Question

I mean to give pointer to variable in struct\memory and then give it to some function and it’ll give me pointer to the start of the struct\memory.
There is a function that does it?

To understand this question:

char* ptr=((char*)malloc(12))+3;
//MemStart(ptr) == ptr-3

MemStart‘ll be the function that’ll give that result in the code.

Answer 1

Unless there is something specific to that structure’s representation in memory (such as it always starts with the bytes 0xff, 0xff), there is no way to determine where a particular structure or array starts. Modern architectures are Von Neuman machines, meaning memory has no inherent meaning.

Note that many architectures have alignment issues or optimizations, meaning structures or arrays may need to start on 16-bit, 32-bit or 64-bit word boundaries, but these are architecture specific.

[Edit: Added the following]

The library may introduce guard bytes at the start or end of the allocated block of memory or fill memory with known constants to look for buffer overflows or errant pointers. However, these are usually omitted in release mode and, even if they are present, may also be valid data.

You could inspect the memory allocation table for dynamically allocated memory but the array/struct could be allocated on the stack rather than the heap. Also, what happens if you have a structure within an array? Which value does it return? If you want to restrict it to just dynamically allocated memory where you want the start of whatever memory was allocated only, Basile has a great answer.