quick raw mbr partition editor

mbrpart.c

#define _CRT_SECURE_NO_WARNINGS
#define _FILE_OFFSET_BITS 64
#include <stdlib.h>
#include <stdint.h>
#include <string.h>
#include <stdbool.h>
#include <stdio.h>

enum { kSectorSize = 512 };

void chs(uint8_t* out, uint32_t lba)
{
    if (lba >= 1024 * 255 * 63)
    {
        memset(out, 0xFF, 3);
        return;
    }
    enum { hpc = 16, spt = 63 };
    uint32_t c = lba / (hpc * spt);
    uint32_t h = (lba / spt) % hpc;
    uint32_t s = lba % spt + 1;
    out[0] = h;
    out[1] = s | (c >> 8 << 6);
    out[2] = c;
}

int main(int argc, char** argv)
{
    if(argc < 5)
    {
        printf("usage: mbrpart <file> <partition 0-3> <a|b|e|s|p> <value>");
        return 1;
    }
    FILE* fp = fopen(argv[1], "rwb");
    uint8_t bs[512];
    fread(bs, 512, 1, fp);
    rewind(fp);
    int vol = atoi(argv[2]);
    int offset = 0x01BE + vol * 16;
    int ishex = argv[4][0] == 'x' ? 1 : 0;
    int val = strtol(ishex ? argv[4] + 1 : argv[4], NULL, ishex ? 16 : 10);
    switch(*(argv[3]))
    {
    case 'a': // active
        *(uint8_t*)(bs + offset + 0) = val ? 0x80 : 0;
        break;
    case 'b': // begin
        *(uint32_t*)(bs + offset + 8) = val;
        chs(bs + offset + 1, val);
        break;
    case 'e': // end
        chs(bs + offset + 5, val);
        *(uint32_t*)(bs + offset + 12) = val - *(uint32_t*)(bs + offset + 8);
        break;
    case 's': // size
        chs(bs + offset + 5, val + *(uint32_t*)(bs + offset + 8));
        *(uint32_t*)(bs + offset + 12) = val;
        break;
    case 'p': // platform
        *(uint8_t*)(bs + offset + 1) = val;
        break;
    }
    fwrite(bs, 512, 1, fp);
    fclose(fp);
    return 0;
}