ソースコード例：FTPクライアント(TCP)

/****************************************/
/* Ftp client for anonymous ftp server	*/
/* Enable commands			*/
/*	pwd				*/
/*	cd [pass]			*/
/*	dir				*/
/*	get [pass]			*/
/*	quit				*/
/****************************************/

#include <stdio.h>
#include <unistd.h>
#include <string.h>
#include <ctype.h>
#include <stdlib.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <arpa/inet.h>
#include <netinet/ip.h>
#include <time.h>
#include <netdb.h>
#include <errno.h>

#define	TIMOUT	4
#define	BUFFSIZ	0x600
#define CMDSIZ	0x200

static char *buf, *cmd, *inp, echo;

static int get_status(int sock) {
	int	n, len, code;
	char	*p, *line;
	int	i;

	buf[1] = 0;
	p = buf;
	do {
		line = ++p;
		if(line[0] == 0) {
			bzero(buf, BUFFSIZ);
			n = read(sock, buf, BUFFSIZ);
			if(n <= 0) return -1;
			line = buf;
		}
		p = strchr(line, '\n');
		*p = 0;
		if(echo) {
			if(isdigit(line[0]) && isdigit(line[1]) && isdigit(line[2])) {
				printf("%s", &line[4]);
			} else {
				printf("%s", line);
			}
		}
	} while(!isdigit(line[0]) || !isdigit(line[1]) || !isdigit(line[2]) || line[3] != ' ');
	putchar('\n');
	sscanf(line, "%d", &code);
	return code;
}

int main(int argc, char **argv) {
	char		*p, *p2, *file;
	int		i, j, ret, code, emport, n, cnt, retry, len;
	unsigned int	servip, ftpsk, datask;
	FILE		*fp;
	struct sockaddr_in target, target2, serv;
	struct hostent	*hp;
	struct timeval	tv;

	echo = 1;
	if(argc != 2) {
		printf("Usage : %s ftpserver\n", argv[0]);
		return -1;
	}
	buf = malloc(BUFFSIZ + CMDSIZ * 2);
	if(buf == NULL) {
		printf("No memory\n");
		return -1;
	}
	cmd = &buf[BUFFSIZ];
	inp = &cmd[CMDSIZ];

	hp = gethostbyname(argv[1]);
	if(hp == NULL) {
		printf("Ftp server not found\n");
		free(buf);
		return -1;
	}
	bcopy(hp->h_addr_list[0], &(serv.sin_addr), hp->h_length);
	ftpsk = socket(AF_INET, SOCK_STREAM, 0);
	if(ftpsk == -1) {
		printf("No more socket\n");
		free(buf);
		return -1;
	}

	getsockopt(ftpsk, SOL_SOCKET, SO_RCVTIMEO, &tv, &len);
	tv.tv_sec = TIMOUT;
	tv.tv_usec = 0;
	setsockopt(ftpsk, SOL_SOCKET, SO_RCVTIMEO, &tv, sizeof(tv));

	serv.sin_family = AF_INET;
	serv.sin_port = htons(21);
	ret = connect(ftpsk, (struct sockaddr *)&serv, sizeof(serv));
	if(ret == -1) {
		printf("FTP server connect error(1)\n");
		close(ftpsk);
		free(buf);
		return -1;
	}
	code = get_status(ftpsk);
	if(code != 220) {
		printf("FTP server connect error(2)\n");
		close(ftpsk);
		free(buf);
		return -1;
	}
	write(ftpsk, "USER anonymous\r\n", 16);
	code = get_status(ftpsk);
	if(code != 230) {
		if(code == 331) {
			write(ftpsk, "PASS MESUser@\r\n", 15);
			code = get_status(ftpsk);
		}
		if(code != 230) {
			printf("User connect error!\n");
			close(ftpsk);
			free(buf);
			return -1;
		}
	}
	for(;;) {
		printf("ftp> ");
		fgets(inp, CMDSIZ, stdin);
		p = strchr(inp, (int)'\n');
		*p = 0;
		if(strcmp(inp, "quit") == 0) {
			break;
		} else if(strcmp(inp, "help") == 0) {
			puts("Enable commands");
			puts("\tpwd\n\tcd [pass]\n\tdir\n\tget [pass]\n\tquit");
			continue;
		} else if(strcmp(inp, "pwd") == 0) {
			write(ftpsk, "PWD\r\n", 5);
			get_status(ftpsk);
		} else if(memcmp(inp, "cd ", 3) == 0) {
			strcpy(cmd, "CWD ");
			strcat(cmd, &inp[3]);
			strcat(cmd, "\r\n");
			write(ftpsk, cmd, strlen(cmd));
			get_status(ftpsk);
		} else if(strcmp(inp, "dir") == 0 || memcmp(inp, "get ", 4) == 0) {
			if(strcmp(inp, "dir") == 0) {
				strcpy(cmd, "LIST\r\n");
				write(ftpsk, "TYPE A\r\n", 8);
				fp = NULL;
			}
			if(memcmp(inp, "get ", 4) == 0) {
				file = &inp[4];
				if(strlen(file) == 0) continue;
				strcpy(cmd, "RETR ");
				strcat(cmd, &inp[4]);
				strcat(cmd, "\r\n");
				fp = fopen(file, "wb");
				if(fp == NULL) continue;
				write(ftpsk, "TYPE I\r\n", 8);
			}
			get_status(ftpsk);
			write(ftpsk, "PASV\r\n", 6);
			get_status(ftpsk);
			p = buf;
			for(i = 0;i < 4;i++) {
				p = strchr(p, ',');
				p++;
			}
			p2 = strchr(p, ',');
			*p2 = 0;
			sscanf(p, "%d", &i);
			p = p2 + 1;
			p2 = strchr(p, ')');
			*p2 = 0;
			sscanf(p, "%d", &j);
			emport = (i << 8) + j;
			datask = socket(AF_INET, SOCK_STREAM, 0);
			if(datask == -1) {
				printf("No more socket\n");
				if(fp != NULL) fclose(fp);
				continue;
			}
			target2.sin_family = AF_INET;
			target2.sin_port = htons(20);
			target2.sin_addr.s_addr = INADDR_ANY;
			ret = bind(datask, (struct sockaddr *)&target2, sizeof(target2));
			target2.sin_family = AF_INET;
			bcopy(hp->h_addr_list[0], &(target2.sin_addr), hp->h_length);
			target2.sin_port = htons(emport);
			ret = connect(datask, (struct sockaddr *)&target2, sizeof(target2));
			if(ret == -1) {
				printf("FTP server connect error\n");
				close(datask);
				if(fp != NULL) fclose(fp);
				continue;
			}
			write(ftpsk, cmd, strlen(cmd));
			code = get_status(ftpsk);
			if((code / 100) == 5) {
				close(datask);
				continue;
			}
			getsockopt(datask, SOL_SOCKET, SO_RCVTIMEO, &tv, &len);
			tv.tv_sec = TIMOUT;
			tv.tv_usec = 0;
			setsockopt(datask, SOL_SOCKET, SO_RCVTIMEO, &tv, sizeof(tv));
			cnt = 0;
			for(;;) {
				cnt++;
				n = read(datask, buf, BUFFSIZ);
				if(n > 0){
					if(strcmp(inp, "dir") == 0) {
						write(1, buf, n);
					}
					if(memcmp(inp, "get ", 4) == 0) {
						fwrite(buf, 1, n, fp);
						if((cnt & 0x0f) == 0) {
							putchar('.');
							fflush(stdout);
						}
					}
				} else {
					break;
				}
			}
			if(fp != NULL) fclose(fp);
			close(datask);
			for(i = 0;i < TIMOUT;i++) {
				code = get_status(ftpsk);
				if(code != -1) break;
				sleep(1);
			}
			if(code == -1) {
				echo = 0;
				write(ftpsk, "ERR\r\n", 5);
				while(get_status(ftpsk) <= 299);
				echo = 1;
			}
		} else {
			printf("Command not found.\n");
		}
	}
	close(ftpsk);
	free(buf);
	return 0;
}