20263

Answer 1

After png_read_image you should technically have a png_read_end call:
// ...
png_read_image(png,(png_byte**)row);
png_infop end_info = png_create_info_struct(png);
png_read_end(png, end_info);
After that the positions should match.
Even the libpng docs (last paragraphs of section 13.7) make it seem unnecessary, though.

Answer 2

It is only necessary if you are interested in the rest of the datastream beyond the PNG (which kaykun is!). However, if you just want to get to the end of the PNG and don't care about the contents of the remaining PNG chunks, as the referenced book says, you can use NULL instead of end_info (and therefore don't need to create the end_info structure). You cannot count on the remainder of the PNG file being exactly 16 bytes; there will be more if the PNG happens to contain text chunks after the last IDAT chunk.