NAME

send, sendto, sendmsg — send a message from a socket

LIBRARY

Standard C Library (libc, -lc)

SYNOPSIS

#include <sys/types.h>
#include <sys/socket.h>

ssize_t
send(int s, const void *msg, size_t len, int flags);

ssize_t
sendto(int s, const void *msg, size_t len, int flags, const struct sockaddr *to, socklen_t tolen);

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

DESCRIPTION

The send(), sendto(), and sendmsg() functions are used to transmit a message to another transport end-point. The send() function can be used only when the socket is in a connected state. See connect(3C). The sendto() and sendmsg() functions can be used at any time. The s socket is created with socket(3C).

The address of the target is supplied by to with a tolen parameter used to specify the size. The length of the message is supplied by the len parameter. For socket types such as SOCK_DGRAM and SOCK_RAW that require atomic messages, the error EMSGSIZE is returned and the message is not transmitted when it is too long to pass atomically through the underlying protocol. The same restrictions do not apply to SOCK_STREAM sockets.

A return value -1 indicates locally detected errors. It does not imply a delivery failure.

If the socket does not have enough buffer space available to hold a message, the send() function blocks the message, unless the socket has been placed in non-blocking I/O mode (see fcntl(2)). The select(3C) or poll(2) call can be used to determine when it is possible to send more data.

The flags parameter is formed from the bitwise OR of zero or more of the following:


MSG_OOB: Send out-of-band data on sockets that support this notion. The underlying protocol must also support out-of-band data. Only SOCK_STREAM sockets created in the AF_INET or the AF_INET6 address family support out-of-band data.

MSG_DONTROUTE: The SO_DONTROUTE option is turned on for the duration of the operation. It is used only by diagnostic or routing programs.

MSG_NOSIGNAL: Don't generate the SIGPIPE signal when a stream-oriented socket is no longer connected.

See recv(3C) for a description of the msghdr structure.

RETURN VALUES

Upon successful completion, these functions return the number of bytes sent. Otherwise, they return -1 and set errno to indicate the error.

ERRORS

The send(), sendto(), and sendmsg() functions return errors under the following conditions:


[EBADF]: s is not a valid file descriptor.

[ECONNRESET]: The s argument refers to a connection oriented socket and the connection was forcibly closed by the peer and is no longer valid. I/O can no longer be performed to filedes.

[EINTR]: The operation was interrupted by delivery of a signal before any data could be buffered to be sent.

[EMSGSIZE]: The message is too large to be sent all at once (as the socket requires), or the msg_iovlen member of the msghdr structure pointed to by message is less than or equal to 0 or is greater than {IOV_MAX}.

[ENOMEM]: Insufficient memory is available to complete the operation.

[ENOSR]: Insufficient STREAMS resources are available for the operation to complete.

[ENOTSOCK]: s is not a socket.

[EWOULDBLOCK]: The socket is marked non-blocking and the requested operation would block. EWOULDBLOCK is also returned when sufficient memory is not immediately available to allocate a suitable buffer. In such a case, the operation can be retried later.

[ECONNREFUSED]: The requested connection was refused by the peer. For connected IPv4 and IPv6 datagram sockets, this indicates that the system received an ICMP “Destination Port Unreachable” message from the peer in response to some prior transmission.

The send() and sendto() functions return errors under the following conditions:


[EINVAL]: The len argument overflows a ssize_t.
Inconsistent port attributes for system call.

The sendto() function returns errors under the following conditions:


[EINVAL]: The value specified for the tolen parameter is not the size of a valid address for the specified address family.

[EISCON]: A destination address was specified and the socket is already connected.

The sendmsg() function returns errors under the following conditions:


[EINVAL]: The msg_iovlen member of the msghdr structure pointed to by msg is less than or equal to 0, or the sum of the iov_len values in the msg_iov array overflows a ssize_t.
One of the iov_len values in the msg_iov array member of the msghdr structure pointed to by msg is negative, or the sum of the iov_len values in the msg_iov array overflows a ssize_t.
msg_iov contents are inconsistent with port attributes.

The send() function returns errors under the following conditions:


[EPIPE]: The socket is shut down for writing, or the socket is connection-mode and is no longer connected. In the latter case, if the socket is of type SOCK_STREAM, the SIGPIPE signal is generated to the calling thread unless the MSG_NOSIGNAL flag is set.

MT-LEVEL

Safe