Erlang -- Socket Usage

# 2 Socket Usage

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2.1 Introduction

The socket interface (module) is basically a "thin" layer on top of the OS socket interface. It is assumed that, unless you have special needs, gen_[tcp|udp|sctp] should be sufficient (when they become available).

Note that just because we have a documented and described option, it does not mean that the OS supports it. So its recommended that the user reads the platform specific documentation for the option used.

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Asynchronous calls

Some functions allow for an asynchronous call (accept/2, connect/3, recv/3,4, recvfrom/3,4, recvmsg/2,3,5, send/3,4, sendmsg/3,4 and sendto/4,5). This is achieved by setting the Timeout argument to nowait. For instance, if calling the recv/3 function with Timeout set to nowait (i.e. recv(Sock, 0, nowait)) when there is actually nothing to read, it will return with:

On Unix

  {select, SelectInfo}

  SelectInfo contains the SelectHandle.

On Windows

  {completion, CompletionInfo}

  CompletionInfo contains the CompletionHandle.

When data eventually arrives a 'select' or 'completion' message will be sent to the caller:

On Unix

  {'$socket', socket(), select, SelectHandle}

  The caller can then make another call to the recv function and now expect data.

  Note that all other users are locked out until the 'current user' has called the function (recv in this case). So either immediately call the function or cancel.

On Windows

  {'$socket', socket(), completion, {CompletionHandle, CompletionStatus}}

  The CompletionStatus contains the result of the operation (read).

The user must also be prepared to receive an abort message:

  {'$socket', socket(), abort, Info}

If the operation is aborted for whatever reason (e.g. if the socket is closed "by someone else"). The Info part contains the abort reason (in this case that the socket has been closed Info = {SelectHandle, closed}).

The general form of the 'socket' message is:

  {'$socket', Sock :: socket(), Tag :: atom(), Info :: term()}

Where the format of Info is a function of Tag:

Tag | Info value type
select | select_handle()
completion | {completion_handle(), CompletionStatus}
abort | {select_handle(), Reason :: term()}

Table 2.1: socket message info value type

The select_handle() is the same as was returned in the SelectInfo.

The completion_handle() is the same as was returned in the CompletionInfo.

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2.2 Socket Registry

The socket registry is how we keep track of sockets. There are two functions that can be used for interaction: socket:number_of/0 and socket:which_sockets/1.

In systems which create and delete many sockets dynamically, it (the socket registry) could become a bottleneck. For such systems, there are a couple of ways to control the use of the socket registry.

Firstly, its possible to effect the global default value when building OTP from source with the two configure options:

    --enable-esock-socket-registry (default) | --disable-esock-socket-registry

Second, its possible to effect the global default value by setting the environment variable ESOCK_USE_SOCKET_REGISTRY (boolean) before starting the erlang.

Third, its possible to alter the global default value in runtime by calling the function use_registry/1.

And finally, its possible to override the global default when creating a socket (with open/2 and open/4) by providing the attribute use_registry (boolean) in the their Opts argument (which effects that specific socket).

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2.3 Socket Options

Options for level otp:

Option Name | Value Type | Set | Get | Other Requirements and comments
assoc_id | integer() | no | yes | type = seqpacket, protocol = sctp, is an association
debug | boolean() | yes | yes | none
iow | boolean() | yes | yes | none
controlling_process | pid() | yes | yes | none
rcvbuf | default | pos_integer() | {pos_integer(), pos_ineteger()} | yes | yes | The tuple format is not allowed on Windows. 'default' only valid for set. The tuple form is only valid for type 'stream' and protocol 'tcp'.
rcvctrlbuf | default | pos_integer() | yes | yes | default only valid for set
sndctrlbuf | default | pos_integer() | yes | yes | default only valid for set
fd | integer() | no | yes | none
use_registry | boolean() | no | yes | the value is set when the socket is created, by a call to open/2 or open/4.

Table 2.2: option levels

Options for level socket:

Option Name | Value Type | Set | Get | Other Requirements and comments
acceptconn | boolean() | no | yes | none
bindtodevice | string() | yes | yes | Before Linux 3.8, this socket option could be set, but not get. Only works for some socket types (e.g. inet). If empty value is set, the binding is removed.
broadcast | boolean() | yes | yes | type = dgram
debug | integer() | yes | yes | may require admin capability
domain | domain() | no | yes | Not on FreeBSD (for instance)
dontroute | boolean() | yes | yes | none
keepalive | boolean() | yes | yes | none
linger | abort | linger() | yes | yes | none
oobinline | boolean() | yes | yes | none
peek_off | integer() | yes | yes | domain = local (unix). Currently disabled due to a possible infinite loop when calling recv([peek]) the second time.
priority | integer() | yes | yes | none
protocol | protocol() | no | yes | Not on (some) Darwin (for instance)
rcvbuf | non_neg_integer() | yes | yes | none
rcvlowat | non_neg_integer() | yes | yes | none
rcvtimeo | timeval() | yes | yes | This option is not normally supported (see why below). OTP has to be explicitly built with the --enable-esock-rcvsndtime configure option for this to be available. Since our implementation is nonblocking, its unknown if and how this option works, or even if it may cause malfunctions. Therefore, we do not recommend setting this option. Instead, use the Timeout argument to, for instance, the recv/3 function.
reuseaddr | boolean() | yes | yes | none
reuseport | boolean() | yes | yes | domain = inet | inet6
sndbuf | non_neg_integer() | yes | yes | none
sndlowat | non_neg_integer() | yes | yes | not changeable on Linux
sndtimeo | timeval() | yes | yes | This option is not normally supported (see why below). OTP has to be explicitly built with the --enable-esock-rcvsndtime configure option for this to be available. Since our implementation is nonblocking, its unknown if and how this option works, or even if it may cause malfunctions. Therefore, we do not recommend setting this option. Instead, use the Timeout argument to, for instance, the send/3 function.
timestamp | boolean() | yes | yes | none
type | type() | no | yes | none

Table 2.3: socket options

Options for level ip:

Option Name | Value Type | Set | Get | Other Requirements and comments
add_membership | ip_mreq() | yes | no | none
add_source_membership | ip_mreq_source() | yes | no | none
block_source | ip_mreq_source() | yes | no | none
drop_membership | ip_mreq() | yes | no | none
drop_source_membership | ip_mreq_source() | yes | no | none
freebind | boolean() | yes | yes | none
hdrincl | boolean() | yes | yes | type = raw
minttl | integer() | yes | yes | type = raw
msfilter | null | ip_msfilter() | yes | no | none
mtu | integer() | no | yes | type = raw
mtu_discover | ip_pmtudisc() | yes | yes | none
multicast_all | boolean() | yes | yes | none
multicast_if | any | ip4_address() | yes | yes | none
multicast_loop | boolean() | yes | yes | none
multicast_ttl | uint8() | yes | yes | none
nodefrag | boolean() | yes | yes | type = raw
pktinfo | boolean() | yes | yes | type = dgram
recvdstaddr | boolean() | yes | yes | type = dgram
recverr | boolean() | yes | yes | none
recvif | boolean() | yes | yes | type = dgram | raw
recvopts | boolean() | yes | yes | type =/= stream
recvorigdstaddr | boolean() | yes | yes | none
recvttl | boolean() | yes | yes | type =/= stream
retopts | boolean() | yes | yes | type =/= stream
router_alert | integer() | yes | yes | type = raw
sendsrcaddr | boolean() | yes | yes | none
tos | ip_tos() | yes | yes | some high-priority levels may require superuser capability
transparent | boolean() | yes | yes | requires admin capability
ttl | integer() | yes | yes | none
unblock_source | ip_mreq_source() | yes | no | none

Table 2.4: ip options

Options for level ipv6:

Option Name | Value Type | Set | Get | Other Requirements and comments
addrform | inet | yes | no | allowed only for IPv6 sockets that are connected and bound to a v4-mapped-on-v6 address
add_membership | ipv6_mreq() | yes | no | none
authhdr | boolean() | yes | yes | type = dgram | raw, obsolete?
drop_membership | ipv6_mreq() | yes | no | none
dstopts | boolean() | yes | yes | type = dgram | raw, requires superuser privileges to update
flowinfo | boolean() | yes | yes | type = dgram | raw, requires superuser privileges to update
hoplimit | boolean() | yes | yes | type = dgram | raw. On some platforms (e.g. FreeBSD) is used to set in order to get hoplimit as a control message heeader. On others (e.g. Linux), recvhoplimit is set in order to get hoplimit.
hopopts | boolean() | yes | yes | type = dgram | raw, requires superuser privileges to update
mtu | boolean() | yes | yes | Get: Only after the socket has been connected
mtu_discover | ipv6_pmtudisc() | yes | yes | none
multicast_hops | default | uint8() | yes | yes | none
multicast_if | integer() | yes | yes | type = dgram | raw
multicast_loop | boolean() | yes | yes | none
recverr | boolean() | yes | yes | none
recvhoplimit | boolean() | yes | yes | type = dgram | raw. On some platforms (e.g. Linux), recvhoplimit is set in order to get hoplimit
recvpktinfo | pktinfo | boolean() | yes | yes | type = dgram | raw. On some platforms (e.g. FreeBSD) is used to set in order to get hoplimit as a control message heeader. On others (e.g. Linux), recvhoplimit is set in order to get hoplimit.
recvtclass | boolean() | yes | yes | type = dgram | raw. On some platforms is used to set (=true) in order to get the tclass control message heeader. On others, tclass is set in order to get tclass control message heeader.
router_alert | integer() | yes | yes | type = raw
rthdr | boolean() | yes | yes | type = dgram | raw, requires superuser privileges to update
tclass | integer() | yes | yes | Set the traffic class associated with outgoing packets. RFC3542.
unicast_hops | default | uint8() | yes | yes | none
v6only | boolean() | yes | no | none

Table 2.5: ipv6 options

Options for level tcp:

Option Name | Value Type | Set | Get | Other Requirements and comments
congestion | string() | yes | yes | none
maxseg | integer() | yes | yes | Set not allowed on all platforms.
nodelay | boolean() | yes | yes | none

Table 2.6: tcp options

Options for level udp:

Option Name | Value Type | Set | Get | Other Requirements and comments
cork | boolean() | yes | yes | none

Table 2.7: udp options

Options for level sctp:

Option Name | Value Type | Set | Get | Other Requirements and comments
associnfo | sctp_assocparams() | yes | yes | none
autoclose | non_neg_integer() | yes | yes | none
disable_fragments | boolean() | yes | yes | none
events | sctp_event_subscribe() | yes | no | none
initmsg | sctp_initmsg() | yes | yes | none
maxseg | non_neg_integer() | yes | yes | none
nodelay | boolean() | yes | yes | none
rtoinfo | sctp_rtoinfo() | yes | yes | none

Table 2.8: sctp options