2 Socket Usage
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 sufficent (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.
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 {select, SelectInfo} (SelectInfo contains the SelectRef). When data eventually arrives a 'select message' will be sent to the caller:
- {'$socket', socket(), select, SelectRef}
The caller can then make another call to the recv function and now expect data.
Note that all other users are locked out until the 'current user' has called the function (recv in this case). So either immediately call the function or cancel.
The user must also be prepared to receive an abort message:
- {'$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 = {SelectRef, 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_ref() |
| abort | {select_ref(), Reason :: term()} |
Table 2.1: socket message info value type
The select_ref() is the same as was received in the SelectInfo.
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).
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 | '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. Therefor, 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. Therefor, 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 | 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, recvtclass is set in order to get tclass control message heeader. |
| 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