VBCC and pointer dereferencing

[PR77]

I'm trying to finalise the code for my 68000 CPU Relocator with FLASH kickstart support (https://github.com/PR77/68000_Relocator_FLASH_Kickstart) however I am stuck on some pointer arithmetic. Here is a simplified version of the code;

fileHandle = Open("test.txt", MODE_OLDFILE);
if (0L != fileHandle)
{                        
  APTR memoryHandle = AllocMem(0x80000, 0);
  if (memoryHandle)
  {
    ULONG totalBytesReadFromFile = 0;
    UWORD *wordToWrite;
                                
    totalBytesReadFromFile = Read(fileHandle, memoryHandle, 0x80000);
    wordToWrite = (UWORD *)memoryHandle

    ….
  }
}

"wordToWrite" should be a pointer to a UWORD however it seems to default to a ULONG boundary. So;

wordToWrite[0] dereferences the LSB of the first ULONG. wordToWrite[1] dereferences the LSB of the second ULONG, and so on.

I put some details in the attached picture. The test.txt file is simply ASCII "1234567890".

The output in the image is generated by the following

printf("Current index: %d, wordInMem: %x, wordToWrite: %x\n", currentWordWritten, (UWORD)memoryHandle[currentWordWritten], (UWORD)wordToWrite[currentWordWritten]);

Any ideas? CPU is 68000.

[a/b]

memoryHandle is a longword pointer, wordToWrite is a word pointer. They both point to the same location (to a "1" in that ASCII string).
When you dereference them via array indexing, you get:
memoryHandle[0] = "1234" (0x31323334)
wordToWrite[0] = "12" (0x3132)
Then, you cast both values (not pointers, values!) to a word, thus cutting off the high-word of "1234", resulting in "34".
Now, ((UWORD*) memoryHandle)[0] would give you "12". That's a pointer cast, not a value cast.

[PR77]

Originally Posted by a/b

memoryHandle is a longword pointer, wordToWrite is a word pointer. They both point to the same location (to a "1" in that ASCII string).
When you dereference them via array indexing, you get:
memoryHandle[0] = "1234" (0x31323334)
wordToWrite[0] = "12" (0x3132)
Then, you cast both values (not pointers, values!) to a word, thus cutting off the high-word of "1234", resulting in "34".
Now, ((UWORD*) memoryHandle)[0] would give you "12". That's a pointer cast, not a value cast.

Thanks, it now works!

[Niklas]

Originally Posted by PR77

Thanks, it now works!

Hi PR77. This is slightly off-topic, but there's a channel, The Lab, on Retro Man Cave's discord (https://discordapp.com/channels/4313...45410385592321) that you might find interesting, where you could ask these kind of question and get answers quickly.

[Samurai_Crow]

Originally Posted by Niklas

Hi PR77. This is slightly off-topic, but there's a channel, The Lab, on Retro Man Cave's discord (https://discordapp.com/channels/4313...45410385592321) that you might find interesting, where you could ask these kind of question and get answers quickly.

You need to be invited to use a Discord channel.

[Niklas]

Originally Posted by Samurai_Crow

You need to be invited to use a Discord channel.

Ah, I see. Here's the invite link: https://discord.gg/7qYtGcz

[Samurai_Crow]

Thanks.

[PR77]

Hey thanks... Just registered...