向上取整字节对齐

向上取整字节对齐宏实现方法

/**

• MEM_ALIGNMENT: should be set to the alignment of the CPU
• 4 byte alignment -> #define MEM_ALIGNMENT 4
• 2 byte alignment -> #define MEM_ALIGNMENT 2
  */
  #define MEM_ALIGNMENT 1
  #define LWIP_MEM_ALIGN_SIZE(size) (((size) + MEM_ALIGNMENT - 1) & ~(MEM_ALIGNMENT-1))

一个简单的对变量size进行操作的宏， 它的作用是将size向上对MEM_ALIGNMENT取整， 即保证size是内存对齐字节数的整数倍。