ARES_SET_SOCKET_FUNCTIONS(3) Library Functions Manual ARES_SET_SOCKET_FUNCTIONS(3)

ares_set_socket_functions, ares_set_socket_functions_ex - Set socket io callbacks

#include <ares.h>
typedef enum {
  ARES_SOCKFUNC_FLAG_NONBLOCKING = 1 << 0
} ares_sockfunc_flags_t;
typedef enum {
  ARES_SOCKET_OPT_SENDBUF_SIZE,
  ARES_SOCKET_OPT_RECVBUF_SIZE,
  ARES_SOCKET_OPT_BIND_DEVICE,
  ARES_SOCKET_OPT_TCP_FASTOPEN
} ares_socket_opt_t;
typedef enum {
  ARES_SOCKET_CONN_TCP_FASTOPEN = 1 << 0
} ares_socket_connect_flags_t;
typedef enum {
  ARES_SOCKET_BIND_TCP = 1 << 0,
  ARES_SOCKET_BIND_CLIENT = 1 << 1
} ares_socket_bind_flags_t;
struct ares_socket_functions_ex {
  unsigned int version; /* ABI Version: must be "1" */
  unsigned int flags;
  ares_socket_t (*asocket)(int domain, int type, int protocol, void *user_data);
  int (*aclose)(ares_socket_t sock, void *user_data);
  int (*asetsockopt)(ares_socket_t sock, ares_socket_opt_t opt, const void *val,
                     ares_socklen_t val_size, void *user_data);
  int (*aconnect)(ares_socket_t sock, const struct sockaddr *address,
                  ares_socklen_t address_len, unsigned int flags,
                  void *user_data);
  ares_ssize_t (*arecvfrom)(ares_socket_t sock, void *buffer, size_t length,
                            int flags, struct sockaddr *address,
                            ares_socklen_t *address_len, void *user_data);
  ares_ssize_t (*asendto)(ares_socket_t sock, const void *buffer, size_t length,
                          int flags, const struct sockaddr *address,
                          ares_socklen_t address_len, void *user_data);
  int (*agetsockname)(ares_socket_t sock, struct sockaddr *address,
                      ares_socklen_t *address_len, void *user_data);
  int (*abind)(ares_socket_t sock, unsigned int flags,
               const struct sockaddr *address, socklen_t address_len,
               void *user_data);
  unsigned int (*aif_nametoindex)(const char *ifname, void *user_data);
  const char *(*aif_indextoname)(unsigned int ifindex, char *ifname_buf,
                                 size_t ifname_buf_len, void *user_data);
};
ares_status_t ares_set_socket_functions_ex(ares_channel_t *channel,
  const struct ares_socket_functions_ex *funcs, void *user_data);
struct ares_socket_functions {
    ares_socket_t (*asocket)(int, int, int, void *);
    int (*aclose)(ares_socket_t, void *);
    int (*aconnect)(ares_socket_t, const struct sockaddr *, ares_socklen_t, void *);
    ares_ssize_t (*arecvfrom)(ares_socket_t, void *, size_t, int,
                              struct sockaddr *, ares_socklen_t *, void *);
    ares_ssize_t (*asendv)(ares_socket_t, const struct iovec *, int, void *);
};
void ares_set_socket_functions(ares_channel_t *channel,
                               const struct ares_socket_functions * functions,
                               void *user_data);

ares_set_socket_functions_ex(3) sets a set of callback functions in the given ares channel handle. Cannot be used when ARES_OPT_EVENT_THREAD is passed to ares_init_options(3). This function replaces the now deprecated ares_set_socket_functions(3) call.

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.

Some callbacks may be optional and are documented as such below, but failing to implement such callbacks will disable certain features within c-ares. It is strongly recommended to implement all callbacks.

All callback functions are expected to operate like their system equivalents, and to set errno(2) or WSASetLastError(2) to an appropriate error code on failure. It is strongly recommended that io callbacks are implemented to be asynchronous and indicated as such in the flags member. The io callbacks can return error codes of EAGAIN, EWOULDBLOCK, or WSAEWOULDBLOCK when they would otherwise block.

The user_data value is provided to each callback function invocation to serve as context.

The ares_set_socket_functions_ex(3) must provide the following structure members and callbacks (which are different from the ares_set_socket_functions(3) members and callbacks):


ABI Version of structure. Must be set to a value of "1".


Flags available are specified in ares_sockfunc_flags_t.


REQUIRED. Creates an endpoint for communication and returns a descriptor. domain, type, and protocol each correspond to the parameters of socket(2). Returns a handle to the newly created socket, or ARES_SOCKET_BAD on error.


REQUIRED. Closes the socket endpoint indicated by fd. See close(2).


REQUIRED. Set socket option. This shares a similar syntax to the BSD setsockopt(2) call, however c-ares uses different options for portability. The value is a pointer to the desired value, and each option has its own data type listed in the options below defined in ares_socket_opt_t.


REQUIRED. Initiate a connection to the address indicated by addr on a socket. Additional flags controlling behavior are in ares_socket_connect_flags_t. See connect(2).


REQUIRED. Receives data from remote socket endpoint, if available. If the addr parameter is not NULL and the connection protocol provides the source address, the callback should fill this in. The flags parameter is currently unused. See recvfrom(2).


REQUIRED. Send data, as provided by the buffer, to the socket endpoint. The flags member may be used on systems that have MSG_NOSIGNAL defined but is otherwise unused. An address is provided primarily to support TCP FastOpen scenarios, which will be NULL in other circumstances. See sendto(2).


Optional. Retrieve the local address of a socket and store it into the provided address buffer. May impact DNS Cookies if not provided. See getsockname(2).


Optional. Bind the socket to an address. This can be used for client connections to bind the source address for packets before connect, or for server connections to bind to an address and port before listening. Currently c-ares only supports client connections. flags from ares_socket_bind_flags_t can be specified. See bind(2).


Optional. Convert an interface name into the interface index. If this callback is not specified, then IPv6 Link-Local DNS servers cannot be used. See if_nametoindex(2).


Optional. Convert an interface index into the interface name. If this callback is not specified, then IPv6 Link-Local DNS servers cannot be used. ifname_buf must be at least IF_NAMESIZE or IFNAMSIZ in size. See if_indextoname(2).

ares_sockfunc_flags_t values:


Used to indicate the implementation of the io functions are asynchronous.

ares_socket_opt_t values:


Set the Send Buffer size. Value is a pointer to an int. (SO_SNDBUF).


Set the Receive Buffer size. Value is a pointer to an int. (SO_RCVBUF).


Set the network interface to use as the source for communication. Value is a C string. (SO_BINDTODEVICE)


Enable TCP Fast Open. Value is a pointer to an ares_bool_t. On some systems this could be a no-op if it is known it is on by default and return success. Other systems may be a no-op if known the system does not support the feature and returns failure with errno set to ENOSYS or WSASetLastError(WSAEOPNOTSUPP);.

ares_socket_connect_flags_t values:


Connect using TCP Fast Open.

ares_socket_bind_flags_t values:


Bind is for a TCP connection.


Bind is for a client connection, not server.

ares_set_socket_functions(3) sets a set of callback functions in the given ares channel handle. Cannot be used when ARES_OPT_EVENT_THREAD is passed to ares_init_options(3). This function is deprecated as of c-ares 1.34.0 in favor of ares_set_socket_functions_ex(3).

ares_set_socket_functions(3) allows you to 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.

All callback functions are expected to operate like their system equivalents, and to set errno(2) or WSASetLastError(2) to an appropriate error code on failure. It is strongly recommended all io functions behave asynchronously and return error codes of EAGAIN, EWOULDBLOCK, or WSAEWOULDBLOCK when they would otherwise block.

The user_data value is provided to each callback function invocation to serve as context.

The ares_set_socket_functions(3) must provide the following callbacks (which are different from the ares_set_socket_functions_ex(3) callbacks):


Creates an endpoint for communication and returns a descriptor. domain, type, and protocol each correspond to the parameters of socket(2). Returns ahandle to the newly created socket, or ARES_SOCKET_BAD on error.


Closes the socket endpoint indicated by fd. See close(2).


Initiate a connection to the address indicated by addr on a socket. See connect(2)


Receives data from remote socket endpoint, if available. If the addr parameter is not NULL and the connection protocol provides the source address, the callback should fill this in. See recvfrom(2)


Send data, as provided by the iovec array data, to the socket endpoint. See writev(2)

The ares_set_socket_functions(3) struct provided is not copied but directly referenced, and must thus remain valid through out the channels and any created socket's lifetime. However, the ares_set_socket_functions_ex(3) struct is duplicated and does not need to survive past the call to the function.

ares_socket_functions added in c-ares 1.13.0, ares_socket_functions_ex added in c-ares 1.34.0

ares_init_options(3), socket(2), close(2), connect(2), recvfrom(2), sendto(2), bind(2), getsockname(2), setsockopt(2), writev(2)

8 Oct 2024