以下是一个简单的UDP客户端和服务端程序示例，用于发送字符。

UDP客户端程序：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>

#define SERVER_IP "127.0.0.1"
#define SERVER_PORT 8888

int main() {
    int sockfd;
    struct sockaddr_in server_addr;

    // 创建UDP套接字
    if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) {
        perror("socket creation failed");
        exit(EXIT_FAILURE);
    }

    memset(&server_addr, 0, sizeof(server_addr));

    server_addr.sin_family = AF_INET;
    server_addr.sin_port = htons(SERVER_PORT);
    if (inet_pton(AF_INET, SERVER_IP, &server_addr.sin_addr) <= 0) {
        perror("inet_pton error");
        exit(EXIT_FAILURE);
    }

    char message[100];
    printf("Enter message: ");
    fgets(message, sizeof(message), stdin);

    // 发送消息到服务器
    if (sendto(sockfd, message, strlen(message), 0, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0) {
        perror("sendto failed");
        exit(EXIT_FAILURE);
    }

    close(sockfd);

    return 0;
}

UDP服务端程序：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <arpa/inet.h>

#define SERVER_PORT 8888

int main() {
    int sockfd;
    struct sockaddr_in server_addr, client_addr;
    socklen_t client_len = sizeof(client_addr);

    // 创建UDP套接字
    if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) {
        perror("socket creation failed");
        exit(EXIT_FAILURE);
    }

    memset(&server_addr, 0, sizeof(server_addr));

    server_addr.sin_family = AF_INET;
    server_addr.sin_addr.s_addr = INADDR_ANY;
    server_addr.sin_port = htons(SERVER_PORT);

    // 绑定套接字到指定的IP地址和端口号
    if (bind(sockfd, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0) {
        perror("bind failed");
        exit(EXIT_FAILURE);
    }

    char buffer[100];

    // 接收来自客户端的消息
    if (recvfrom(sockfd, buffer, sizeof(buffer), 0, (struct sockaddr *)&client_addr, &client_len) < 0) {
        perror("recvfrom failed");
        exit(EXIT_FAILURE);
    }

    printf("Received message: %s\n", buffer);

    close(sockfd);

    return 0;
}

你可以将上述代码分别保存为udp_client.c和udp_server.c，然后使用C编译器编译它们：

gcc udp_client.c -o udp_client
gcc udp_server.c -o udp_server

然后分别运行udp_client和udp_server，就可以发送字符消息了。请确保在运行UDP客户端之前先运行UDP服务端。