Socket 命令行 大小写转换 C语言实现

问题：使用C语言 通过socket的命令行通信 实现字符大小写转换
我目前的代码如下：

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#define PORT 10291
#define MAXLINE 9999

// Client side code
int main(int argc, char *argv[])
{
    int sockfd;
    char buffer[MAXLINE];
    char clientMsg;
    struct sockaddr_in servaddr;

    // Creating socket file descriptor
    if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0)
    {
        perror("socket creation failed");
        exit(EXIT_FAILURE);
    }

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

    // Filling server information
    servaddr.sin_family = AF_INET;
    servaddr.sin_port = htons(PORT);
    servaddr.sin_addr.s_addr = INADDR_ANY;

    //Connect to the Server
    connect(sockfd, (struct sockaddr *) &servaddr, sizeof(servaddr));
    printf("Connected to the Server. \n");

    int n, len;

    // Getting Input From Client
    /*
    printf("Please Enter Your Message: ");
    scanf("%s", &clientMsg);
    */

    if (argc == 2)
    {
        for (int i = 0; argc; i++)
        {
            //                      send(sockfd, &argv[i], 10000, 0);
            sendto(sockfd, (const char *) &clientMsg, strlen(&clientMsg),
                MSG_CONFIRM, (const struct sockaddr *) &servaddr,
                sizeof(servaddr));
        }

        printf("Message sent to server. \n");
    }
    else
    {
        printf("Error: Please enter the correct number of arguement. \n");
        close(sockfd);
        printf("Disconneted from the Server. \n");
        exit(1);
    }

    /*
    // Sending message to server
    sendto(sockfd, (const char *) &clientMsg, strlen(&clientMsg),
                    MSG_CONFIRM, (const struct sockaddr *) &servaddr,
            sizeof(servaddr));
    printf("Message sent to server. \n");
    */

    // Receive message from server
    n = recvfrom(sockfd, (char*) buffer, MAXLINE,
        MSG_WAITALL, (struct sockaddr *) &servaddr, &len);
    buffer[n] = '\0';
    printf("Message Received From Server: %s\n", buffer);

    //Disconnet from the Server
    close(sockfd);
    printf("Disconneted from the Server. \n");

    return 0;
}

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <netinet/in.h>
#define PORT 10291
#define MAXLINE 9999

// Server Side code
int main(int argc, char *argv[])
{
    int sockfd;
    char buffer[MAXLINE];
    struct sockaddr_in servaddr, cliaddr;

    // Creating socket file descriptor
    if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0)
    {
        perror("socket creation failed");
        exit(EXIT_FAILURE);
    }

    memset(&servaddr, 0, sizeof(servaddr));
    memset(&cliaddr, 0, sizeof(cliaddr));

    // Filling server information
    servaddr.sin_family = AF_INET;    // IPv4
    servaddr.sin_addr.s_addr = INADDR_ANY;
    servaddr.sin_port = htons(PORT);

    // Bind the socket with the server address
    if (bind(sockfd, (const struct sockaddr *) &servaddr, sizeof(servaddr)) < 0)
    {
        perror("bind failed");
        exit(EXIT_FAILURE);
    }

    int len, n;
    len = sizeof(cliaddr);    //len is value/resuslt
    n = recvfrom(sockfd, (char*) buffer, MAXLINE,
        MSG_WAITALL, (struct sockaddr *) &cliaddr, &len);
    buffer[n] = '\0';

    char c;
    while (buffer[c] != '\0')
    {
        char ch = buffer[c];

        if (ch >= 'A' && ch <= 'Z')
            buffer[c] = buffer[c] + 32;
        else if (ch >= 'a' && ch <= 'z')
            buffer[c] = buffer[c] - 32;
        c++;
    }

    //Returning Encrypted String Received By Client
    sendto(sockfd, (const char *) buffer, strlen(buffer),
        MSG_CONFIRM, (const struct sockaddr *) &cliaddr, len);

    close(sockfd);
    return 0;
}

我得出的输出：

要求的输出：

请问要如何改正？谢谢！