Variable Length Packet

[Marcia Hon]

Hi,

I am using read and send of the socket library. I would like to send packets
that have variable length, and I would like to read these packets. I have
designated the first 2 bytes to state the size of the packet.

So far, my program works when I force the packet size to be the size of the
buffer. However, when I try and change the buffer size to correspond to the
packet size, the receiving end does not work. I try to receive the first two
bytes in order to read the size. This works. However, when I read from the
buffer again to get the rest of the packet it does not work.

Bytes_sent = send(server_socket, sbuffer, *sbuffer, 0);



Where *sbuffer stores the message size, thereby sending the full message.



Let's say that the message is 7 bytes, where the first 2 bytes state the
message size.

On the receiving end I want to read the message size, and then read
according to how many bytes are left. So I have the following:



Bytes_received = read(client_socket, rbuffer, 2);

Size_of_packet = *rbuffer;

Bytes_received += read(client_socket, (rbuffer+2), (Size_of_packet-2));



This code does not work. I am not able to read the packet.



Please help.

Thanks,

Marcia Hon

 

[James Hu]

I am using read and send of the socket library. I would like to send packets
that have variable length, and I would like to read these packets. I have
designated the first 2 bytes to state the size of the packet.

So far, my program works when I force the packet size to be the size of the
buffer. However, when I try and change the buffer size to correspond to the
packet size, the receiving end does not work. I try to receive the first two
bytes in order to read the size. This works. However, when I read from the
buffer again to get the rest of the packet it does not work.

Socket programming is not topical in this newsgroup. You might try
comp.unix.programmer.

However, as a general debugging aid, make sure that the size of the
packet you received matches the size of the packet that you sent.

There are several C related anomalies in your program sample. First,
you did not provide a full description of the types of your variables.
It matters what the types of Size_of_packet, sbuffer, and rbuffer
are, and what the prototypes to send and recv are. I will assume:

int server_socket;
int Bytes_sent;
char *sbuffer;
int send (int, const void *, int, int);

int client_socket;
int Size_of_packet;
int Bytes_received;
char *rbuffer;
int read (int, void *, int);

and that the variables get initialized properly at some point.

Given these assumptions note that *sbuffer is of type char, and is
thus only one byte long. Thus, this code:

Bytes_sent = send(server_socket, sbuffer, *sbuffer, 0);

would send the wrong number of bytes if the size really occupied more
than 255 bytes. One possible solution is:

uint16_t Bytes_to_send;
int Bytes_sent;
/* ... code that initializes Bytes_to_send ... */
memcpy(sbuffer, &Bytes_to_send, sizeof(Bytes_to_send));
/* ... initialize sbuffer+sizeof(Bytes_to_send) with rest of data ... */
Bytes_sent = send(server_socket, sbuffer, Bytes_to_send, 0);

You might want to check to see if Bytes_sent matches Bytes_to_send.

Similar problems occur here:

Bytes_received = read(client_socket, rbuffer, 2);

Size_of_packet = *rbuffer;

Although you read the first two bytes in for the message size, you are
only storing the first byte of rbuffer into Size_of_packet. If you
followed my suggestion above, the the mirror on the receiving end would
be:

uint16_t Bytes_to_recv;
/* ... */
Bytes_received = read(client_socket, rbuffer, sizeof(Bytes_to_recv));
/* ... make sure Bytes_received equals sizeof(Bytes_to_recv) ... */
memcpy(&Bytes_to_recv, rbuffer, sizeof(Bytes_to_recv));
/* ... some code to sanitize the value of Bytes_to_recv ... */
Bytes_received = read(client_socket, rbuffer+sizeof(Bytes_to_recv),
Bytes_to_recv-sizeof(Bytes_to_recv));

Even if all these precautions are taken, your program may still fail if
the receiving side of the program is running on a different machine from
the sending side. Please follow-up to with comp.unix.programmer if you
need further assistance.

-- James