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2025-09-24

hexdump pattern

I find myself surprisingly often in situations where I want to inspect byte buffers.

Luckily my first programming lesson from my cs studies taught me a pretty good hexdump pattern:

func hexdump(buf []byte) {
	// the hexdump print loop, for each row
	for row := 0; row < len(buf); row += 16 {
		fmt.Printf("%04x | ", row)

		// print hex value of each field in the current row
		for field := row; field < row+16 && field < len(buf); field += 1 {

			// separate 8 byte chunks (offset by 8 to not print the separater
			// before the first byte of a row)
			if (field+8)%16 == 0 {
				fmt.Printf(" ")
			}
			fmt.Printf("%02x ", buf[field])
		}

		fmt.Print("| ")

		// print ascii value of each field of the current if printable
		for field := row; field < row+16 && field < len(buf); field += 1 {
			if (field+8)%16 == 0 {
				fmt.Printf(" ")
			}

			// space (20) and tilde (126) are the boundaries of the printable
			// ascii table
			if buf[field] >= ' ' && buf[field] <= '~' {
				fmt.Printf("%c", buf[field])
			} else {
				fmt.Print(".")
			}

		}
		fmt.Println("")
	}
}

Let’s say we generate an exemplary buffer with numbers 0 to a full byte of 255 (0xff):

package main

import "fmt"

func main() {

	// exemplary bytes from 0x00 to 0xff
	buf := make([]byte, 0x100)

	for i := 0; i < len(buf); i += 1 {
		buf[i] = byte(i % 0x100)
	}

	hexdump(buf)
}

Output:

0000 | 00 01 02 03 04 05 06 07  08 09 0a 0b 0c 0d 0e 0f | ........ ........
0010 | 10 11 12 13 14 15 16 17  18 19 1a 1b 1c 1d 1e 1f | ........ ........
0020 | 20 21 22 23 24 25 26 27  28 29 2a 2b 2c 2d 2e 2f |  !"#$%&' ()*+,-./
0030 | 30 31 32 33 34 35 36 37  38 39 3a 3b 3c 3d 3e 3f | 01234567 89:;<=>?
0040 | 40 41 42 43 44 45 46 47  48 49 4a 4b 4c 4d 4e 4f | @ABCDEFG HIJKLMNO
0050 | 50 51 52 53 54 55 56 57  58 59 5a 5b 5c 5d 5e 5f | PQRSTUVW XYZ[\]^_
0060 | 60 61 62 63 64 65 66 67  68 69 6a 6b 6c 6d 6e 6f | `abcdefg hijklmno
0070 | 70 71 72 73 74 75 76 77  78 79 7a 7b 7c 7d 7e 7f | pqrstuvw xyz{|}~.
0080 | 80 81 82 83 84 85 86 87  88 89 8a 8b 8c 8d 8e 8f | ........ ........
0090 | 90 91 92 93 94 95 96 97  98 99 9a 9b 9c 9d 9e 9f | ........ ........
00a0 | a0 a1 a2 a3 a4 a5 a6 a7  a8 a9 aa ab ac ad ae af | ........ ........
00b0 | b0 b1 b2 b3 b4 b5 b6 b7  b8 b9 ba bb bc bd be bf | ........ ........
00c0 | c0 c1 c2 c3 c4 c5 c6 c7  c8 c9 ca cb cc cd ce cf | ........ ........
00d0 | d0 d1 d2 d3 d4 d5 d6 d7  d8 d9 da db dc dd de df | ........ ........
00e0 | e0 e1 e2 e3 e4 e5 e6 e7  e8 e9 ea eb ec ed ee ef | ........ ........
00f0 | f0 f1 f2 f3 f4 f5 f6 f7  f8 f9 fa fb fc fd fe ff | ........ ........

+hf