NAME

sendto — send a message on a socket

SYNOPSIS

#include <sys/socket.h>

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


DESCRIPTION

The sendto() function shall send a message through a connection-mode or connectionless-mode socket.

If the socket is a connectionless-mode socket, the message shall be sent to the address specified by dest_addr if no pre-specified peer address has been set. If a peer address has been pre-specified, either the message shall be sent to the address specified by dest_addr (overriding the pre-specified peer address), or the function shall return -1 and set errno to [EISCONN].

If the socket is connection-mode, dest_addr shall be ignored.

The sendto() function takes the following arguments:

socket

Specifies the socket file descriptor.

message

Points to a buffer containing the message to be sent.

length

Specifies the size of the message in bytes.

flags

Specifies the type of message transmission. Values of this argument are formed by logically OR'ing zero or more of the following flags:

MSG_EOR

Terminates a record (if supported by the protocol).

MSG_OOB

Sends out-of-band data on sockets that support out-of-band data. The significance and semantics of out-of-band data are protocol-specific.

MSG_NOSIGNAL

Requests not to send the SIGPIPE signal if an attempt to send is made on a stream-oriented socket that is no longer connected. The [EPIPE] error shall still be returned.

dest_addr

Points to a sockaddr structure containing the destination address. The length and format of the address depend on the address family of the socket.

dest_len

Specifies the length of the sockaddr structure pointed to by the dest_addr argument.

If the address family of the socket is AF_UNIX, the application shall ensure that a null terminator after the pathname is included in the sun_path member of dest_addr as a sockaddr_un structure, and that dest_len is at least offsetof(struct sockaddr_un, sun_path) + 1 plus the length of the pathname.

If the socket protocol supports broadcast and the specified address is a broadcast address for the socket protocol, sendto() shall fail if the SO_BROADCAST option is not set for the socket.

The dest_addr argument specifies the address of the target.

The length argument specifies the length of the message.

Successful completion of a call to sendto() does not guarantee delivery of the message. A return value of -1 indicates only locally-detected errors.

If space is not available at the sending socket to hold the message to be transmitted and the socket file descriptor does not have O_NONBLOCK set, sendto() shall block until space is available or a timeout occurs (see SO_SNDTIMEO in 2.10.16 Use of Options). If space is not available at the sending socket to hold the message to be transmitted and the socket file descriptor does have O_NONBLOCK set, sendto() shall fail.

The socket in use may require the process to have appropriate privileges to use the sendto() function.

RETURN VALUE

Upon successful completion, sendto() shall return the number of bytes sent. Otherwise, -1 shall be returned and errno set to indicate the error.

ERRORS

The sendto() function shall fail if:

[EAFNOSUPPORT]

Addresses in the specified address family cannot be used with this socket.

[EAGAIN] or [EWOULDBLOCK]

The socket's file descriptor is marked O_NONBLOCK and the requested operation would block. See also SO_SNDTIMEO in 2.10.16 Use of Options.

[EBADF]

The socket argument is not a valid file descriptor.

[ECONNRESET]

A connection was forcibly closed by a peer.

[EINTR]

A signal interrupted sendto() before any data was transmitted.

[EMSGSIZE]

The message is too large to be sent all at once, as the socket requires.

[ENOTCONN]

The socket is connection-mode but is not connected.

[ENOTSOCK]

The socket argument does not refer to a socket.

[EOPNOTSUPP]

The socket argument is associated with a socket that does not support one or more of the values set in flags.

[EPIPE]

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 or SOCK_SEQPACKET and the MSG_NOSIGNAL flag is not set, the SIGPIPE signal is generated to the calling thread.

If the address family of the socket is AF_UNIX, then sendto() shall fail if:

[EIO]

An I/O error occurred while reading from or writing to the file system.

[ELOOP]

A loop exists in symbolic links encountered during resolution of the pathname in the socket address.

[ENAMETOOLONG]

The length of a component of a pathname is longer than {NAME_MAX}.

[ENOENT]

A component of the pathname does not name an existing file or the pathname is an empty string.

[ENOTDIR]

A component of the path prefix of the pathname in the socket address names an existing file that is neither a directory nor a symbolic link to a directory, or the pathname in the socket address contains at least one non-<slash> character and ends with one or more trailing <slash> characters and the last pathname component names an existing file that is neither a directory nor a symbolic link to a directory.

The sendto() function may fail if:

[EACCES]

Search permission is denied for a component of the path prefix; or write access to the named socket is denied.

[EDESTADDRREQ]

The socket is not connection-mode and does not have its peer address set, and no destination address was specified.

[EHOSTUNREACH]

The destination host cannot be reached (probably because the host is down or a remote router cannot reach it).

[EINVAL]

The dest_len argument is not a valid length for the address family.

[EIO]

An I/O error occurred while reading from or writing to the file system.

[EISCONN]

A destination address was specified and the socket is already connected.

[ENETDOWN]

The local network interface used to reach the destination is down.

[ENETUNREACH]

No route to the network is present.

[ENOBUFS]

Insufficient resources were available in the system to perform the operation.

[ENOMEM]

Insufficient memory was available to fulfill the request.

If the address family of the socket is AF_UNIX, then sendto() may fail if:

[ELOOP]

More than {SYMLOOP_MAX} symbolic links were encountered during resolution of the pathname in the socket address.

[ENAMETOOLONG]

The length of a pathname exceeds {PATH_MAX}, or pathname resolution of a symbolic link produced an intermediate result with a length that exceeds {PATH_MAX}.

EXAMPLES

None.

APPLICATION USAGE

The select() and poll() functions can be used to determine when it is possible to send more data.

For AF_UNIX sockets, some implementations support an extension where dest_len does not have to include a null terminator for the pathname stored in sun_path, which in turn allows a pathname to be one byte longer. However, such usage is not portable, and carries a risk of accessing beyond the intended bounds of the pathname length.

RATIONALE

None.

FUTURE DIRECTIONS

None.

SEE ALSO

getsockopt, poll, pselect, recv, recvfrom, recvmsg, send, sendmsg, setsockopt, shutdown, socket

XBD <sys/socket.h>

CHANGE HISTORY

First released in Issue 6. Derived from the XNS, Issue 5.2 specification.

The wording of the mandatory [ELOOP] error condition is updated, and a second optional [ELOOP] error condition is added.

Issue 7

Austin Group Interpretations 1003.1-2001 #035 and #073 are applied, updating the [EISCONN] error and the DESCRIPTION.

Austin Group Interpretation 1003.1-2001 #143 is applied, clarifying the [ENAMETOOLONG] error condition.

The MSG_NOSIGNAL flag is added from The Open Group Technical Standard, 2006, Extended API Set Part 2.

The [EPIPE] error is modified.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0545 [324] is applied.

Issue 8

Austin Group Defect 561 is applied, changing the requirements for the sun_path member of the sockaddr_un structure.

Austin Group Defect 1429 is applied, clarifying the behavior on timeout by adding references to 2.10.16 Use of Options.