ソースコード例：TFTPクライアント(UDP)

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>
#include <unistd.h>
#include <string.h>
#include <sys/ioctl.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <net/if.h>
#include <netdb.h>

#define TFTP_SIZE	512

#define	TFTP_NOFILE	0
#define	TFTP_ACCESS	2
#define	TFTP_ILLEGAL	4

#define	TFTP_READREQ	1
#define	TFTP_WRITEREQ	2
#define	TFTP_DATA	3
#define TFTP_ACK	4
#define TFTP_ERROR	5

static int make_req_packet(char *packet, char *name, char command) {
	int	index;
	short	*data;

	index = 0;
	data = (short*)&packet[index];
	data[0] = htons(command);
	index += sizeof(short);
	strcpy(&packet[index], name);
	index += strlen(name) + 1;
	strcpy(&packet[index], "octet");
	index += strlen("octet") + 1;
	return index;
}

static int ack_get(int sock,struct sockaddr_in *target) {
	unsigned short	ack[2];
	int		ret, addrlen;

	ack[0] = 0;
	ret = recvfrom(sock, (char*)ack, 4, 0, (struct sockaddr *)target, &addrlen);
	if(ret == -1) return -1;
	if(ntohs(ack[0]) == TFTP_ACK) return ntohs(ack[1]);  // block number
	return -1;
}

/*static void ack_put(int sock,struct sockaddr_in *target, unsigned short pnum) {
	unsigned short ack[2];

	ack[0] = htons(TFTP_ACK);
	ack[1] = htons(pnum);  // block number
	sendto(sock,  (char*)ack, 4, 0, (struct sockaddr *)&target, sizeof(target));
}*/

static int tftp_common(int ip, char *filename, char command, void (*func)()) {
	int		ack, i, c, sock, size, filesize, count;
	int		mode, fullsize, ret, retry, addrlen, len;
	unsigned int	myip, servip, mask;
	unsigned char	mac[6];
	unsigned char	*packet;
	unsigned short	*data, pnum;
	const int	on = 1;
	FILE		*fp;
	struct sockaddr_in target;
	struct timeval	tv;

	switch(command) {
	case TFTP_WRITEREQ:
		fullsize = TFTP_SIZE;
		fp = fopen(filename, "rb");
		break;
	case TFTP_READREQ:
		fullsize = TFTP_SIZE + 4;
		fp = fopen(filename, "wb");
		break;
	default:
		return -1;
	}
	if(fp == NULL) {
		printf("Open error.\n");
		return -1;
	}
	servip = ip;
	packet = malloc(TFTP_SIZE + 4);
	if(packet == 0) {
		printf("malloc error.\n");
		return -1;
	}
	sock = socket(AF_INET, SOCK_DGRAM, 0);
	if(sock == -1) {
		printf("socket error.\n");
		free(packet);
		return -1;
	}
	getsockopt(sock, SOL_SOCKET, SO_RCVTIMEO, &tv, &len);
	tv.tv_sec = 3;
	tv.tv_usec = 0;
	setsockopt(sock, SOL_SOCKET, SO_RCVTIMEO, &tv, sizeof(tv));
	bzero(packet, TFTP_SIZE + 4);
	target.sin_family = AF_INET;
	target.sin_addr.s_addr = servip;
	target.sin_port = htons(69);
	bind(sock, (struct sockaddr *)&target, sizeof(target));
	size = make_req_packet(packet, filename, command);
	data = (short*)packet;
	sendto(sock, packet, size, 0, (struct sockaddr *)&target, sizeof(target));
	ack = 0;
	if(command == TFTP_WRITEREQ) ack = ack_get(sock, &target);
	ret = -1;
	count = 0;
        if(ack == 0) {
        	ret = 0;
	        pnum = 1;
		do {
			bzero(packet, TFTP_SIZE + 4);
			switch(command) {
			case TFTP_WRITEREQ:
				size = fread(&packet[4], 1, TFTP_SIZE, fp);
				for(retry = 0;retry < 4;retry++) {
					if(command == TFTP_WRITEREQ) {
						data[0] = htons(TFTP_DATA);
						data[1] = htons(pnum);
						sendto(sock, packet, size + 4, 0, (struct sockaddr *)&target, sizeof(target));
						ack = ack_get(sock, &target);
//						if(ntohs(target.sin_port) < 1024) continue;
						if(ack == -1) ret = -1;
						if(ack == pnum || ack == -1) break;
					}
				}
				if(retry == 4) ret = -1;
				break;
			case TFTP_READREQ:
				for(retry = 0;retry < 4;retry++) {
					size = recvfrom(sock, packet, TFTP_SIZE + 4, 0, (struct sockaddr *)&target, &addrlen);
					if(size == -1) {
						ret = -1;
						break;
					}
//					if(ntohs(target.sin_port) < 1024) continue;
					if(pnum == ntohs(data[1])) {
						data[0] = htons(TFTP_ACK);
						sendto(sock, packet, 4, 0, (struct sockaddr *)&target, sizeof(target));
						fwrite(&packet[4], 1, size - 4, fp);
						break;
					}
				}
				if(retry == 4) ret = -1;
				break;
			}
			pnum++;
			if((count++ % 10) == 0) if(func != 0) (*func)();
		} while(size == fullsize && ret == 0);
	}
	fclose(fp);
	close(sock);
	free(packet);
	return ret;
}

static int tftpsave(int ip, char *filename, void (*func)()) {
	return tftp_common(ip, filename, TFTP_WRITEREQ, func);
}

static int tftpload(int ip, char *filename, void (*func)()) {
	return tftp_common(ip, filename, TFTP_READREQ, func);
}

int main(int argc, char** argv) {
	struct sockaddr_in	addr, saddr;
	struct hostent		*hp;
	int			ip, ret;
	char			*name;

	if((hp = gethostbyname(argv[1])) == NULL) {
		if((hp = gethostbyname(argv[2])) == NULL) {
			printf("host not found.\n");
			return -1;
		}
		bcopy(hp->h_addr_list[0], &(addr.sin_addr), hp->h_length);
		name = strrchr(argv[1], '/');
		if(name == 0) name = argv[1];
		if(tftpload(addr.sin_addr.s_addr, name, 0) == -1) {
			printf("download error.\n");
			return -1;
		}
	} else {
		bcopy(hp->h_addr_list[0], &(addr.sin_addr), hp->h_length);
		name = strrchr(argv[2], '/');
		if(name == 0) name = argv[2];
		if(tftpsave(addr.sin_addr.s_addr, name, 0) == -1) {
			printf("upload error.\n");
			return -1;
		}
	}
	return 0;
}