为什么当元素大小大于实际值大小时，`g_array_append_val`会出现段错误？

Question

My program segfaulted in a code that looks like the following:

user_t user;
users = g_array_sized_new(FALSE, TRUE, sizeof(*user), nb_results);
g_array_append_val(users, user);

At line 3, instead of adding user by value I add it by pointer. This will segfault sometime. The correct does initialize the GArray to host pointers:

users = g_array_sized_new(FALSE, TRUE, sizeof(user_t *), nb_results);
g_array_append_val(users, user);

I don't understand why the first version of this code does segfault since sizeof(*user) is bigger than sizeof(user_t *) .

Answer 1

For simplicity's sake, lets say that sizeof *user is six bytes, and a pointer is four bytes. That means an array of two users have 12 bytes, enough space for three pointers. But when you get an element from that array it will get a six-byte element, which will contain one and a half pointer.

When you try to store this value into a pointer, you will get half of the first pointer and half of the other pointer. That will not be valid and when you dereference it you will get undefined behavior .