recv

Name

recv, recvfrom, recvmsg
- receive a message from a socket

Library

libc.lib

Synopsis

#include <sys/socket.h>

ssize_t recv (int s, void *buf, size_t len, int flags);

ssize_t recvfrom (int s, void * restrict buf, size_t len, int flags, struct sockaddr * restrict from, socklen_t * restrict fromlen);

ssize_t recvmsg (int s, struct msghdr *msg, int flags);

Return values

These calls return the number of bytes received, or -1 if an error occurred.

Detailed description

The recvfrom and recvmsg system calls are used to receive messages from a socket, and may be used to receive data on a socket whether or not it is connection-oriented.

If from is not a null pointer and the socket is not connection-oriented, the source address of the message is filled in. The fromlen argument is a value-result argument, initialized to the size of the buffer associated with from, and modified on return to indicate the actual size of the address stored there.

The recv function is normally used only on a connected socket (see connect and is identical to recvfrom with a null pointer passed as its from argument. As it is redundant, it may not be supported in future releases.

All three routines return the length of the message on successful completion. If a message is too long to fit in the supplied buffer, excess bytes may be discarded depending on the type of socket the message is received from (see socket

If no messages are available at the socket, the receive call waits for a message to arrive, unless the socket is nonblocking (see fcntl in which case the value -1 is returned and the external variable errno set to EAGAIN. The receive calls normally return any data available, up to the requested amount, rather than waiting for receipt of the full amount requested; this behavior is affected by the socket-level options SO_RCVLOWAT and SO_RCVTIMEO described in getsockopt.

The select system call may be used to determine when more data arrive.

The flags argument to a recv function is formed by or Ap ing one or more of the values:

`MSG_OOB process out-of-band data` `MSG_PEEK peek at incoming message` `MSG_WAITALL wait for full request or error` `MSG_DONTWAIT do not block`

The MSG_OOB flag requests receipt of out-of-band data that would not be received in the normal data stream. Some protocols place expedited data at the head of the normal data queue, and thus this flag cannot be used with such protocols. The MSG_PEEK flag causes the receive operation to return data from the beginning of the receive queue without removing that data from the queue. Thus, a subsequent receive call will return the same data. The MSG_WAITALL flag requests that the operation block until the full request is satisfied. The MSG_DONTWAIT flag requests the call to return when it would block otherwise. If no data is available, errno is set to EAGAIN.

The recvmsg system call uses a msghdr structure to minimize the number of directly supplied arguments. This structure has the following form, as defined in

#include <sys/socket.h:>

struct msghdr {
        caddr_t msg_name;       /* optional address */
        u_int   msg_namelen;    /* size of address */
        struct  iovec *msg_iov; /* scatter/gather array */
        u_int   msg_iovlen;     /* # elements in msg_iov */
        caddr_t msg_control;    /* ancillary data, see below */
        u_int   msg_controllen; /* ancillary data buffer len */
        int     msg_flags;      /* flags on received message */
};

Here msg_name and msg_namelen specify the destination address if the socket is unconnected; msg_name may be given as a null pointer if no names are desired or required. The msg_iov and msg_iovlen arguments describe scatter gather locations, as discussed in read. The msg_control argument, which has length msg_controllen, points to a buffer for other protocol control related messages or other miscellaneous ancillary data. The messages are of the form:

struct cmsghdr {
        u_int   cmsg_len;       /* data byte count, including hdr */
        int     cmsg_level;     /* originating protocol */
        int     cmsg_type;      /* protocol-specific type */
/* followed by
        u_char  cmsg_data[]; */
};

As an example, one could use this to learn of changes in the data-stream in XNS/SPP, or in ISO, to obtain user-connection-request data by requesting a recvmsg with no data buffer provided immediately after an accept system call.

Open file descriptors are now passed as ancillary data for AF_UNIX domain sockets, with cmsg_level set to SOL_SOCKET and cmsg_type set to SCM_RIGHTS.

Process credentials can also be passed as ancillary data for AF_UNIX domain sockets using a cmsg_type of SCM_CREDS. In this case, cmsg_data should be a structure of type cmsgcred, which is defined in

#include <sys/socket.h>as follows:

struct cmsgcred {
        pid_t   cmcred_pid;             /* PID of sending process */
        uid_t   cmcred_uid;             /* real UID of sending process */
        uid_t   cmcred_euid;            /* effective UID of sending process */
        gid_t   cmcred_gid;             /* real GID of sending process */
        short   cmcred_ngroups;         /* number or groups */
        gid_t   cmcred_groups[CMGROUP_MAX];     /* groups */
};

The kernel will fill in the credential information of the sending process and deliver it to the receiver.

The msg_flags field is set on return according to the message received. MSG_EOR indicates end-of-record; the data returned completed a record (generally used with sockets of type SOCK_SEQPACKET). MSG_TRUNC indicates that the trailing portion of a datagram was discarded because the datagram was larger than the buffer supplied. MSG_CTRUNC indicates that some control data were discarded due to lack of space in the buffer for ancillary data. MSG_OOB is returned to indicate that expedited or out-of-band data were received.

Examples

#include <sys/socket.h>
#include <netinet/in.h>
#include <unistd.h>
void Recv()
{
   struct sockaddr_in serv_addr;
   int sock_fd;
   char line[10];
   int size = 10;
   serv_addr.sin_family = AF_INET;
   serv_addr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
   serv_addr.sin_port = htons(5000);
   sock_fd = socket(AF_INET,SOCK_STREAM,IPPROTO_TCP);
   connect(sock_fd,(struct sockaddr*)&serv_addr,sizeof(serv_addr));
   recv(sock_fd, line, size, 0);
   close(sock_fd);
}

#include <sys/socket.h>
#include <netinet/in.h>
#include <unistd.h>
void Sendto()
{
   struct sockaddr_in sender_addr;
   int sock_fd;
   char line[15] = "Hello World!";
   unsigned int size = sizeof(sender_addr);
   sock_fd = socket(AF_INET,SOCK_DGRAM,IPPROTO_UDP);
   sender_addr.sin_family = AF_INET;
   sender_addr.sin_addr.s_addr = htonl(INADDR_LOOPBACK);
   sender_addr.sin_port = htons(5000);
   recvfrom(sock_fd,line,13,0,(struct sockaddr*)&sender_addr,&size);
   close(sock_fd);
}

#include <sys/socket.h>
#include <netinet/in.h>
#include <unistd.h>
void SendMsgRecvMsg()
{
   int sock_fd;
   unsigned int sender_len;
   struct msghdr msg;
   struct iovec iov;
   struct sockaddr_in receiver_addr,sender_addr;
   char line[10];
   sock_fd = socket(AF_INET,SOCK_DGRAM,IPPROTO_UDP);
   receiver_addr.sin_family = AF_INET;
   receiver_addr.sin_addr.s_addr = htonl(INADDR_ANY);
   receiver_addr.sin_port = htons(5000);
   bind(sock_fd,(struct sockaddr*)&receiver_addr,sizeof(receiver_addr));
   sender_len = sizeof(sender_addr);
   msg.msg_name = &sender_addr;
   msg.msg_namelen = sender_len;
   msg.msg_iov = &iov;
   msg.msg_iovlen = 1;
   msg.msg_iov->iov_base = line;
   msg.msg_iov->iov_len = 10;
   msg.msg_control = 0;
   msg.msg_controllen = 0;
   msg.msg_flags = 0;
   recvmsg(sock_fd,&msg,0);
   close(sock_fd);
}

Errors

The calls fail if:

[`EBADF`]
	The argument s is an invalid descriptor.
[`ECONNRESET`]
	The remote socket end is forcibly closed.
[`ENOTCONN`]
	The socket is associated with a connection-oriented protocol and has not been connected (see connect and accept
[`ENOTSOCK`]
	The argument s does not refer to a socket.
[`EMSGSIZE`]
	The `recvmsg` system call was used to receive rights (file descriptors) that were in flight on the connection. However, the receiving program did not have enough free file descriptor slots to accept the them. In this case the descriptors are closed, any pending data can be returned by another call to `recvmsg.`
[`EAGAIN`]
	The socket is marked non-blocking, and the receive operation would block, or a receive timeout had been set, and the timeout expired before data were received.
[`EFAULT`]
	The receive buffer pointer(s) point outside the process’s address space.