I got a snippet from internet for send data through a socket .
Here is the code .
u32_t nLength = 0;
u32_t nOffset = 0;
do {
nLength = nFullLength - nOffset;
status = Socket->Send(((u8_t*) buff) + nOffset, &nLength);
if (status != ERROR_SUCCESS) {
break;
}
nOffset += nLength;
} while (nOffset < nFullLength);
My doubts are :
When send(sock_fd, buf+bytes, buflen-bytes, flags); function running , it will send the entire data ?
Let's assume i have a buff with 45 byte length . So it will send like
send(buf+0, 45-0) = send(buf+0, 45);
So it will send complete data with length 45 ? what is the use of length here ? initially it will 45 . Isn't ?
Well, no. There's no guarantee that it will send all the data you ask it to send, that's why the code looks the way it does.
The manual page for send() states this pretty clearly:
Return Value
On success, these calls return the number of characters sent. On error, -1
is returned, and errno is set appropriately.
The same is true for e.g. a regular write() to a local file, by the way. It might never happen, but the way the interface is designed you're supposed to handle partial sends (and writes) if they do happen.
TCP is a streaming transport. There is no guarantee that a given send() operation will accept all of the bytes given to it at one time. It depends on the available kernel buffer space, the I/O mode of the socket (blocking vs non-blocking), etc. send() returns the number of bytes it actually accepted and put into the kernel buffer for subsequent transmission.
In the code example shown, it appears that Socket->Send() expects nLength to be initially set to the total number of bytes to sent, and it will then update nLength with the number of bytes actually sent. The code is adjusting its nOffset variable accordingly, looping just in case send() returns fewer bytes than requested, so it can call send() as many times as it takes to send the full number of bytes.
So, for example, lets assume the kernel accepts up to 20 bytes at a time. The loop would call send() 3 times:
send(buf+0, 45-0) // returns 20
send(buf+20, 45-20) // returns 20
send(buf+40, 45-40) // returns 5
// done
This is typical coding practice for TCP programming, given the streaming nature of TCP.
Related
I'm building a multi-client<->server messaging application over TCP.
I created a non blocking server using epoll to multiplex linux file descriptors.
When a fd receives data, I read() /or/ recv() into buf.
I know that I need to either specify a data length* at the start of the transmission, or use a delimiter** at the end of the transmission to segregate the messages.
*using a data length:
char *buffer_ptr = buffer;
do {
switch (recvd_bytes = recv(new_socket, buffer_ptr, rem_bytes, 0)) {
case -1: return SOCKET_ERR;
case 0: return CLOSE_SOCKET;
default: break;
}
buffer_ptr += recvd_bytes;
rem_bytes -= recvd_bytes;
} while (rem_bytes != 0);
**using a delimiter:
void get_all_buf(int sock, std::string & inStr)
{
int n = 1, total = 0, found = 0;
char c;
char temp[1024*1024];
// Keep reading up to a '\n'
while (!found) {
n = recv(sock, &temp[total], sizeof(temp) - total - 1, 0);
if (n == -1) {
/* Error, check 'errno' for more details */
break;
}
total += n;
temp[total] = '\0';
found = (strchr(temp, '\n') != 0);
}
inStr = temp;
}
My question: Is it OK to loop over recv() until one of those conditions is met? What if a client sends a bogus message length or no delimiter or there is packet loss? Wont I be stuck looping recv() in my program forever?
Is it OK to loop over recv() until one of those conditions is met?
Probably not, at least not for production-quality code. As you suggested, the problem with looping until you get the full message is that it leaves your thread at the mercy of the client -- if a client decides to only send part of the message and then wait for a long time (or even forever) without sending the last part, then your thread will be blocked (or looping) indefinitely and unable to serve any other purpose -- usually not what you want.
What if a client sends a bogus message length
Then you're in trouble (although if you've chosen a maximum-message-size you can detect obviously bogus message-lengths that are larger than that size, and defend yourself by e.g. forcibly closing the connection)
or there is packet loss?
If there is a reasonably small amount of packet loss, the TCP layer will automatically retransmit the data, so your program won't notice the difference (other than the message officially "arriving" a bit later than it otherwise would have). If there is really bad packet loss (e.g. someone pulled the Ethernet cable out of the wall for 5 minutes), then the rest of the message might be delayed for several minutes or more (until connectivity recovers, or the TCP layer gives up and closes the TCP connection), trapping your thread in the loop.
So what is the industrial-grade, evil-client-and-awful-network-proof solution to this dilemma, so that your server can remain responsive to other clients even when a particular client is not behaving itself?
The answer is this: don't depend on receiving the entire message all at once. Instead, you need to set up a simple state-machine for each client, such that you can recv() as many (or as few) bytes from that client's TCP socket as it cares to send to you at any particular time, and save those bytes to a local (per-client) buffer that is associated with that client, and then go back to your normal event loop even though you haven't received the entire message yet. Keep careful track of how many valid received-bytes-of-data you currently have on-hand from each client, and after each recv() call has returned, check to see if the associated per-client incoming-data-buffer contains an entire message yet, or not -- if it does, parse the message, act on it, then remove it from the buffer. Lather, rinse, and repeat.
I'm very new to C++, but I'm trying to learn some basics of TCP socket coding. Anyway, I've been able to send and receive messages, but I want to prefix my packets with the length of the packet (like I did in C# apps I made in the past) so when my window gets the FD_READ command, I have the following code to read just the first two bytes of the packet to use as a short int.
char lengthBuffer[2];
int rec = recv(sck, lengthBuffer, sizeof(lengthBuffer), 0);
short unsigned int toRec = lengthBuffer[1] << 8 | lengthBuffer[0];
What's confusing me is that after a packet comes in the 'rec' variable, which says how many bytes were read is one, not two, and if I make the lengthBuffer three chars instead of two, it reads three bytes, but if it's four, it also reads three (only odd numbers). I can't tell if I'm making some really stupid mistake here, or fundamentally misunderstanding some part of the language or the API. I'm aware that recv doesn't guarantee any number of bytes will be read, but if it's just two, it shouldn't take multiple reads.
Because you cannot assume how much data will be available, you'll need to continuously read from the socket until you have the amount you want. Something like this should work:
ssize_t rec = 0;
do {
int result = recv(sck, &lengthBuffer[rec], sizeof(lengthBuffer) - rec, 0);
if (result == -1) {
// Handle error ...
break;
}
else if (result == 0) {
// Handle disconnect ...
break;
}
else {
rec += result;
}
}
while (rec < sizeof(lengthBuffer));
Streamed sockets:
The sockets are generally used in a streamed way: you'll receive all the data sent, but not necessarily all at once. You may as well receive pieces of data.
Your approach of sending the length is hence valid: once you've received the length, you cann then load a buffer, if needed accross successive reads, until you got everything that you expected. So you have to loop on receives, and define a strategy on how to ahandle extra bytes received.
Datagramme (packet oriented) sockets:
If your application is really packet oriented, you may consider to create a datagramme socket, by requesting linux or windows socket(), the SOCK_DGRAM, or better SOCK_SEQPACKET socket type.
Risk with your binary size data:
Be aware that the way you send and receive your size data appers to be assymetric. You have hence a major risk if the sending and receiving between machine with CPU/architectures that do not use the same endian-ness. You can find here some hints on how to ame your code platform/endian-independent.
TCP socket is a stream based, not packet (I assume you use TCP, as to send length of packet in data does not make any sense in UDP). Amount of bytes you receive at once does not have to much amount was sent. For example you may send 10 bytes, but receiver may receive 1 + 2 + 1 + 7 or whatever combination. Your code has to handle that, be able to receive data partially and react when you get enough data (that's why you send data packet length for example).
I have two programs that use socket programming to communicate. Initially I will specify the no. of hops as to how many time they have to exchange messages between each other. Each time it receives a message, it will append its id to it. Hence the string grows in size every time. My program is working fine for 8000 hops, but after it crosses 8000, although program p1 sends a string of length 16388, p2 identifies that there are only 16385 in the socket ready to be read. I use ioctl() to determine the amount of characters ready to recv() in the socket, and then recv it in a char * variable...
Is it because there is a delay in the send () in p1 and recv() in p2 , that p2 identifies only 16385 characters in the socket ?
For ex:
If P1 sends length(16388)
P2 receives only the following length(16385)
Say I'm trying to send you 8 pumpkins. I put 6 of them on the table. You think, "I'm expecting 8 pumpkins, not 6. I'll wait until he puts the last two on the table." I think, "I don't want too many pumpkins 'in flight' at once. I'll wait until he takes 2 of these 6 before I put the last 2 on the table." We're stuck. We're each waiting for the other. We'll wait forever.
You are not permitted to wait until more bytes are received before accepting the bytes that have already been received. The reason for this is simple: No network protocol can allow each side to wait for the other. Since TCP permits the sending side to wait in this context, it cannot permit the receiving side to wait as well.
So accept the bytes as they are received. Don't wait for the other side to send all of them before accepting any of them. Otherwise, what happens if the other side is waiting for you to accept the first one before it sends any more?
You're probably hitting a kernel buffer limit. You can probably increase SO_RCVBUF on the receiver and it will work as you expect: SIOCINQ will eventually return the full size of the unread data.
But you shouldn't do that to ensure proper Functioning. messing with buffers should only be done when you want to tweak performance.
You should restructure the code so that you never have to ask the kernel how many bytes are available. Just read up to a reasonable limit(like 4096) and deal with one application-level message being broken up in multiple pieces. If you need message lengths/boundaries then you MUST implement them yourself on top of TCP.
Here's some silly code to read a message with a length header:
int ret, len = 0, have_read;
have_read = 0;
while (have_read < sizeof(len)) {
// This will likely always return sizeof(len) the first time.
ret = read(fd, ((char*)&len) + have_read, sizeof(len) - have_read);
if (ret <= 0) {
// Handle error.
}
have_read += ret;
}
char* buf = malloc(len);
if (!buf) {
// Handle error.
}
have_read = 0;
while (have_read < len) {
ret = read(fd, buf + have_read, len - have_read);
if (ret <= 0) {
// Handle error.
}
have_read += ret;
}
// Handle message in buf.
I'm trying to get to know how many bytes there are readable at my TCP socket. I am calling ioctl with the Flag "FIONREAD" which should actually give me this value.
When I call the function I get as return val 0 ( so no Error ) but also my integer argument gets the value 0. That would be no problem but when I call the recv() method I actually read some Bytes out of the socket. What am I doing wrong?
// here some Code:
char recBuffer[BUFFERLENGTH] = {0};
int bytesAv = 0;
int bytesRead = 0;
int flags = 0;
if ( ioctl (m_Socket,FIONREAD,&bytesAv) < 0 )
{
// Error
}
if ( bytesAv < 1 )
{
// No Data Available
}
bytesRead = recv(m_Socket,recBuffer,BUFFERLENGTH,flags);
When I call the recv function i acutally read some valid Data ( which I expected )
It's happening very quickly, that's why you don't see anything. What you're doing:
ioctl: Is there data for me ? No, nothing yet
recv: Block until there is data for me. Some (short) time later: Here is your data
So if you really want to see FIONREAD, just wait for it.
/* Try FIONREAD until we get *something* or ioctl fails. */
while (!bytesAv && ioctl (m_Socket,FIONREAD,&bytesAv) >= 0)
sleep(1);
The real answer here is to use select(2) like cnicutar said. Toby, what you aren't understanding is that you have a race condition. First you look at the socket and ask how many bytes are there. Then, while your code is processing the "no data here" block, bytes are being received by the hardware & OS asynchronous to your application. So, by the time that the recv() function is called, the answer of "no bytes are available" is no longer true...
if ( ioctl (m_Socket,FIONREAD,&bytesAv) < 0 )
{ // Error
}
// BYTES MIGHT BE RECEIVED BY HARDWARE/OS HERE!
if ( bytesAv < 1 ) // AND HERE!
{
// No Data Available
// BUT BYTES MIGHT BE RECEIVED BY HARDWARE/OS HERE!
}
// AND MORE BYTES MIGHT BE RECEIVED BY HARDWARE/OS HERE!
bytesRead = recv(m_Socket,recBuffer,BUFFERLENGTH,flags);
// AND NOW bytesRead IS NOT EQUAL TO 0!
Sure, a small sleep probably fixed your program two years ago, but it also taught you terrible coding practice and you lost out on an opportunity to learn how to use sockets correctly by using select().
Further, as Karoly Horvath said, you can tell recv to not read more bytes than you can store in the buffer that the user passed in. Then your function interface becomes "This fn will return as many bytes as are available on the socket, but not more than [buffer size you passed in]".
This means that this function doesn't need to worry about clearing the buffer any more. The caller can call your function as many times as necessary to clear all of the bytes out of it (or you can provide a separate fn that discards the data wholesale and not tie up that functionality in any specific data gather function). Your function is more flexible by not doing too many things. You can then create a wrapper function that is smart to your data transfer needs of a particular application, and that fn calls the get_data fn and the clear_socket fn as needed for that specific app. Now you are building a library you can carry around from project to project, and maybe job to job if you're so lucky as to have an employer that lets you take code with you.
Use select() then ioctl(FIONREAD) then recv()
You're doing nothing wrong, if you are using blocking I/O recv() will block untill the data is available.
The recv() library function man page mention that:
It returns the number of bytes received. It normally returns any data available, up to the requested amount, rather than waiting for receipt of the full amount requested.
If we are using blocking recv() call and requested for 100 bytes:
recv(sockDesc, buffer, size, 0); /* Where size is 100. */
and only 50 bytes are send by the server then this recv() is blocked until 100 bytes are available or it will return receiving 50 bytes.
The scenario could be that:
server crashes after sendign only 50 bytes
bad protocol design where server is only sending 50 bytes while client is expecting 100 and server is also waiting for client's reply (i.e. socket close connection has not been initiated by server in which recv will return)
I am interested on Linux / Solaris platform. I don't have the development environment to check it out myself.
recv will return when there is data in the internal buffers to return. It will not wait until there is 100 bytes if you request 100 bytes.
If you're sending 100 byte "messages", remember that TCP does not provide messages, it is just a stream. If you're dealing with application messages, you need to handle that at the application layer as TCP will not do it.
There are many, many conditions where a send() call of 100 bytes might not be read fully on the other end with only one recv call when calling recv(..., 100); here's just a few examples:
The sending TCP stack decided to bundle together 15 write calls, and the MTU happened to be 1460, which - depending on timing of the arrived data might cause the clients first 14 calls to fetch 100 bytes and the 15. call to fetch 60 bytes - the last 40 bytes will come the next time you call recv() . (But if you call recv with a buffer of 100 , you might get the last 40 bytes of the prior application "message" and the first 60 bytes of the next message)
The sender buffers are full, maybe the reader is slow, or the network is congested. At some point, data might get through and while emptying the buffers the last chunk of data wasn't a multiple of 100.
The receiver buffers are full, while your app recv() that data, the last chunk it pulls up is just partial since the whole 100 bytes of that message didn't fit the buffers.
Many of these scenarios are rather hard to test, especially on a lan where you might not have a lot of congestion or packet loss - things might differ as you ramp up and down the speed at which messages are sent/produced.
Anyway. If you want to read 100 bytes from a socket, use something like
int
readn(int f, void *av, int n)
{
char *a;
int m, t;
a = av;
t = 0;
while(t < n){
m = read(f, a+t, n-t);
if(m <= 0){
if(t == 0)
return m;
break;
}
t += m;
}
return t;
}
...
if(readn(mysocket,buffer,BUFFER_SZ) != BUFFER_SZ) {
//something really bad is going on.
}
The behavior is determined by two things. The recv low water mark and whether or not you pass the MSG_WAITALL flag. If you pass this flag the call will block until the requested number of bytes are received, even if the server crashes. Other wise it returns as soon as at least SO_RCVLOWAT bytes are available in the socket's receive buffer.
SO_RCVLOWAT
Sets the minimum number of bytes to
process for socket input operations.
The default value for SO_RCVLOWAT is
1. If SO_RCVLOWAT is set to a larger value, blocking receive calls normally
wait until they have received the
smaller of the low water mark value or
the requested amount. (They may return
less than the low water mark if an
error occurs, a signal is caught, or
the type of data next in the receive
queue is different than that returned,
e.g. out of band data). This option
takes an int value. Note that not all
implementations allow this option to
be set.
If you read the quote precisely, the most common scenario is:
the socket is receiving data. That 100 bytes will take some time.
the recv() call is made.
If there are more than 0 bytes in the buffer, recv() returns what is available and does not wait.
While there are 0 bytes available it blocks and the granularity of the threading system determines how long that is.