此宏功能有什么作用？

Question

This fucntion:

FIFO_ELEM_AT(p_fifo, index) ((uint8_t*) ((uint8_t*) p_fifo->elem_array) + (p_fifo->elem_size) * (index))

and "p_fifo" was defined as:

typedef struct
{
    void* elem_array;
    uint32_t elem_size;
    uint32_t array_len;
    uint32_t head;
    uint32_t tail;
    fifo_memcpy memcpy_fptr; /* must be a valid function or NULL */
  } fifo_t;

Overall it's trying to do something to a static fifo data structure. I can't really tell what you can possibly achieve by the calculation described in the function. Please help!

Answer 1

You should read the documentation for the code you are looking at.
Otherwise you are stuck with guessing. You seem to ask for help with guessing.
Here are my guesses:

• The mentioned struct type describes all the information for handling an instance of a special data construct.
• Because of the name, it is probably a FIFO aka "ringbuffer", aka "queue" (as commented by MM)
• It needs an accompanying memory, to be used as the actual FIFO.
• The memory to use is referenced by a pointer to it, "elem_array".
• "elem_size" is the size of one element in the FIFO
• "head" references the next element to be returned from FIFO
• "tail" references the last element added to the FIFO
• or the other way around
• "memcpy_fptr" references a potentially optimized function to move elements to and from the FIFO
• the quoted macro provides access to the element in the FIFO at a given index
• in the form of a pointer to uint8_t; instead of a pointer-to-type-of-element
• the macro gets the address to the FIFO, does some pointer arithmetic (including the size of elements and the given index) and thereby evaluates to the result: uint8_t-pointer to indexed element

On top of my guesses, here are my opinion-based recommendations:

• when using the result (pointer to uint8_t), cast it to appropriate type
• when using the macro, make sure that you give an index to an existing element, at least an element which has been added, with a meaningful value
• when using the macro, give an index to an element which has not been returned from the FIFO yet
• do not use the macro, it seems to go against the idea of a FIFO,
  with the possible exception to get info on the content of the FIFO for debugging purposes