Identify file type from the first bytes

Challenge

Given the first 64 bytes of a file, return the file format as an integer:

• 1 = PNG (\x89PNG\r\n\x1a\n)
• 2 = JPEG (\xff\xd8\xff)
• 3 = GIF (GIF87a or GIF89a)
• 4 = PDF (%PDF-)
• 0 = unknown

Implement int sniff_format(const unsigned char *buf, int len).

Why this matters

Upload pipelines that trust the file extension get owned. Magic-byte sniffing is the second line of defence.

Input format

Byte buffer + length.

Output format

Format code 0-4.

Constraints

Read at most the first 8 bytes; bound-check len.

Starter code

int sniff_format(const unsigned char *buf, int len) { /* TODO */ (void)buf; (void)len; return 0; }

Common mistakes

Matching PDF anywhere instead of at offset 0.

Edge cases to handle

Very short buffer; len < 4.

Complexity

O(1).

Background lessons

• ELF header — the binary's calling card
• Input validation

Up next

• Identify the file format from the first 8 bytes