Question

My program/process exited abnormally? I doubt something is wrong with the following code:

There are two typedef TYPE1, TYPE2. (TYPE1’s size is bigger than TYPE2)

TYPE1 var1;
TYPE2 var2;
....
....var2 has been assigned.
....
memset(&var1, 0, sizeof(var1));
memcpy(&var1, (TYPE1 *)&var2, sizeof(TYPE1)）;
printf("....");

1. Cast var2 may cause accessing the illegal memory which may lead to segmentation fault? I thought it may mess up the data following the var2 in memory, but cannot make segmentation fault? I found my program exit here, since the following printf() did not print anything.

2. Cast will ruin the data following var2 in memory even if it is only read (var2 is just source of memcpy().

3. what happens if I change it like memcpy(&var1, (TYPE1 *)&var2, sizeof(TYPE2)）;

Thanks.

Answer 1

So first: you don’t need to cast &var2 to TYPE1* when using memcpy. memcpy expects void*, and any pointer type can be cast to void*.

The problem here is, when TYPE1 is bigger than TYPE2 and you are copying the data from var2 to var1 using sizeof(TYPE1) as the size, memcpy will eventually read from beyond the memory occupied by var2. As a consequence, you are accessing memory you are not allowed to access. Hence, a segmentation fault is the best you could have gotten there, since that error might also have be gone unnoticed for quite some time until it really could have caused trouble (undefined behavior is the key phrase here).

When you copy only sizeof(TYPE2) bytes from var2 to var1, you are ok in terms of memory access (as long as TYPE1 is really bigger than TYPE2).