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Subject: Re: Python recv loop
From: Ihsan Junaidi Ibrahim <ihsan@grep.my>
In-Reply-To: <511847C7.1040008@davea.name>
Date: Mon, 11 Feb 2013 23:02:44 +0800
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References: <B52841CC-778D-41AF-AFCC-05A5F9C26A92@grep.my> <511847C7.1040008@davea.name>
To: Dave Angel <davea@davea.name>
Cc: python-list@python.org
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Xref: csiph.com comp.lang.python:38690

Hi Dave,

On Feb 11, 2013, at 9:22 AM, Dave Angel <davea@davea.name> wrote:

> Exactly how are you sending "hexadecimal" ?  If that 0xad (which is =
only one byte, what about the other 3 ?) is intended to be a C =
description, then it's certainly not hex, it's binary.  And probably =
little-endian, to boot.  That's a mistake, as network protocols almost =
always use big-endian.
>=20
> So what is the range of values for the length, and how are they =
actually encoded?  Are they uint32 in native binary format?   And are =
you permitted to change the encoding, to fix it?  (If it were my choice, =
I'd make it a printable decimal value, first choice, or printable hex, =
second choice.)

They are ASCII stream, actually JSON with the exception of the first 4 =
bytes. I avoided using struct class for this as it's overkill for my =
application.

So the idea is that i code the length to a max of 0xff (max length of =
256 bytes), I would only have to assign 4 bytes to it. If i need 1k =
length, i just need to increase it to 6 bytes or 4 if i decide to strip =
the 0x.

the code for the function is as follows:

def request_get_session(sock, jmsg):
    # append message length
    plen =3D hex(len(jmsg))
    msg =3D '{0}{1}'.format(plen, jmsg)

    print 'sending data'
    n =3D sock.send(msg)
    str =3D '{0} bytes sent: {1}'.format(n, msg)
    print str

    # receive message length
    print 'receiving data'
    mlen =3D sock.recv(4)
    try:
        nbuf =3D int(mlen, 16)
    except ValueError as e:
        print 'invalid length type'
        return -1

    print 'message length is {0}'.format(nbuf)

    while True:
        buf =3D sock.recv(nbuf)

        if not buf:
            break

    slen =3D len(buf)
    str =3D "{0} bytes received: {1}".format(slen, buf)
    print str
    return 0
>=20
>>=20
>> I've managed to receive and translate the message length until I =
reach my second recv which I readjusted the buffer size to include the =
new message length.
>>=20
>> However that failed and recv received 0 bytes. I implemented the same =
algorithm on the server side using C and it work so appreciate if you =
can help me on this.
>>=20
>> # receive message length
>>     print 'receiving data'
>>     mlen =3D sock.recv(4)
>>     try:
>>         nbuf =3D int(mlen, 16)
>=20
> That supposes the count is being sent as a printable string of hex =
digits.  That's not what I concluded above.

That is what I meant. it just an ascii string.