how to read from a TCP socket line by line

[unknown]

Hello.

How does one do when there's the need to read from a TCP socket line by
line? I've been looking through the docs but found nothing.

Something like this would be nice:

{ok, Socket} = gen_tcp:connect(...),
Line = io:get_line(Socket, ...)

Maybe I should look at how the inets webserver parses HTTP headers
(which BTW is exactly what I want to do, except on the client side).

[unknown]

Post by unknown
Hello.
How does one do when there's the need to read from a TCP socket line by
line? I've been looking through the docs but found nothing.
{ok, Socket} = gen_tcp:connect(...),
Line = io:get_line(Socket, ...)
Maybe I should look at how the inets webserver parses HTTP headers
(which BTW is exactly what I want to do, except on the client side).

You can use the [{packet, line}] option in gen_tcp:connect/3.

For example:

1> gen_tcp:connect("mail", 25, [{packet, line}, {active, true}]).
{ok,#Port<0.8>}
2> flush().
Shell got {tcp,#Port<0.8>,
"220 sharks.alteon.com ESMTP Sendmail 8.11.3/8.11.3; Tue, 14 Aug 2001 06:30:31 -0700 (PDT)\r\n"}
ok

I think this was added somewhat recently, but it looks like it's there
in R7B.

Cheers,
Luke

[unknown]

Post by unknown
You can use the [{packet, line}] option in gen_tcp:connect/3.
...
I think this was added somewhat recently, but it looks like it's
there in R7B.

Ah yes, now I see it in the docs, but they say:
"line mode, a packet is a line terminated with newline, lines longer
than the receive buffer are truncated"

What does the last part mean? If "truncated" means what I think it
means, how do I ensure my buffer is large enough when reading binary
data?

Note that after reading the HTTP response header line by line I should
switch to buffer mode (using gen_tcp:recv/3), since the response body
may be binary data.

In Modula-3 I've solved this using "read one line" and "read one
buffer" methods on the same reader. But in Erlang it seems I either
open the socket in active mode and get lines as messages, or open the
socket in passive mode and get buffers.

[unknown]

Post by unknown
"line mode, a packet is a line terminated with newline, lines longer
than the receive buffer are truncated"
What does the last part mean? If "truncated" means what I think it
means, how do I ensure my buffer is large enough when reading binary
data?

You can use the {buffer, ByteSize} option when opening the socket to
say how long the buffer should be. If a line is longer than that, then
you'll get it in several parts (only the last containing a
newline). So for long lines maybe you'll need to concatenate them
yourself, I'm not sure if there's a practical limit on the socket
buffer size.

Post by unknown
Note that after reading the HTTP response header line by line I should
switch to buffer mode (using gen_tcp:recv/3), since the response body
may be binary data.
In Modula-3 I've solved this using "read one line" and "read one
buffer" methods on the same reader. But in Erlang it seems I either
open the socket in active mode and get lines as messages, or open the
socket in passive mode and get buffers.

You can do packet-based reads both as messages and with
gen_tcp:recv. The trick with recv is to have the socket in packet
mode, and pass a length of 0.

Here's an example, using a buffer size too small to get the whole
line:

13> {ok, S} = gen_tcp:connect("mail.bluetail.com", 25, [{active, false}, {packet, line}, {buffer, 40}]).
{ok,#Port<0.10>}
14> gen_tcp:recv(S, 0).
{ok,"220 mail.bluetail.com ESMTP BLUETAIL Mai"}
15> gen_tcp:recv(S, 0).
{ok,"l Robustifier (2.2.2/3.1.2); Wed, 15 Aug"}
16> gen_tcp:recv(S, 0).
{ok," 2001 17:16:59 +0200\r\n"}

When you've finished reading the headers, you can take the socket out
of line mode with:

inet:setopts(S, [{packet, raw}])

Then you can read the body.

There are also some nifty tricks in the inet module:

You can find out the default/current settings for a socket, e.g:

32> {ok, S} = gen_tcp:connect("mail.bluetail.com", 25, []).
{ok,#Port<0.14>}
33> inet:getopts(S, [buffer, active, packet]).
{ok,[{buffer,1024},{active,true},{packet,0}]}

If you want to see all options, then you can call inet:options() to
get the complete list and pass that in for the second argument of
getopts/2.

Another nice thing is inet:i() which is like a "netstat" for erlang
showing all open sockets:

35> inet:i().
Port Module Recv Sent Owner Local Address Foreign Address State
13 inet_tcp 0 0 <0.54.0> 192.168.128.43:4371 192.168.128.251:smtp CONNECTED
14 inet_tcp 102 0 <0.54.0> 192.168.128.43:4373 192.168.128.251:smtp CONNECTED
Port Module Recv Sent Owner Local Address Foreign Address State
ok

[unknown]

Post by unknown
Maybe I should look at how the inets webserver parses HTTP headers
(which BTW is exactly what I want to do, except on the client side).

In this case, you should check out
http://www.erlang.org/contrib/www_tools-1.0.tgz

CU, SN
------
The UNIX equivalent of the "Blue Screen of Death" would be called
"kernel panic". It obviously exists, since I have heard and read about it,
but I've never been witness to it in my professional career.
John Kirch,
Networking Consultant and Microsoft Certified Professional (Windows NT)