前言
记录一下在学习使用axi-uart官方example过程的一些问题和困惑
问题和困惑
RecvBuffer是一个数组，为什么可以直接用数组名和变量RecvCount相加，就可以给XUartLite_Recv第二个参数传递了？
点击查看代码
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/**
*
* @file xuartlite_polled_example.c
*
* This file contains a design example using the UartLite driver (XUartLite) and
* hardware device using the polled mode.
*
* @note
*
* The user must provide a physical loopback such that data which is
* transmitted will be received.
*
* MODIFICATION HISTORY:
* <pre>
* Ver Who Date Changes
* ----- ---- -------- -----------------------------------------------
* 1.00a jhl 02/13/02 First release
* 1.00a sv 06/13/05 Minor changes to comply to Doxygen and coding guidelines
* 2.00a ktn 10/20/09 Updated this example to wait for valid data in receive
* fifo instead of Tx fifo empty to update receive buffer
* and minor changes as per coding guidelines.
* 3.2 ms 01/23/17 Added xil_printf statement in main function to
* ensure that "Successfully ran" and "Failed" strings
* are available in all examples. This is a fix for
* CR-965028.
* </pre>
******************************************************************************//***************************** Include Files *********************************/#include "xparameters.h"
#include "xstatus.h"
#include "xuartlite.h"
#include "xil_printf.h"/************************** Constant Definitions *****************************//** The following constants map to the XPAR parameters created in the* xparameters.h file. They are defined here such that a user can easily* change all the needed parameters in one place.*/
#define UARTLITE_DEVICE_ID XPAR_UARTLITE_0_DEVICE_ID/** The following constant controls the length of the buffers to be sent* and received with the UartLite, this constant must be 16 bytes or less since* this is a single threaded non-interrupt driven example such that the* entire buffer will fit into the transmit and receive FIFOs of the UartLite.*/
#define TEST_BUFFER_SIZE 16/**************************** Type Definitions *******************************//***************** Macros (Inline Functions) Definitions *********************//************************** Function Prototypes ******************************/int UartLitePolledExample(u16 DeviceId);/************************** Variable Definitions *****************************/XUartLite UartLite; /* Instance of the UartLite Device *//** The following buffers are used in this example to send and receive data* with the UartLite.*/
u8 SendBuffer[TEST_BUFFER_SIZE]; /* Buffer for Transmitting Data */
u8 RecvBuffer[TEST_BUFFER_SIZE]; /* Buffer for Receiving Data *//*****************************************************************************/
/**
*
* Main function to call the Uartlite polled example.
*
* @param None.
*
* @return XST_SUCCESS if successful, otherwise XST_FAILURE.
*
* @note None.
*
******************************************************************************/
int main(void)
{int Status;/** Run the UartLite polled example, specify the Device ID that is* generated in xparameters.h*/Status = UartLitePolledExample(UARTLITE_DEVICE_ID);if (Status != XST_SUCCESS) {xil_printf("Uartlite polled Example Failed\r\n");return XST_FAILURE;}xil_printf("Successfully ran Uartlite polled Example\r\n");return XST_SUCCESS;}/****************************************************************************/
/**
* This function does a minimal test on the UartLite device and driver as a
* design example. The purpose of this function is to illustrate
* how to use the XUartLite component.
*
* This function sends data and expects to receive the data thru the UartLite
* such that a physical loopback must be done with the transmit and receive
* signals of the UartLite.
*
* This function polls the UartLite and does not require the use of interrupts.
*
* @param DeviceId is the Device ID of the UartLite and is the
* XPAR_<uartlite_instance>_DEVICE_ID value from xparameters.h.
*
* @return XST_SUCCESS if successful, XST_FAILURE if unsuccessful.
*
*
* @note
*
* This function calls the UartLite driver functions in a blocking mode such that
* if the transmit data does not loopback to the receive, this function may
* not return.
*
****************************************************************************/
int UartLitePolledExample(u16 DeviceId)
{int Status;unsigned int SentCount;unsigned int ReceivedCount = 0;int Index;/** Initialize the UartLite driver so that it is ready to use.*/Status = XUartLite_Initialize(&UartLite, DeviceId);if (Status != XST_SUCCESS) {return XST_FAILURE;}/** Perform a self-test to ensure that the hardware was built correctly.*/Status = XUartLite_SelfTest(&UartLite);if (Status != XST_SUCCESS) {return XST_FAILURE;}/** Initialize the send buffer bytes with a pattern to send and the* the receive buffer bytes to zero.*/for (Index = 0; Index < TEST_BUFFER_SIZE; Index++) {SendBuffer[Index] = Index;RecvBuffer[Index] = 0;}/** Send the buffer through the UartLite waiting til the data can be sent* (block), if the specified number of bytes was not sent successfully,* then an error occurred.*/SentCount = XUartLite_Send(&UartLite, SendBuffer, TEST_BUFFER_SIZE);if (SentCount != TEST_BUFFER_SIZE) {return XST_FAILURE;}/** Receive the number of bytes which is transfered.* Data may be received in fifo with some delay hence we continuously* check the receive fifo for valid data and update the receive buffer* accordingly.*/while (1) {ReceivedCount += XUartLite_Recv(&UartLite,RecvBuffer + ReceivedCount,TEST_BUFFER_SIZE - ReceivedCount);if (ReceivedCount == TEST_BUFFER_SIZE) {break;}}/** Check the receive buffer data against the send buffer and verify the* data was correctly received.*/for (Index = 0; Index < TEST_BUFFER_SIZE; Index++) {if (SendBuffer[Index] != RecvBuffer[Index]) {return XST_FAILURE;}}return XST_SUCCESS;
}解释
核心原理：C语言中数组名会退化成指针
- 数组名 → 指针
当你把 RecvBuffer 用在表达式里（除了 sizeof 和 & 等少数场合），它会自动转换成一个指向首元素的指针，类型是 u8（即 unsigned char）。
也就是说：
点击查看代码
RecvBuffer // 等价于 &RecvBuffer[0]，类型 u8*
- 指针 + 整数 = 偏移后的指针
C语言允许指针进行加减整数运算，实际效果是：
点击查看代码
RecvBuffer + RecvCount // 等价于 &RecvBuffer[RecvCount]
它给出的是数组中第 RecvCount 个元素的地址，类型仍然是 u8*。因为 u8 是1字节大小，所以这里的“偏移”刚好就是 RecvCount 个字节。
- 函数签名也对得上
XUartLite_Recv 的第二个参数声明为 u8 DataBufferPtr，它需要的就是一个指向 u8 的指针。
RecvBuffer + RecvCount 的类型正是 u8，完美匹配。
一句话总结
RecvBuffer + RecvCount 就是 “数组首地址往后偏移 RecvCount 个字节的指针”，每次调用 Recv 时，新接收的数据就从这个位置开始存放，从而逐步填满整个接收缓冲区。