echoclient.c

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <netdb.h>

static int open_connection(char *host, char *service);

int
main(int argc, char *argv[])
{
    if (argc != 3) {
        fprintf(stderr, "%s: error: wrong number of argument: %d for 2\n", argv[0], argc - 1);
        fprintf(stderr, "Usage: %s HOST MESSAGE\n", argv[0]);
        exit(1);
    }
    char *host = argv[1];
    char *msg = argv[2];

    int sock = open_connection(host, "echo");
    FILE *f = fdopen(sock, "w+");
    if (!f) {
        perror("fdopen(3)");
        exit(1);
    }

    fprintf(f, "%s\n", msg);
    fflush(f);

    char buf[1024];
    fgets(buf, sizeof buf, f);
    fclose(f);
    fputs(buf, stdout);
    exit(0);
}

static int
open_connection(char *host, char *service)
{
    struct addrinfo hints, *res, *ai;
    int err;

    memset(&hints, 0, sizeof(struct addrinfo));
    hints.ai_family = AF_UNSPEC;
    hints.ai_socktype = SOCK_STREAM;
    if ((err = getaddrinfo(host, service, &hints, &res)) != 0) {
        fprintf(stderr, "getaddrinfo(3): %s\n", gai_strerror(err));
        exit(1);
    }
    for (ai = res; ai; ai = ai->ai_next) {
        int sock = socket(ai->ai_family, ai->ai_socktype, ai->ai_protocol);
        if (sock < 0) {
            continue;
        }
        if (connect(sock, ai->ai_addr, ai->ai_addrlen) < 0) {
            close(sock);
            continue;
        }
        /* success */
        freeaddrinfo(res);
        return sock;
    }
    fprintf(stderr, "socket(2)/connect(2) failed");
    freeaddrinfo(res);
    exit(1);
}