.\" Copyright (C) Daniel Stenberg
.\" SPDX-License-Identifier: MIT
.TH ARES_SET_SOCKET_FUNCTIONS 3 "13 Dec 2016"
.SH NAME
ares_set_socket_functions \- Set socket io callbacks
.SH SYNOPSIS
.nf
#include <ares.h>

struct ares_socket_functions {
    ares_socket_t (*\fIasocket\fP)(int, int, int, void *);
    int (*\fIaclose\fP)(ares_socket_t, void *);
    int (*\fIaconnect\fP)(ares_socket_t, const struct sockaddr *, ares_socklen_t, void *);
    ares_ssize_t (*\fIarecvfrom\fP)(ares_socket_t, void *, size_t, int,
                              struct sockaddr *, ares_socklen_t *, void *);
    ares_ssize_t (*\fIasendv\fP)(ares_socket_t, const struct iovec *, int, void *);
};

void ares_set_socket_functions(ares_channel_t *\fIchannel\fP,
                               const struct ares_socket_functions * \fIfunctions\fP,
                               void *\fIuser_data\fP);
.fi
.SH DESCRIPTION
.PP
This function sets a set of callback \fIfunctions\fP in the given ares channel handle.
Cannot be used when \fBARES_OPT_EVENT_THREAD\fP is passed to \fIares_init_options(3)\fP.

These callback functions will be invoked to create/destroy socket objects and perform
io, instead of the normal system calls. A client application can override normal network
operation fully through this functionality, and provide its own transport layer. You
can choose to only implement some of the socket functions, and provide NULL to any
others and c-ares will use its built-in system functions in that case.
.PP
All callback functions are expected to operate like their system equivalents, and to
set
.BR errno(3)
to an appropriate error code on failure. C-ares also expects all io functions to behave
asynchronously, i.e. as if the socket object has been set to non-blocking mode. Thus
read/write calls (for TCP connections) are expected to often generate
.BR EAGAIN
or
.BR EWOULDBLOCK.

.PP
The \fIuser_data\fP value is provided to each callback function invocation to serve as
context.
.PP
The
.B ares_socket_functions
must provide the following callbacks:
.TP 18
.B \fIasocket\fP
.B ares_socket_t(*)(int \fIdomain\fP, int \fItype\fP, int \fIprotocol\fP, void * \fIuser_data\fP)
.br
Creates an endpoint for communication and returns a descriptor. \fIdomain\fP, \fItype\fP, and \fIprotocol\fP
each correspond to the parameters of
.BR socket(2).
Returns ahandle to the newly created socket, or -1 on error.
.TP 18
.B \fIaclose\fP
.B int(*)(ares_socket_t \fIfd\fP, void * \fIuser_data\fP)
.br
Closes the socket endpoint indicated by \fIfd\fP. See
.BR close(2)
.TP 18
.B \fIaconnect\fP
.B int(*)(ares_socket_t \fIfd\fP, const struct sockaddr * \fIaddr\fP, ares_socklen_t \fIaddr_len\fP, void * \fIuser_data\fP)
.br
Initiate a connection to the address indicated by \fIaddr\fP on a socket. See
.BR connect(2)

.TP 18
.B \fIarecvfrom\fP
.B ares_ssize_t(*)(ares_socket_t \fIfd\fP, void * \fIbuffer\fP, size_t \fIbuf_size\fP, int \fIflags\fP, struct sockaddr * \fIaddr\fP, ares_socklen_t * \fIaddr_len\fP, void * \fIuser_data\fP)
.br
Receives data from remote socket endpoint, if available. If the \fIaddr\fP parameter is not NULL and the connection protocol provides the source address, the callback should fill this in. See
.BR recvfrom(2)

.TP 18
.B \fIasendv\fP
.B ares_ssize_t(*)(ares_socket_t \fIfd\fP, const struct iovec * \fIdata\fP, int \fIlen\fP, void * \fIuser_data\fP)
.br
Send data, as provided by the iovec array \fIdata\fP, to the socket endpoint. See
.BR writev(2),

.PP
The
.B ares_socket_functions
struct provided is not copied but directly referenced,
and must thus remain valid through out the channels and any created socket's lifetime.
.SH AVAILABILITY
Added in c-ares 1.13.0
.SH SEE ALSO
.BR ares_init_options (3),
.BR socket (2),
.BR close (2),
.BR connect (2),
.BR recv (2),
.BR recvfrom (2),
.BR send (2),
.BR writev (2)