[Solved] why using sizeof in malloc?

Question

sizeof doesn’t return the size of the memory block that was allocated (C does NOT have a standard way to get that information); it returns the size of the operand based on the operand’s type. Since your pointer is of type struct A*, the sizeof operand is of type struct A, so sizeof always returns 8.

So, even if you allocate 1 byte for a 10000 byte structure, you will still see sizeof return 10000.

If you don’t allocate enough memory for that object (e.g. because sizeof(int) < sizeof(struct A)) but you try to use the object anyway, you’ll encounter undefined behaviour – your program is no longer well defined and anything could happen (nothing, crashing, memory corruption, hackers owning your computer).

Accepted Answer

sizeof doesn’t return the size of the memory block that was allocated (C does NOT have a standard way to get that information); it returns the size of the operand based on the operand’s type. Since your pointer is of type struct A*, the sizeof operand is of type struct A, so sizeof always returns 8.

So, even if you allocate 1 byte for a 10000 byte structure, you will still see sizeof return 10000.

If you don’t allocate enough memory for that object (e.g. because sizeof(int) < sizeof(struct A)) but you try to use the object anyway, you’ll encounter undefined behaviour – your program is no longer well defined and anything could happen (nothing, crashing, memory corruption, hackers owning your computer).