On Sunday, 4 December 2022 at 16:33:35 UTC, rempas wrote:
struct MemoryBlock {
char* ptr;
ulong length;
}
(MemoryBlock.sizeof is 16 on my 64-bit system).
void* ptr = cast(void*)0x7a7;
void* right() {
return cast(MemoryBlock*)(ptr + MemoryBlock.sizeof); // Cast
the whole expression between paranthesis. Got the right value!
}
The above adds 16 bytes to ptr.
void* wrong() {
return cast(MemoryBlock*)ptr + MemoryBlock.sizeof; // First
cast the `ptr` variable and then add the number. Got a wronge
value...
}
The above adds 16 * MemoryBlock.sizeof bytes (16 * 16) to ptr,
because ptr is cast first. Should be `+ 1` to be equivalent.
https://dlang.org/spec/expression.html#pointer_arithmetic
"the resulting value is the pointer plus (or minus) the second
operand **multiplied by the size of the type pointed to by the
first operand**."
char* return_address_wrong() {
MemoryBlock* local_ptr = cast(MemoryBlock*)ptr;
return cast(char*)(local_ptr + MemoryBlock.sizeof); // Casted
the whole expression. BUT GOT THE WRONG VALUE!!!! Why???
}
Because you are adding to a pointer that points to a 16-byte
block, rather than a void* which points to a single byte.
char* return_address_right() {
MemoryBlock* local_ptr = cast(MemoryBlock*)ptr;
return cast(char*)local_ptr + MemoryBlock.sizeof; // Now I
first casted the `local_ptr` variable and then added the number
but this time this gave me the right value....
}
The casted pointer points to a single byte.
