send(), sendto(), sendmsg()
initiate a connection on a socket
#include 'sys/type.h' #include 'sys/socket.h' ssize_t send (int sockfd, const void *buf, size_t len , int flags ); ssize_t sendto( int sockfd, const void *buf, size_t len, int flags, const struct sockaddr *dest_addr, socklen_t addrlen); ssize_t sendmsg(int sockfd, const struct msghdr *msg, int flags);

DESCRIPTION :

The system calls send(), sendto(), and sendmsg() are used to transmit a message to another socket.

The send() call may be used only when the socket is in a connected state (so that the intended recipient is known). The only differ‐ ence between send() and write(2) is the presence of flags. With a zero flags argument, send() is equivalent to write(2). Also, the following call

send(sockfd, buf, len, flags);
is equivalent to

sendto(sockfd, buf, len, flags, NULL, 0);

The argument sockfd is the file descriptor of the sending socket.

If sendto() is used on a connection-mode (SOCK_STREAM, SOCK_SEQPACKET) socket, the arguments dest_addr and addrlen are ignored (and the error EISCONN may be returned when they are not NULL and 0), and the error ENOTCONN is returned when the socket was not actually con‐ nected. Otherwise, the address of the target is given by dest_addr with addrlen specifying its size. For sendmsg(), the address of the target is given by msg.msg_name, with msg.msg_namelen specifying its size.

For send() and sendto(), the message is found in buf and has length len. For sendmsg(), the message is pointed to by the elements of the array msg.msg_iov. The sendmsg() call also allows sending ancillary data (also known as control information).

If the message is too long to pass atomically through the underlying protocol, the error EMSGSIZE is returned, and the message is not transmitted.

No indication of failure to deliver is implicit in a send(). Locally detected errors are indicated by a return value of -1.

When the message does not fit into the send buffer of the socket, send() normally blocks, unless the socket has been placed in non‐ blocking I/O mode. In nonblocking mode it would fail with the error EAGAIN or EWOULDBLOCK in this case. The select(2) call may be used to determine when it is possible to send more data.

The definition of the msghdr structure follows. See recv(2) and below for an exact description of its fields.

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

Return Value

On success, these calls return the number of characters sent. On error, -1 is returned, and errno is set appropriately.