117笔记问答在Linux网络编程中，sendmsg函数是用于向指定的套接字发送数据的函数。下面是一些实际案例分析：
- 发送UDP数据包：
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);
- 发送TCP数据流：
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);
- 发送文件描述符：
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);
这些案例展示了sendmsg函数的不同用法，包括发送UDP数据包、发送TCP数据流以及发送文件描述符等情况。在实际的网络编程中，sendmsg函数可以根据具体的需求进行灵活应用，为程序的网络通信提供了更多的选择和控制。