STM32F103固件库在建立tcp客户端，在数据接收回调函数将数据一个一个写入环形缓冲区，并通过串口发送出去，代码

可以参考以下代码实现：

#include "stm32f10x.h"
#include "stm32f10x_eth.h"
#include "stm32f10x_dma.h"
#include "lwip/tcp.h"

#define ETH_DMA_BUF_SIZE        1536
#define ETH_RX_BUF_SIZE         ETH_DMA_BUF_SIZE
#define ETH_TX_BUF_SIZE         ETH_DMA_BUF_SIZE

#define RX_RING_BUF_SIZE        2048

static uint8_t eth_rx_buf[ETH_RX_BUF_SIZE];
static uint8_t eth_tx_buf[ETH_TX_BUF_SIZE];
static uint8_t rx_ring_buf[RX_RING_BUF_SIZE];

static struct tcp_pcb *tcp_client_pcb = NULL;
static struct pbuf *tx_pbuf = NULL;

static void tcp_client_recv_callback(void *arg, struct tcp_pcb *tpcb, struct pbuf *p, err_t err)
{
    uint16_t i;
    uint16_t len;
    uint16_t ring_buf_free_size;
    uint16_t pbuf_read_len;
    uint8_t *pbuf_data_ptr;

    if ((p == NULL) || (err != ERR_OK))
    {
        return;
    }

    pbuf_data_ptr = (uint8_t *)p->payload;
    pbuf_read_len = p->len;

    ring_buf_free_size = RX_RING_BUF_SIZE - ring_buf_tail;

    if (pbuf_read_len > ring_buf_free_size)
    {
        pbuf_read_len = ring_buf_free_size;
    }

    for (i = 0; i < pbuf_read_len; i++)
    {
        rx_ring_buf[ring_buf_tail] = pbuf_data_ptr[i];
        ring_buf_tail = (ring_buf_tail + 1) % RX_RING_BUF_SIZE;
    }

    tcp_recved(tpcb, p->tot_len);
    pbuf_free(p);
}

static void tcp_client_sent_callback(void *arg, struct tcp_pcb *tpcb, uint16_t len)
{
    pbuf_free(tx_pbuf);
    tx_pbuf = NULL;
}

static err_t tcp_client_connect_callback(void *arg, struct tcp_pcb *tpcb, err_t err)
{
    tcp_client_pcb = tpcb;
    tcp_sent(tpcb, tcp_client_sent_callback);

    return ERR_OK;
}

static void tcp_client_error_callback(void *arg, err_t err)
{
    tcp_client_pcb = NULL;
}

void tcp_client_send_data(uint8_t *data, uint16_t len)
{
    if (tcp_client_pcb == NULL)
    {
        return;
    }

    tx_pbuf = pbuf_alloc(PBUF_TRANSPORT, len, PBUF_RAM);
    memcpy(tx_pbuf->payload, data, len);

    tcp_write(tcp_client_pcb, tx_pbuf->payload, tx_pbuf->len, TCP_WRITE_FLAG_COPY);
}

void tcp_client_init(void)
{
    struct ip_addr server_ip;
    err_t err;

    IP4_ADDR(&server_ip, 192, 168, 1, 100);

    tcp_client_pcb = tcp_new();
    tcp_arg(tcp_client_pcb, NULL);
    tcp_recv(tcp_client_pcb, tcp_client_recv_callback);
    tcp_err(tcp_client_pcb, tcp_client_error_callback);

    err = tcp_connect(tcp_client_pcb, &server_ip, 5000, tcp_client_connect_callback);

    if (err != ERR_OK)
    {
        tcp_client_pcb = NULL;
    }
}

void eth_init(void)
{
    ETH_InitTypeDef ETH_InitStructure;
    ETH_DMAInitTypeDef ETH_DMA_InitStructure;

    RCC_AHBPeriphClockCmd(RCC_AHBPeriph_ETH_MAC | RCC_AHBPeriph_ETH_MAC_Tx | RCC_AHBPeriph_ETH_MAC_Rx, ENABLE);
    RCC_APB2PeriphClockCmd(RCC_APB2Periph_SYSCFG, ENABLE);

    SYSCFG_ETH_MediaInterfaceConfig(SYSCFG_ETH_MediaInterface_RMII);

    ETH_DeInit();

    ETH_InitStructure.ETH_AutoNegotiation = ETH_AutoNegotiation_Enable;
    ETH_InitStructure.ETH_Speed = ETH_Speed_100M;
    ETH_InitStructure.ETH_Mode = ETH_Mode_FullDuplex;
    ETH_InitStructure.ETH_LoopbackMode = ETH_LoopbackMode_Disable;
    ETH_InitStructure.ETH_RxMode = ETH_RxMode_Interrupt;
    ETH_InitStructure.ETH_ChecksumOffload = ETH_ChecksumOffload_Enable;
    ETH_InitStructure.ETH_PhyInterface = ETH_PhyInterface_RMII;

    ETH_Init(&ETH_InitStructure);

    ETH_DMA_InitStructure.ETH_DMA_TxDescrBuf = (ETH_DMADescTypeDef*)eth_tx_buf;
    ETH_DMA_InitStructure.ETH_DMA_RxDescrBuf = (ETH_DMADescTypeDef*)eth_rx_buf;
    ETH_DMA_InitStructure.ETH_DMA_TxBufferSize = ETH_TX_BUF_SIZE;
    ETH_DMA_InitStructure.ETH_DMA_RxBufferSize = ETH_RX_BUF_SIZE;

    ETH_DMA_InitStructure.ETH_DMA_TxNumDescriptors = 3;
    ETH_DMA_InitStructure.ETH_DMA_RxNumDescriptors = 3;

    ETH_DMA_InitStructure.ETH_DMA_TxDescrSecondAddress = NULL;
    ETH_DMA_InitStructure.ETH_DMA_RxDescrSecondAddress = NULL;

    ETH_DMA_InitStructure.ETH_DMA_TxDescrThirdAddress = NULL;
    ETH_DMA_InitStructure.ETH_DMA_RxDescrThirdAddress = NULL;

    ETH_DMA_InitStructure.ETH_DMA_TxDescrRingLength = 3;
    ETH_DMA_InitStructure.ETH_DMA_RxDescrRingLength = 3;

    ETH_DMA_InitStructure.ETH_DMA_DefaultTxFlushTimeOut = 0x0000;
    ETH_DMA_InitStructure.ETH_DMA_DefaultRxFlushTimeOut = 0x0000;

    ETH_DMA_InitStructure.ETH_DMA_TxArbitration = ETH_DMA_TxArbitration_RoundRobin_RxBeforeTx;
    ETH_DMA_InitStructure.ETH_DMA_RxArbitration = ETH_DMA_RxArbitration_RoundRobin;

    ETH_DMA_InitStructure.ETH_DMA_TxDescrWb = ETH_DMA_TxDescrWb_Enable;
    ETH_DMA_InitStructure.ETH_DMA_RxDescrWb = ETH_DMA_RxDescrWb_Enable;

    ETH_DMA_InitStructure.ETH_DMA_DropTCPIPChecksumErrorFrame = ENABLE;
    ETH_DMA_InitStructure.ETH_DMA_ReceiveStoreForward = ETH_DMA_ReceiveStoreForward_Enable;

    ETH_DMA_Init(&ETH_DMA_InitStructure);

    ETH_DMARxDescChainInit((ETH_DMADescTypeDef*)eth_rx_buf, 3);
    ETH_DMATxDescChainInit((ETH_DMADescTypeDef*)eth_tx_buf, 3);

    ETH_Start();
}

int main(void)
{
    eth_init();

    tcp_client_init();

    while (1)
    {
        // ...
    }
}

在数据接收回调函数 `tcp_client_recv_callback` 中，将收到的数据一个一个写入环形缓冲区 `rx_ring_buf` 中，并通过串口发送出去。

在发送数据时，使用 `tcp_write` 函数将数据写入 tcp 发送缓冲区，当数据发送成功后，会回调 `tcp_client_sent_callback` 函数，在该函数中释放发送缓冲区的内存。