How to Get the BIOS UUID

biosuuid.cpp

#include <windows.h>
#include <sysinfoapi.h>

#include "biosuuid.h"


/*!
  SMBIOS Structure header as described at
  https://www.dmtf.org/sites/default/files/standards/documents/DSP0134_3.3.0.pdf 
  (para 6.1.2)
*/
struct dmi_header
{
  BYTE type;
  BYTE length;
  WORD handle;
  BYTE data[1];
};

/*
  Structure needed to get the SMBIOS table using GetSystemFirmwareTable API.
  see https://docs.microsoft.com/en-us/windows/win32/api/sysinfoapi/nf-sysinfoapi-getsystemfirmwaretable
*/
struct RawSMBIOSData {
  BYTE  Used20CallingMethod;
  BYTE  SMBIOSMajorVersion;
  BYTE  SMBIOSMinorVersion;
  BYTE  DmiRevision;
  DWORD  Length;
  BYTE  SMBIOSTableData[1];
};

/*
  Get BIOS UUID in a 16 byte array.

  Return true if successful, 0 otherwise
*/
bool biosuuid (unsigned char *uuid)
{
  bool result = false;
  
  RawSMBIOSData *smb=nullptr;
  BYTE *data;

  DWORD size = 0;

  // Get size of BIOS table
  size = GetSystemFirmwareTable ('RSMB', 0, smb, size);
  smb = (RawSMBIOSData*)malloc (size);

  // Get BIOS table
  GetSystemFirmwareTable ('RSMB', 0, smb, size);

  //Go through BIOS structures
  data = smb->SMBIOSTableData;
  while (data < smb->SMBIOSTableData + smb->Length)
  {
    BYTE *next;
    dmi_header *h = (dmi_header*)data;

    if (h->length < 4)
      break;

    //Search for System Information structure with type 0x01 (see para 7.2)
    if (h->type == 0x01 && h->length >= 0x19)
    {
      data += 0x08; //UUID is at offset 0x08

      // check if there is a valid UUID (not all 0x00 or all 0xff)
      bool all_zero = true, all_one = true;
      for (int i = 0; i < 16 && (all_zero || all_one); i++)
      {
        if (data[i] != 0x00) all_zero = false;
        if (data[i] != 0xFF) all_one = false;
      }
      if (!all_zero && !all_one)
      {
        /* As off version 2.6 of the SMBIOS specification, the first 3 fields
        of the UUID are supposed to be encoded on little-endian. (para 7.2.1) */
        *uuid++ = data[3];
        *uuid++ = data[2];
        *uuid++ = data[1];
        *uuid++ = data[0];
        *uuid++ = data[5];
        *uuid++ = data[4];
        *uuid++ = data[7];
        *uuid++ = data[6];
        for (int i = 8; i < 16; i++)
          *uuid++ = data[i];

        result = true;
      }
      break;
    }

    //skip over formatted area
    next = data + h->length;

    //skip over unformatted area of the structure (marker is 0000h)
    while (next < smb->SMBIOSTableData + smb->Length && (next[0] != 0 || next[1] != 0))
      next++;
    next += 2;

    data = next;
  }
  free (smb);
  return result;
}

biosuuid.h

// Retrieve BIOS UUUID. Return true if successful
bool biosuuid (unsigned char *uuid);

main.cpp

/*
  Test program for BIOSUUID function
*/

#include <windows.h>
#include <stdio.h>

#include "biosuuid.h"

char *hexbyte (BYTE b, char *ptr)
{
  static const char *digits{ "0123456789ABCDEF" };
  *ptr++ = digits[b >> 4];
  *ptr++ = digits[b & 0x0f];
  return ptr;
}

int main (int argc, char **argv)
{
  BYTE uuid[16];
  if (biosuuid (uuid))
  {
    char uuid_str[40];
    char *ptr = uuid_str;
    int i;
    for (i = 0; i < 4; i++)
      ptr = hexbyte (uuid[i], ptr);
    *ptr++ = '-';
    for (; i< 6; i++)
      ptr = hexbyte (uuid[i], ptr);
    *ptr++ = '-';
    for (; i < 8; i++)
      ptr = hexbyte (uuid[i], ptr);
    *ptr++ = '-';
    for (; i < 10; i++)
      ptr = hexbyte (uuid[i], ptr);
    *ptr++ = '-';
    for (; i < 16; i++)
      ptr = hexbyte (uuid[i], ptr);
    *ptr++ = 0;

    printf ("BIOS UUID: %s\n", uuid_str);
    return 0;
  }
  fprintf (stderr, "Failed to get BIOS UUID\n");
  return 1;
}

Thanks for providing such a good reference!
I just reworked this to make more readable for C++ guys, may be that would help someone.
Code: https://gist.github.com/vadimpiven/618b720324e9f54c01075fcb8675f2c4

Good stuff! Thanks for letting me now.