SEND(2) System Calls Manual SEND(2)

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

#include <sys/socket.h>

ssize_t
send(int socket, const void *buffer, size_t length, int flags);

ssize_t
sendmsg(int socket, const struct msghdr *message, int flags);

ssize_t
sendto(int socket, const void *buffer, size_t length, int flags, const struct sockaddr *dest_addr, socklen_t dest_len);

(), (), and sendmsg() are used to transmit a message to another socket. send() may be used only when the socket is in a state, while sendto() and sendmsg() may be used at any time.

The address of the target is given by dest_addr with dest_len specifying its size. The length of the message is given by length. 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 (). Locally detected errors are indicated by a return value of -1.

If no messages space is available at the socket to hold the message to be transmitted, then () normally blocks, unless the socket has been placed in non-blocking I/O mode. The select(2) call may be used to determine when it is possible to send more data.

The flags parameter may include one or more of the following:

#define	MSG_OOB        0x1  /* process out-of-band data */
#define	MSG_DONTROUTE  0x4  /* bypass routing, use direct interface */

The flag MSG_OOB is used to send “out-of-band” data on sockets that support this notion (e.g. SOCK_STREAM); the underlying protocol must also support “out-of-band” data. MSG_DONTROUTE is usually used only by diagnostic or routing programs.

The () system call uses a msghdr structure to minimize the number of directly supplied arguments. The msg_iov and msg_iovlen fields of message specify zero or more buffers containing the data to be sent. msg_iov points to an array of iovec structures; msg_iovlen shall be set to the dimension of this array. In each iovec structure, the iov_base field specifies a storage area and the iov_len field gives its size in bytes. Some of these sizes can be zero. The data from each storage area indicated by msg_iov is sent in turn. See recv(2) for a complete description of the msghdr structure.

Upon successful completion, the number of bytes which were sent is returned. Otherwise, -1 is returned and the global variable errno is set to indicate the error.

The send(), sendmsg(), and sendto() system calls will fail if:

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The SO_BROADCAST option is not set on the socket and a broadcast address is given as the destination.
[]
The socket is marked non-blocking and the requested operation would block.
[]
An invalid descriptor is specified.
[]
A connection is forcibly closed by a peer.
[]
An invalid user space address is specified for a parameter.
[]
The destination address specifies an unreachable host.
[]
A signal interrupts the system call before any data is transmitted.
[]
The socket requires that message be sent atomically, and the size of the message to be sent makes this impossible. IOV_MAX.
[]
The local network interface used to reach the destination is down.
[]
No route to the network is present.
[]
The system is unable to allocate an internal buffer. The operation may succeed when buffers become available.
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The output queue for a network interface is full. This generally indicates that the interface has stopped sending, but may be caused by transient congestion.
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The argument socket is not a socket.
[]
socket does not support (some of) the option(s) specified in flags.
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The socket is shut down for writing or the socket is connection-mode and is no longer connected. In the latter case, and if the socket is of type SOCK_STREAM, the SIGPIPE signal is generated to the calling thread.
[]
The specified address is not available or no longer available on this machine.

The sendmsg() and sendto() system calls will fail if:

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Addresses in the specified address family cannot be used with this socket.
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The socket is not connection-mode and does not have its peer address set, and no destination address is specified.
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A destination address was specified and the socket is already connected.
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A component of the pathname does not name an existing file or the path name is an empty string.
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Insufficient memory is available to fulfill the request.
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The socket is connection-mode, but is not connected.
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A component of the path prefix of the pathname in the socket address is not a directory.

The send() system call will fail if:

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The socket is not connection-mode and no peer address is set.
[]
The socket is not connected or otherwise has not had the peer pre-specified.

The sendmsg() system call will fail if:

[]
The sum of the iov_len values overflows an ssize_t.
[]
The socket requires that message be sent atomically, and the size of the message to be sent makes this impossible, 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.

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

The include file <sys/types.h> is necessary.

fcntl(2), getsockopt(2), recv(2), select(2), socket(2), write(2), compat(5)

The send() function call appeared in 4.2BSD.

February 21, 1994 BSD 4.2