#include <sys/socket.h>ssize_t recv(int sockfd, void *buff, size_t nbytes, int flags);
ssize_t send(int sockfd, const void *buff, size_t nbytes, int flags);
返回：成功返回读入或写出的字节数，出错返回-1前三个参数与read和write相同，flags的值或为0，或是一个或多个常值的逻辑或构成MSG_DONTROUTE:不查路由表MSG_DONTWAIT:本操作不阻塞MSG_OOB:发送或接收带外数据MSG_PEEK:查看外来的消息MSG_WAITTALL:等待所有数据

#include <sys/uio.h>ssize_t readv(int filedes, const struct iovec *iov, int iovcnt);
ssize_t writev(int filedes, const struct iovec *iov, int iovcnt);
返回：读到或写出的字节数，出错时为-1
struct iovec {void *iov_base;      /* starting address of buffer */size_t iov_len;      /* size of buffer */
};

#include <sys/socket.h>ssize_t recvmsg(int sockfd, struct msghdr *msg, int flags);
ssize_t sendmsg(int sockfd, struct msghdr *msg, int flags);
返回：成功时为读入或写出的字符数，出错-1struct msghdr {void *msg_name;                 /* protocol address */用于未经连接的套接口socklen_t msg_namelen;          /* size of protocol address */未经连接的套接口struct iovec *msg_iov;          /* scatter/gather array */指明输入或输出的缓冲区数组size_t msg_iovlen;              /* # elements in msg_iov */void *msg_control;   /* ancillary data, must be aligned for a cmsghdr structure */socklen_t msg_controllen;       /* length of ancillary data */指明可选辅助数据的位置和大小int msg_flags;                  /* flags returned by recvmsg() */标志变量
};

struct cmsghdr{socklen_t cmsg_len;    /* length in bytes, including this structure */字节长度int cmsg_level;        /* originating protocol */初始协议int cmsg_type;         /* protocol-specific type */协议指定类型/* followed by unsigned char cmsg_data[] */真正的控制消息数据
};

#include <sys/socket.h>
#include <sys/param.h>    /* for ALIGN macro on many implementations */
struct cmsghdr *CMSG_FIRSTHDR(struct msghdr *mhdrptr);
返回：指向第一个cmsghdr结构的指针，无辅助数据时为NULL
struct cmsghdr *CMSG_NXTHDR(struct msghdr *mhdrptr, struct cmsghdr *cmsgptr);
返回：指向下一个cmsghdr结构的指针，不再有辅助数据对象时为NULL
unsigned char *CMSG_DATA(struct cmsghdr *cmsgptr);
返回：指向与cmsghdr结构关联的数据的第一个字节的指针
unsigned int CMSG_LEN(unsigned in length);
返回：给定数量下存储在cmsg_len中的值
unsigned int CMSG_SPACE(unsigned int length);
返回：给定数据量下一个辅助数据对象的总大小

一些使用例子：struct msghdr msg;struct cmsghdr *cmptr;

cmptr = CMSG_FIRSTHDR(&msg);cmptr->cmsg_len = CMSG_LEN(sizeof(int));cmptr->cmsg_level = SOL_SOCKET;cmptr->cmsg_type = SCM_RIGHTS;    /* send file description */*((int *)CMSG_DATA(cmptr)) = sendfd;

 1 #include <stdio.h>
 2 #include <stdlib.h>
 3 #include <unistd.h>
 4 #include <errno.h>
 5 #include <string.h>
 6 #include <sys/types.h>
 7 #include <sys/socket.h>
 8
 9 int main(int argc, char *argv[])
10 {
11     int ret;
12     int sock[2];
13     struct msghdr msg;
14     struct iovec iov[1];
15     char send_buf[100] = "Hello";
16
17     struct msghdr msgr;
18     struct iovec iovr[1];
19     char recv_buf[100];
20
21     ret = socketpair(AF_LOCAL, SOCK_STREAM, 0, sock);
22     if(ret == -1) {
23         printf("socketpair err\n");
24         return 1;
25     }
26
27     /* sock[1] send data to local */
28     bzero(&msg, sizeof(msg));
29     msg.msg_name = NULL;
30     msg.msg_namelen = 0;
31     iov[0].iov_base = send_buf;
32     iov[0].iov_len = sizeof(send_buf);
33     msg.msg_iov = iov;
34     msg.msg_iovlen = 1;
35
36     printf("Starting send data: \n");
37     printf("Send data: %s\n", send_buf);
38     ret = sendmsg(sock[1], &msg, 0);
39     if(ret == -1) {
40         printf("sendmsg err\n");
41         return -1;
42     }
43     printf("Send successful\n");
44
45     /* sock[0] recv data to local */
46     bzero(&msgr, sizeof(msgr));
47     msgr.msg_name = NULL;
48     msgr.msg_namelen = 0;
49     iovr[0].iov_base = recv_buf;
50     iovr[0].iov_len  = sizeof(recv_buf);
51     msgr.msg_iov = iovr;
52     msgr.msg_iovlen = 1;
53     ret = recvmsg(sock[0], &msgr, 0);
54     if(ret == -1) {
55         printf("recvmsg err\n");
56         return -1;
57     }
58     printf("Recv successful\n");
59     printf("Recv data: %s\n", recv_buf);
60
61     close(sock[0]);
62     close(sock[1]);
63
64     return 0;
65 }

$ ./a.outStarting send data:
Send data: Hello
Send successful
Recv successful
Recv data: Hello