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.

Answer 2

Alignment requirements specify what address offsets can be assigned to what types. This is completely implementation-dependent, but is generally based on word size. For instance, some 32-bit architectures require all int variables start on a multiple of four. On some architectures, alignment requirements are absolute. On others (e.g. x86) flouting them only comes with a performance penalty.

malloc is required to return an address suitable for any alignment requirement. In other words, the returned address can be assigned to a pointer of any type. From C99 §7.20.3 (Memory management functions):

The pointer returned if the allocation succeeds is suitably aligned so that it may be assigned to a pointer to any type of object and then used to access such an object or an array of such objects in the space allocated (until the space is explicitly deallocated).