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// ================================================================================================
// Basic JPEG decoder. Does no bounds checking on inputs.
//
// ref: https://en.wikipedia.org/wiki/JPEG#Syntax_and_structure
// ref: ITU-T T.81 (1992) — ISO/IEC 10918-1:1994
//
// Test files:
// $ ffmpeg -f lavfi -i testsrc=size=800x600:rate=1 -frames:v 1 test.jpg
//
// Changelog:
// ??/??/2026: Initial release
//
// License:
// Copyright (c) 2026 Hunter Kvalevog
//
// Permission to use, copy, modify, and/or distribute this software for any
// purpose with or without fee is hereby granted.
//
// THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
// WITH REGARD TO THIS SOFTWARE.
// ================================================================================================
#include <assert.h>
#include <stdbool.h>
#include <stdint.h>
#include <stdio.h>
#include <stdlib.h>
// Marker codes - Table B.1
enum
{
MC_SOF0 = 0xC0,
MC_SOF1 = 0xC1,
MC_SOF2 = 0xC2,
MC_SOF3 = 0xC3,
MC_SOF5 = 0xC5,
MC_SOF6 = 0xC6,
MC_SOF7 = 0xC7,
MC_SOF8 = 0xC8,
MC_SOF9 = 0xC9,
MC_SOFA = 0xCA,
MC_SOFB = 0xCB,
MC_SOFC = 0xCC,
MC_SOFD = 0xCD,
MC_SOFE = 0xCE,
MC_DHT = 0xC4,
MC_DAC = 0xCC,
MC_RST0 = 0xD0,
MC_RST1 = 0xD1,
MC_RST2 = 0xD2,
MC_RST3 = 0xD3,
MC_RST4 = 0xD4,
MC_RST5 = 0xD5,
MC_RST6 = 0xD6,
MC_RST7 = 0xD7,
MC_SOI = 0xD8,
MC_EOI = 0xD9,
MC_SOS = 0xDA,
MC_DQT = 0xDB,
MC_DNL = 0xDC,
MC_DRI = 0xFF,
MC_DHP = 0xDE,
MC_EXP = 0xDF,
MC_APP0 = 0xE0,
MC_APP1 = 0xE1,
MC_APP2 = 0xE2,
MC_APP3 = 0xE3,
MC_APP4 = 0xE4,
MC_APP5 = 0xE5,
MC_APP6 = 0xE6,
MC_APP7 = 0xE7,
MC_APP8 = 0xE8,
MC_APP9 = 0xE9,
MC_APPA = 0xEA,
MC_APPB = 0xEB,
MC_APPC = 0xEC,
MC_APPD = 0xED,
MC_APPE = 0xEE,
MC_APPF = 0xEF,
MC_COM = 0xFE,
};
int main(int argc, const char **argv)
{
uint8_t *bbuf = 0;
size_t blen = 0;
{
if (argc != 2) {
printf("Supply a file\n");
return 0;
}
FILE *fp = fopen(argv[1], "rb");
if (!fp) {
perror("Couldn't open file");
}
fseek(fp, 0, SEEK_END);
blen = ftell(fp);
fseek(fp, 0, SEEK_SET);
bbuf = malloc(blen);
fread(bbuf, 1, blen, fp);
fclose(fp);
}
const uint8_t *pbuf = bbuf;
// File should start with SOI
assert(pbuf[0] == 0xFF && pbuf[1] == MC_SOI && "not a jpeg");
pbuf += 2;
#define READ_LEN() ((pbuf[0] << 8) | pbuf[1])
while (true) {
uint8_t mc0 = pbuf[0];
uint8_t mc1 = pbuf[1];
pbuf += 2;
assert(mc0 == 0xFF);
switch (mc1) {
case MC_APP0:
case MC_APP1:
case MC_APP2:
case MC_APP3:
case MC_APP4:
case MC_APP5:
case MC_APP6:
case MC_APP7:
case MC_APP8:
case MC_APP9:
case MC_APPA:
case MC_APPB:
case MC_APPC:
case MC_APPD:
case MC_APPE:
case MC_APPF: {
printf("MC_APP%d\n", mc1 & 0xF);
uint16_t len = READ_LEN();
pbuf += len;
} break;
case MC_COM: {
printf("MC_COM\n");
uint16_t len = READ_LEN();
pbuf += len;
} break;
case MC_DHT: {
printf("MC_DHT\n");
uint16_t len = READ_LEN();
pbuf += len;
} break;
case MC_DQT: {
printf("MC_DQT\n");
uint16_t len = READ_LEN();
pbuf += len;
} break;
case MC_SOF0: {
printf("MC_SOF0 (Baseline DCT)\n");
uint16_t len = READ_LEN();
pbuf += len;
} break;
case MC_SOF2: {
printf("MC_SOF2 (Progressive DCT)\n");
uint16_t len = READ_LEN();
pbuf += len;
} break;
case MC_SOS: {
// Section B.2.3
printf("MC_SOS\n");
uint16_t len = READ_LEN();
pbuf += len;
} break;
default: {
printf("Got unknown marker code 0x%X\n", mc1);
exit(1);
} break;
}
}
}
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