posix_memalign 与 malloc 对比

1. 原因原理

编程中的类型对齐问题主要是处于性能考虑，如果不做对齐，那么单个数据元素的访问很容易跨在多个时钟周期上，从而导致性能下降。
内建数据类型的对齐，是由编译器和C语言库的API实现中自动完成的，这对于用户是透明的，比如常用的 malloc。
有时候需要使用大块数据，同时需要提高性能，可能需要 64 bytes对齐，按照机器的cache line对齐等，这时候可以使用 posix_memalign：

Function: int posix_memalign (void **memptr, size_t alignment, size_t size)

Preliminary: | MT-Safe | AS-Unsafe lock | AC-Unsafe lock fd mem | See POSIX Safety Concepts.

The posix_memalign function is similar to the memalign function in that it returns a buffer of size bytes aligned to a multiple of alignment. But it adds one requirement to the parameter alignment: the value must be a power of two multiple of sizeof (void *).

If the function succeeds in allocation memory a pointer to the allocated memory is returned in *memptr and the return value is zero. Otherwise the function returns an error value indicating the problem. The possible error values returned are:

ENOMEM

There was insufficient memory available to satisfy the request.

EINVAL

alignment is not a power of two multiple of sizeof (void *).

This function was introduced in POSIX 1003.1d. Although this function is superseded by aligned_alloc, it is more portable to older POSIX systems that do not support ISO C11.

更多内存对齐的API的内容可参考：

Aligned Memory Blocks (The GNU C Library)

2. 测试代码

#include 
#include 

int main()
{
//      posix_memalign, (, void **, p,. size_t, alignment,. size_t, size. )
        float* p = nullptr;
        int res;

        for(int i=0; i<10; i++)
        {
                res = 99;
                res = posix_memalign((void**)&p, 64, 100*sizeof(float));
                p[99] = 0.333f + i;
                printf("res=%d, p[99] = %7.4f , p = %p\n", res, p[99], p);

//              free(p);
        }

        for(int i=0; i<10; i++)
        {
                p = (float*)malloc(100*sizeof(float));
                p[99] = 10.333f + i;
                printf("res=%d, p[99] = %7.4f , p = %p\n", p[99], p);

//                free(p);
        }

        return 0;
}

Makefile:

EXE := hello_posix_memalign

$(EXE):

.PHONY: clean
clean:
        -rm -rf $(EXE)

3. 运行效果