Question

When working with char buffers in C, sometimes it would be useful and more efficient to able to work with int-sized chunks of data at a time. To do this I can cast my char * to an int * and use that pointer instead. However I’m not entirely confident that this works the way I think it does.

For example, suppose I have char *data, does *(int32_t *)data = -1 always overwrite the bytes data[0], data[1], data[2] and data[3] and no other bytes?

Answer 1

Expanding on my comment.

There are two major issues here:

• It violates strict-aliasing.
• You might break alignment.

---

Violating strict-aliasing is technically undefined behavior. You are allowed to alias any datatype with char*, but not the other way around.

You can get around the issue with f[no-]strict-aliasing on GCC.

---

The other issue is alignment. The char pointer might not be properly aligned. Accessing misaligned data may lead to performance degradation or even a hardware exception if the hardware doesn’t support misaligned access.

---

If performance isn’t an issue, the full-proof way is to memcpy() to an int array buffer.

Once these two issues are resolved, your example with:

*(int32_t *)data = -1

overwriting data[0], data[1], data[2], and data[3] should work as expected if sizeof(int32_t) == 4. Just pay attention to the endianness…