fails to parse from string

[Brad Lira]

I have a c++ client/server application that sends messages using
protocol buffers, however, at the server side, when i call

address_book.ParseFromString(mystr)

it returns false, but it actually gets the message correctly from
client side, so i am not sure why it thinks that parsing has failed.
any ideas?

thanks,
brad

[Evan Jones]

On Nov 9, 2010, at 16:11 , Brad Lira wrote:
> it returns false, but it actually gets the message correctly from
> client side, so i am not sure why it thinks that parsing has failed.
> any ideas?

How are you putting data into mystr? Protocol buffers contain null
bytes, so you must pass both a char* and a length:

mystr.assign(data, length);

Evan

--
Evan Jones
http://evanjones.ca/

[Kenton Varda]

It sounds like you probably have extra bytes at the end of mystr which are not part of the protobuf.  The parser parses everything before those bytes just fine, but then chokes when it gets to the bytes it doesn't recognize.  Please make sure you only pass in the exact bytes which came out of the serializer at the other end, no more, no less.

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[Brad Lira]

client side:

address_book.SerializeToString(&mystr)
strncpy(buf, mystr.c_str(), strlen(mystr.c_str()));
sendto(socket, buf, ....)

server side:

recvfrom(socket, buf, ....)
mystr.assign(buf, strlen(buf));

if (address_book.ParseFromString(mystr) == false)
{
print "deserialization failed"
}

I get deserialization failed all the time, but get the message correctly though.

Maybe this method is not the right way to send string across socket.
I tried using SerializeToFileDescriptor(socket), that worked on the
client side, but on the server side, i never get the message with UDP sockets.
is there a better way of sending data across network?

thanks,

[Evan Jones]

Brad Lira wrote:
> address_book.SerializeToString(&mystr)
> strncpy(buf, mystr.c_str(), strlen(mystr.c_str()));

strlen will return a shorter length than the real length, due to null
characters. Use mystr.size()

> Maybe this method is not the right way to send string across socket.
> I tried using SerializeToFileDescriptor(socket), that worked on the
> client side, but on the server side, i never get the message with UDP sockets.
> is there a better way of sending data across network?

You probably want to use TCP sockets, since it provides retransmissions
for you. Also, you'll need to prepend a length. See:

http://code.google.com/apis/protocolbuffers/docs/techniques.html#streaming

Or search the group archives for threads such as:

http://groups.google.com/group/protobuf/browse_thread/thread/3af587ab16132a3f

Good luck,

[Brad Lira]

thanks,

yes it was the null character, on the server side when copying buffer
into string, i had add 1 to the
size of the buffer (i guess for the null), then the parsing was ok
with no error.

[Henner Zeller]

On Wed, Nov 10, 2010 at 08:23, Brad Lira <snmp....@gmail.com> wrote:
> client side:
>
> address_book.SerializeToString(&mystr)
> strncpy(buf, mystr.c_str(), strlen(mystr.c_str()));

This is an error - you only copy to the first \0 byte (strlen looks
for a nul-terminated string) -- however, the string contains binary
data.

And: why do you copy the data in the first place to some buffer ? This
is an additional (potentially expensive) memory copy; why not use the
buffer directly from the string ?

sendto(socket, mystr.data(), mystr.size() ...)

[Evan Jones]

On Nov 10, 2010, at 14:13 , Brad Lira wrote:
> yes it was the null character, on the server side when copying buffer
> into string, i had add 1 to the
> size of the buffer (i guess for the null), then the parsing was ok
> with no error.

Just adding 1 is still probably not correct. You have similar
incorrect code on the receive side:

> recvfrom(socket, buf, ....)
> mystr.assign(buf, strlen(buf));

strlen(buf) is not going to give you the right thing. You should be
using the return value from recvfrom(), which gives you the number of
bytes that were read from the network.

Note: If you are using UDP, it will end up not working as soon as you
have a message which is bigger than either your buffer, or the maximum
UDP packet size, whichever comes first.

[Brad Lira]

hmmm, i have to use UDP in my case, TCP is not an option.