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I'm using an TCP/IP stack called lwip . I have implemented a function below to send data packets, inspired from a similar callback function that receives data packets. Each time a packet is received, I create a buffer using the pbuf_alloc function. Then, I send the packet using udp_sendto . Finally, I free the buffer using pbuf_free . (See the code below.) For some reason, pbuf_free is not freeing the buffer. (I get a buffer overflow after n packets, where n is the pool size.) The lwip wiki warns that: The network driver may also not assume that the pbuf memory is actually freed when it calls pbuf_free. How can I force pbuf_free to free my buffer? How is the buffer overflow avoided? (My implementation below.) static err_t IAP_tftp_send_data_packet(struct udp_pcb *upcb, struct ip_addr *to, int to_port, int block) { err_t err; struct pbuf *pkt_buf; char packet[TFTP_DATA_PKT_LEN_MAX]; int bytesRead; int bytesToSend; /* Specify that we are sending data. */ IAP_tftp_set_opcode(packet, TFTP_DATA); /* Specify the block number that we are sending. */ IAP_tftp_set_block(packet, block); bytesRead = IAP_tftp_set_data(packet, block); if(bytesRead != 0) { bytesToSend = TFTP_DATA_PKT_LEN_MAX - (512 - bytesRead + 1); } else { bytesToSend = TFTP_DATA_PKT_LEN_MAX - 512; } pkt_buf = pbuf_alloc(PBUF_TRANSPORT, bytesToSend, PBUF_POOL); if (!pkt_buf) { print("(TFTP) Buffer overflow!\r\n"); } /* Copy the file data onto pkt_buf. */ memcpy(pkt_buf->payload, packet, bytesToSend); err = udp_sendto(upcb, pkt_buf, to, to_port); /* free the buffer pbuf */ printf("%d\n\r", pbuf_free(pkt_buf)); retu

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