在Linux網絡編程中，sendmsg函數是用于向指定的套接字發送數據的函數。下面是一些實際案例分析：

1. 發送UDP數據包：

struct sockaddr_in serv_addr;
int sockfd = socket(AF_INET, SOCK_DGRAM, 0);

memset(&serv_addr, 0, sizeof(serv_addr));
serv_addr.sin_family = AF_INET;
serv_addr.sin_port = htons(PORT);
inet_pton(AF_INET, SERVER_IP, &serv_addr.sin_addr);

struct msghdr msg = { 0 };
struct iovec iov[1];
char buffer[BUF_SIZE] = "Hello, world!";
iov[0].iov_base = buffer;
iov[0].iov_len = strlen(buffer);
msg.msg_name = &serv_addr;
msg.msg_namelen = sizeof(serv_addr);
msg.msg_iov = iov;
msg.msg_iovlen = 1;

sendmsg(sockfd, &msg, 0);

2. 發送TCP數據流：

struct sockaddr_in serv_addr;
int sockfd = socket(AF_INET, SOCK_STREAM, 0);

memset(&serv_addr, 0, sizeof(serv_addr));
serv_addr.sin_family = AF_INET;
serv_addr.sin_port = htons(PORT);
inet_pton(AF_INET, SERVER_IP, &serv_addr.sin_addr);

struct msghdr msg = { 0 };
struct iovec iov[1];
char buffer[BUF_SIZE] = "Hello, world!";
iov[0].iov_base = buffer;
iov[0].iov_len = strlen(buffer);
msg.msg_name = &serv_addr;
msg.msg_namelen = sizeof(serv_addr);
msg.msg_iov = iov;
msg.msg_iovlen = 1;

sendmsg(sockfd, &msg, 0);

3. 發送文件描述符：

struct sockaddr_in serv_addr;
int sockfd = socket(AF_INET, SOCK_STREAM, 0);

memset(&serv_addr, 0, sizeof(serv_addr));
serv_addr.sin_family = AF_INET;
serv_addr.sin_port = htons(PORT);
inet_pton(AF_INET, SERVER_IP, &serv_addr.sin_addr);

struct msghdr msg = { 0 };
struct iovec iov[1];
char buffer[BUF_SIZE] = "Hello, world!";
iov[0].iov_base = buffer;
iov[0].iov_len = strlen(buffer);
msg.msg_name = &serv_addr;
msg.msg_namelen = sizeof(serv_addr);
msg.msg_iov = iov;
msg.msg_iovlen = 1;

int fd = open("file.txt", O_RDONLY);
struct cmsghdr *cmsg;
size_t cmsglen = CMSG_LEN(sizeof(int));
cmsg = (struct cmsghdr *)malloc(cmsglen);
cmsg->cmsg_len = CMSG_LEN(sizeof(int));
cmsg->cmsg_level = SOL_SOCKET;
cmsg->cmsg_type = SCM_RIGHTS;
*((int *)CMSG_DATA(cmsg)) = fd;
msg.msg_control = cmsg;
msg.msg_controllen = cmsglen;

sendmsg(sockfd, &msg, 0);

這些案例展示了sendmsg函數的不同用法，包括發送UDP數據包、發送TCP數據流以及發送文件描述符等情況。在實際的網絡編程中，sendmsg函數可以根據具體的需求進行靈活應用，為程序的網絡通信提供了更多的選擇和控制。