Detect if PNG file is corrupted in Objective C

Question

I'm downloading jpgs and pngs using NSURLRequest. This works ok but sometimes the files are corrupted. I have seen Catching error: Corrupt JPEG data: premature end of data segment and have this working for jpgs. Does anyone know of a way to do the same for pngs? ie Programatically check if the png data is valid...

Answer 1

The PNG format has several built in checks. Each "chunk" has a CRC32 check, but to check that you'd need to read the full file.

A more basic check (not foolproof, of course) would be to read the start and ending of the file.

The first 8 bytes should always be the following (decimal) values { 137, 80, 78, 71, 13, 10, 26, 10 } (ref). In particular, the bytes second-to-fourth correspond to the ASCII string "PNG".

In hexadecimal:

89 50 4e 47 0d 0a 1a 0a
.. P  N  G  ...........

You can also check the last 12 bytes of the file (IEND chunk). The middle 4 bytes should correspond to the ASCII string "IEND". More specifically the last 12 bytes should be (in hexa):

00 00 00 00 49 45 4e 44 ae 42 60 82
........... I  E  N  D  ...........

(Strictly speaking, it's not really obligatory for a PNG file to end with those 12 bytes, the IEND chunk itself signals the end of the PNG stream and so a file could in principle have extra trailing bytes which would be ignored by the PNG reader. In practice, this is extremely improbable).