StackOverflow2013

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plurals

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Id
16780784
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2013-05-27T21:30:33.320
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2013-05-27T21:30:33.320
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text
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I found this in other question and put together what solved the issue for me. Hope you find this helpful. 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: <pre><code>89 50 4e 47 0d 0a 1a 0a .. P N G ........... </code></pre> 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): <pre><code>00 00 00 00 49 45 4e 44 ae 42 60 82 ........... I E N D ........... </code></pre> (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). Here is an implementation: <pre><code>- (BOOL)dataIsValidPNG:(NSData *)data { if (!data || data.length < 12) { return NO; } NSInteger totalBytes = data.length; const char *bytes = (const char *)[data bytes]; return (bytes[0] == (char)0x89 && // PNG bytes[1] == (char)0x50 && bytes[2] == (char)0x4e && bytes[3] == (char)0x47 && bytes[4] == (char)0x0d && bytes[5] == (char)0x0a && bytes[6] == (char)0x1a && bytes[7] == (char)0x0a && bytes[totalBytes - 12] == (char)0x00 && // IEND bytes[totalBytes - 11] == (char)0x00 && bytes[totalBytes - 10] == (char)0x00 && bytes[totalBytes - 9] == (char)0x00 && bytes[totalBytes - 8] == (char)0x49 && bytes[totalBytes - 7] == (char)0x45 && bytes[totalBytes - 6] == (char)0x4e && bytes[totalBytes - 5] == (char)0x44 && bytes[totalBytes - 4] == (char)0xae && bytes[totalBytes - 3] == (char)0x42 && bytes[totalBytes - 2] == (char)0x60 && bytes[totalBytes - 1] == (char)0x82); } </code></pre>
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1. This table or related slice is empty.
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1. POPNG validation on iOS
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1. PTAnswer
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1. USGlamdring
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