send -- send a message from a socket


cc [options] file -lsocket -lnsl
#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, size_t tolen);

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


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

The address of the target is given by to with tolen specifying its size. The length of the message is given by len. If the message is too long to pass automatically through the underlying protocol, then the error EMSGSIZE is returned, and the message is not transmitted.

No indication of failure to deliver is implicit in a send. Return values of -1 indicate some locally detected errors.

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

If the socket protocol supports broadcasting and the specified address is a broadcast address, sendto and sendmsg will fail if the SO_BROADCAST option is not set for the socket.

The flags parameter is formed by ORing one or more of the following:

Requests not to send SIGPIPE on errors on stream oriented sockets when the other end breaks the connection. The EPIPE error is still returned.

Send out-of-band data on sockets that support this notion. The underlying protocol must also support out-of-band data. Currently, only SOCK_STREAM sockets created in the AF_INET and AF_INET6 address families support out-of-band data.

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

Enables non-blocking operation; if the operation would block, EAGAIN is returned.

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



Return values

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


The calls fail if:

No search permission for a component of the specified path, or no write permission on a named socket (AF_UNIX only).

The specified address family cannot be used with the socket.

s is an invalid descriptor.

The peer forcibly closed the connection.

The socket is not connection-oriented and a peer address is not set.

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

tolen is not the size of a valid address for the specified address family.

An I/O error occurred while writing to or reading from the filesystem (AF_UNIX only).

Too many symbolic links when translating the pathname in the socket address (AF_UNIX only).

A component of the pathname or the entire pathname was too long (AF_UNIX only).

The socket requires that message be sent atomically, and the message was too long.

A component of the pathname or the entire pathname does not refer to an existing file (AF_UNIX only).

There was insufficient user memory available for the operation to complete.

s is a descriptor for a file, not a socket.

The socket is not connected or the peer has not been prespecified.

A component of the pathname is not a directory (AF_UNIX only).

The socket does not support a value set in flags.

The socket is connection-oriented and the peer is closed or shut down for reading (generates SIGPIPE to calling process if type is SOCK_STREAM), or the socket is shut down for writing.

There were insufficient STREAMS resources available for the operation to complete.

The socket is marked non-blocking and the requested operation would block.


connect(3sock), fcntl(2), getsockopt(3sock), recv(3sock), socket(3sock), write(2)

RFC 2133


The type of address structure passed to sendto and sendmsg depends on the address family. UNIX® domain sockets (address family AF_UNIX) require a sockaddr_un structure as defined in sys/un.h; Internet domain IPv4 sockets (address family AF_INET) require a struct sockaddr_in structure as defined in netinet/in.h; Internet domain IPv6 sockets (address family AF_INET6) require a struct sockaddr_in6 structure as defined in netinet/in.h. Other address families may require other structures. Use the structure appropriate to the address family; cast the structure address to a struct sockaddr* in the call to sendto and pass the size of the structure in the fromlen argument. If you are using sendmsg, set msghdr.msg_name to point to the appropriate structure and msghdr.msg_namelen to the length of the structure.

In UnixWare 7 the sockaddr structure has been modified to support variable length sockets. The net result of this modification is that the family member has been shortened to 8 bits and a new 8-bit member inserted before it called len. For more information on the new sockaddr structures, see: unix(7sock) and inet(7tcp).

© 2004 The SCO Group, Inc. All rights reserved.
UnixWare 7 Release 7.1.4 - 25 April 2004