malloc - What is aligned memory allocation?

Question

I also want to know whether glibc malloc() does this.

Answer 1

Suppose that you have the structure.

struct S {
    short a;
    int b;
    char c, d;
};

Without alignment, it would be laid out in memory like this (assuming a 32-bit architecture):

 0 1 2 3 4 5 6 7
|a|a|b|b|b|b|c|d|  bytes
|       |       |  words

The problem is that on some CPU architectures, the instruction to load a 4-byte integer from memory only works on word boundaries. So your program would have to fetch each half of b with separate instructions.

But if the memory was laid out as:

 0 1 2 3 4 5 6 7 8 9 A B
|a|a| | |b|b|b|b|c|d| | |
|       |       |       |

Then access to b becomes straightforward. (The disadvantage is that more memory is required, because of the padding bytes.)

Different data types have different alignment requirements. It's common for char to be 1-byte aligned, short to be 2-byte aligned, and 4-byte types (int, float, and pointers on 32-bit systems) to be 4-byte aligned.

malloc is required by the C standard to return a pointer that's properly aligned for any data type.

glibc malloc on x86-64 returns 16-byte-aligned pointers.