sendmsg系统调用允许在用户空间构造消息头和控制信息，用此函数可以发送多个数据缓冲区的数据，并支持控制信息；当调用进入内核后，会将用户端的user_msghdr对应拷贝到内核的msghdr中，然后进行数据发送；
1 SYSCALL_DEFINE3(sendmsg, int, fd, struct user_msghdr __user *, msg, unsigned int, flags) 2 { 3 /* 不支持64位采用32位兼容标记 */ 4 if (flags & MSG_CMSG_COMPAT) 5 return -EINVAL; 6 7 /* 调用__sys_sendmsg */ 8 return __sys_sendmsg(fd, msg, flags); 9 }
1 /* 2 * BSD sendmsg interface 3 */ 4 5 long __sys_sendmsg(int fd, struct user_msghdr __user *msg, unsigned flags) 6 { 7 int fput_needed, err; 8 struct msghdr msg_sys; 9 struct socket *sock; 10 11 /* 查找socket */ 12 sock = sockfd_lookup_light(fd, &err, &fput_needed); 13 if (!sock) 14 goto out; 15 16 /* 发送数据 */ 17 err = ___sys_sendmsg(sock, msg, &msg_sys, flags, NULL, 0); 18 19 fput_light(sock->file, fput_needed); 20 out: 21 return err; 22 }
1 static int ___sys_sendmsg(struct socket *sock, struct user_msghdr __user *msg, 2 struct msghdr *msg_sys, unsigned int flags, 3 struct used_address *used_address, 4 unsigned int allowed_msghdr_flags) 5 { 6 struct compat_msghdr __user *msg_compat = 7 (struct compat_msghdr __user *)msg; 8 struct sockaddr_storage address; 9 struct iovec iovstack[UIO_FASTIOV], *iov = iovstack; 10 unsigned char ctl[sizeof(struct cmsghdr) + 20] 11 __aligned(sizeof(__kernel_size_t)); 12 /* 20 is size of ipv6_pktinfo */ 13 unsigned char *ctl_buf = ctl; 14 int ctl_len; 15 ssize_t err; 16 17 msg_sys->msg_name = &address; 18 19 /* 需要做64位采用32位兼容 */ 20 if (MSG_CMSG_COMPAT & flags) 21 /* 从64位消息头拷贝数据到32位消息头 */ 22 err = get_compat_msghdr(msg_sys, msg_compat, NULL, &iov); 23 else 24 /* 从用户空间拷贝消息数据到消息头 */ 25 err = copy_msghdr_from_user(msg_sys, msg, NULL, &iov); 26 if (err < 0) 27 return err; 28 29 err = -ENOBUFS; 30 31 /* 控制信息长度错误 */ 32 if (msg_sys->msg_controllen > INT_MAX) 33 goto out_freeiov; 34 35 /* 拷贝控制信息 */ 36 flags |= (msg_sys->msg_flags & allowed_msghdr_flags); 37 ctl_len = msg_sys->msg_controllen; 38 if ((MSG_CMSG_COMPAT & flags) && ctl_len) { 39 err = 40 cmsghdr_from_user_compat_to_kern(msg_sys, sock->sk, ctl, 41 sizeof(ctl)); 42 if (err) 43 goto out_freeiov; 44 ctl_buf = msg_sys->msg_control; 45 ctl_len = msg_sys->msg_controllen; 46 } else if (ctl_len) { 47 BUILD_BUG_ON(sizeof(struct cmsghdr) != 48 CMSG_ALIGN(sizeof(struct cmsghdr))); 49 if (ctl_len > sizeof(ctl)) { 50 ctl_buf = sock_kmalloc(sock->sk, ctl_len, GFP_KERNEL); 51 if (ctl_buf == NULL) 52 goto out_freeiov; 53 } 54 err = -EFAULT; 55 /* 56 * Careful! Before this, msg_sys->msg_control contains a user pointer. 57 * Afterwards, it will be a kernel pointer. Thus the compiler-assisted 58 * checking falls down on this. 59 */ 60 if (copy_from_user(ctl_buf, 61 (void __user __force *)msg_sys->msg_control, 62 ctl_len)) 63 goto out_freectl; 64 msg_sys->msg_control = ctl_buf; 65 } 66 67 /* 设置发送标记 */ 68 msg_sys->msg_flags = flags; 69 70 /* 设置非阻塞标记 */ 71 if (sock->file->f_flags & O_NONBLOCK) 72 msg_sys->msg_flags |= MSG_DONTWAIT; 73 /* 74 * If this is sendmmsg() and current destination address is same as 75 * previously succeeded address, omit asking LSM's decision. 76 * used_address->name_len is initialized to UINT_MAX so that the first 77 * destination address never matches. 78 */ 79 /* 如果这次发送的地址跟上次成功发送的一致 */ 80 if (used_address && msg_sys->msg_name && 81 used_address->name_len == msg_sys->msg_namelen && 82 !memcmp(&used_address->name, msg_sys->msg_name, 83 used_address->name_len)) { 84 /* 无需进行检查，直接发送 */ 85 err = sock_sendmsg_nosec(sock, msg_sys); 86 goto out_freectl; 87 } 88 89 /* 进行安全模块检查后发送 */ 90 err = sock_sendmsg(sock, msg_sys); 91 /* 92 * If this is sendmmsg() and sending to current destination address was 93 * successful, remember it. 94 */ 95 96 /* 发送成功需要更新成功地址记录 */ 97 if (used_address && err >= 0) { 98 used_address->name_len = msg_sys->msg_namelen; 99 if (msg_sys->msg_name) 100 memcpy(&used_address->name, msg_sys->msg_name, 101 used_address->name_len); 102 } 103 104 out_freectl: 105 if (ctl_buf != ctl) 106 sock_kfree_s(sock->sk, ctl_buf, ctl_len); 107 out_freeiov: 108 kfree(iov); 109 return err; 110 }
TCP层的sendmsg实现为tcp_sendmsg，详情请移步<TCP层sendmsg系统调用的实现分析>；