Am back with C sockets question.
I am trying to receive a char array from a client via TCP. I have the following recv code:
char buffer[2048];
int recvBytes = 0;
while (1) {
if ((clntSockfd = accept(servSockfd, (struct sockaddr *) &clntAddr, &clntSockAddrLen)) == -1) {
printf("Unable to accept client connection\n");
exit(1);
}
else {
printf("Client connection accepted!\n");
}
/* clear buffer */
memset(buffer, 0, sizeof(buffer));
recvBytes = recv (clntSockfd, buffer, sizeof(buffer), 0);
printf("Bytes received: %i\n", recvBytes);
printf("Char array received: %s\n", buffer);
}
I noted that upon execution, recvBytes shows zero in the first iteration. It's only after the second iteration that it picks up the incoming char array.
I would like to find out will recv block until I receive data from the incoming socket? How should I discard the unwanted chars in the buffer - i.e. drop the chars received from the first iteration.
Many thanks!
-- edit ---
Hi all, I have appended the console output from eclipse.
**... <- initialization stuff
Binding socket to port
Socket binded
Start listening on port 8081.
Client connection accepted! *while loop - first iteration
Bytes received: 0 *while loop - first iteration
Char array received: *while loop - first iteration
Client connection accepted! *while loop - second iteration
Bytes received: 22 *while loop - second iteration
Char array received: 1~Message~HowAreYou? *while loop - second iteration**
Hope the above console output illustrates my questions clearer, regarding the first and second iterations. From the above, the first iteration is not receiving any bytes but recv does not block. I thought recv was supposed to block until it receives bytes?
I believe the recv only returns 0 if the remote end closes the connection (for stream sockets such as TCP) or if a packet with no payload is received (datagram sockets such as UDP).
If the socket has been set to non-blocking mode (default is blocking), and there's no data yet, you'd get an error code, either EAGAIN or EWOULDBLOCK depending on the OS.
In any case, each time recv returns, the new data will be stored at the beginning of the buffer. It won't append to previously received data unless you calculate a pointer designed to store new data after the old.
If your socket is a blocking socket, then recv function will block till you get some data. recv function will return 0 only when the other side closes the connection on most systems.
I suggest you recheck your code and execution and let us know the exact output.
Refer here and here for more details.
If you know what to expect of the data that you receive you can drop the first set of characters. For example if you know what will the length of the packet be or the source of the message or the header information then you can validate and ignore the first set of characters.
Related
I'm trying to get a server to receive messages from a client in TCP.
The problem is, I only receive the messages on the server side once I close the socket on the client side.
Here is the read function on the server side:
char *read_socket(int fd){
int bytesRcvd, aux;
char *buffer=(char*)malloc(BUFFSIZE*sizeof(char));
bytesRcvd=read(fd, buffer , BUFFSIZE);
aux=bytesRcvd;
while(bytesRcvd>0){
if((bytesRcvd = read(fd, &buffer[aux], BUFFSIZE))<0){
printf("read() failed!: %s\n", strerror(errno));
exit(1);
}
aux+=bytesRcvd;
}
return &buffer[0];
}
I know (by printfs) that it gets stuck on the line:
bytesRcvd = read(fd, &buffer[aux], BUFFSIZE)
Any help will be greatly appreciated.
Your program is reading the data in the line:
bytesRcvd = read(fd, &buffer[aux], BUFFSIZE)
Your while loop receives the data that the client sends. If the client doesn't have data to send read will block until the connection is closed where read will return 0 and you will exit from while loop.
This is the reason that you think that the data are send in the end (when connection is closed). This is not right, if you print the data you read in the while loop you will see them immediately and not all of them in the end.
Though you can't return the data before the connection is closed due to read() blocking.
For one client your program may seems ok if you don't have the problem of getting data at the end (as I said you could just print them inside while loop) but imagine have two or more clients then you would firstly close connection with first client to go on and read data from second and so on.
One solution(to both problems) is to use select() system call, this will go on to read only if there are data.( Though select() is not safe when using fork()- doesn't guarantee that a parent of child process will not block in read() ).
I am sending a file through TCP, and have the server sending a message containing "END_OF_MESSAGE" to alert the client that they have received the whole file and can close the socket. The file is being sent, and the client receives the "END_OF_MESSAGE" string, however, when I use strcmp to compare the received information to "END_OF_MESSAGE", it never says that they match. I have tried strncmp and memcmp but am confused as to why strcmp does not tell me the strings match.
Code snippets:
Server:
char endMessage[MESSAGESIZE] = "END_OF_MESSAGE";
if ((send(clntSocket, endMessage, sizeof endMessage, 0))!= sizeof endMessage) DieWithError("Sending failed");
The above code snippet does get sent.
Client:
if ((bytesRcvd = recv(sock, echoBuffer, RCVBUFSIZE - 1, 0)) <= 0)
DieWithError("recv() failed or connection closed prematurely");
totalBytesRcvd += bytesRcvd; /* Keep tally of total bytes */
echoBuffer[bytesRcvd] = '\0'; /* Terminate the string! */
if (!(strcmp(echoBuffer, "END_OF_MESSAGE")==0)){
printf(echoBuffer); /* Print the echo buffer */
printf("\n");
}else{
break; //break out of while loop
}
the strcmp of the echoBuffer and "END_OF_MESSAGE" never returns 0, even though "END_OF_MESSAGE" is what I am sending from the server..I have tried strncmp to compare the first 3 characters ("END") to no avail.
Note: when I print out the echoBuffer, the very last one does print out END_OF_MESSAGE which is just adding to my confusion.
Does anyone have any insights into what I am doing wrong?
Thank you.
am sending a file through TCP, and have the server sending a message containing "END_OF_MESSAGE" to alert the client that they have received the whole file and can close the socket.
Why? Just close the socket. That will tell the client exactly the same thing..
What you're attempting is fraught with difficulty. What happens if the file contains END_OF_MESSAGE? You're going to need an escape convention, and an escape for the escape, and inspect all the data when both sending and receiving.
The actual problem that you're seeing is that END_OF_MESSAGE can arrive along with the last bit of the file, so it isn't at the start of the buffer.
But it's all pointless. Just close the socket.
I am working on an program for school and having some issues with sockets. I have pasted the write and read commands from my program below since I think these are the problem. The program should take the plaintext file and encrypt it using the key provided.
MY PROBLEM: When I execute the program using "client [plaintext] [key] [port]" the program returns "Reading data from client -- 140 bytes" and then just hangs. I can hit ctrl-c and the program prints the correct output for ptext and ktext and that 37 bytes were sent back to the client (which is the correct number of bytes). I feel like the encrypted text should print as well but it does not.
TWO QUESTIONS:
1) Why does the program hang?
2) Why does it seem like data is written from the server to the client but the client does not read any of the data?
Thank you in advance for any help you can offer.
CLIENT
n = write(sockfd,ptext,strlen(ptext));
bzero(crypt_text, BUF_MAX);
bzero(buffer, BUF_MAX);
while((n = read(sockfd,buffer,BUF_MAX))>0){
printf("Reading data from Server -- %d bytes\n",n);
strcat(crypt_text, buffer);
bzero(buffer,BUF_MAX);
}
if (n < 0){
error("ERROR reading from socket");
}
printf("%s", crypt_text);
SERVER
while((n = read(newsockfd,buffer,512))>0){
printf("Reading data from client -- %d bytes\n",n);
strcat(full_text, buffer);
bzero(buffer,BUF_MAX);
}
if (n < 0){
error("ERROR reading from socket");
}
bzero (ptext,BUF_MAX);
bzero (ktext, BUF_MAX);
strcpy(ptext, strtok(full_text,"["));
strcpy(ktext, strtok(NULL, "["));
printf("ptext length ==%s %d\n\n",ptext,strlen(ptext)); //Prints the correct plain text
printf("ktext length ==%s %d\n\n",ktext,strlen(ktext)); //prints the correct key
crypt_text = encrypt(ptext, ktext);
n = write(newsockfd,crypt_text,strlen(crypt_text));
printf("WRITE TO CILENT ==== %d",n); //This returns the correct number of bytes that should be sent back to client
if (n < 0){
error("ERROR writing to socket");
}
As is, your client and server will always hang waiting for each other. This is because read() blocks by default until new data is available to fetch from the file (in this case, a socket).
Look carefully at the code:
The client writes once into the socket before entering the read loop
The server only reads from the socket (well, further down there is a write(), but it will never reach it). The first time the loop runs on the server, it will read the data that the client initially wrote into the socket.
The server processes the data it just read and concatenates it to full_text. Then it goes back to the loop condition, where it calls read() again. read() blocks because there is nothing else to read from the socket at this point.
The client enters a similar loop where it attempts to read from the socket, expecting messages from the server.
At this point, both the server and the client are blocked waiting for messages from each other, which will never happen.
Tu put it another way: you only wrote to the socket once, and somehow you expect to read it multiple times.
You have to rethink your design. Go back to the problem description, work your way through a simple protocol, dry run it on paper, and then implement it - that's how it's done in the real world :)
There are other bugs in your code. For example you write this:
strcat(full_text, buffer);
But buffer is not NUL terminated. n bytes have been read, the rest of the buffer is indeterminate. You should set a '\0' byte at offset n and only try reading BUF_MAX-1 bytes to keep a byte available for all cases and do this:
buffer[n] = '\0';
strcat(full_text, buffer);
Furthermore, you do not test if there is enough room available in full_text for the n+1 bytes strcat will copy at the end.
On another front, packets can be sliced and diced into chunked of different sizes when received by the server. Buffering is required to ensure a reliable client / server communication. To enable this buffering, you need to devise a protocol to determine when a packet has been fully received: a simple protocol is to transmit lines terminated by '\n'.
I read in MSDN about the send() and recv() function, and there is one thing that I'm not sure I understand.
If I send a buffer of size 256 for example, and receive first 5 bytes, so the next time I call the recv() function, it will point to the 6th byte and get the data from there?
for example :
char buff[256];
memcpy(buff,"hello world",12);
send(sockfd, buffer, 100) //sending 100 bytes
//server side:
char buff[256];
recv(sockfd, buff, 5) // now buffer contains : "Hello"?
recv(socfd, buff,5) // now I ovveride the data and the buffer contains "World"?
thanks!
The correct way to receive into a buffer in a loop from TCP in C is as follows:
char buffer[8192]; // or whatever you like, but best to keep it large
int count = 0;
int total = 0;
while ((count = recv(socket, &buffer[total], sizeof buffer - total, 0)) > 0)
{
total += count;
// At this point the buffer is valid from 0..total-1, if that's enough then process it and break, otherwise continue
}
if (count == -1)
{
perror("recv");
}
else if (count == 0)
{
// EOS on the socket: close it, exit the thread, etc.
}
You have missed the principal detail - what kind of socket is used and what protocol is requested. With TCP, data is octet granulated, and, yes, if 256 bytes was sent and you have read only 5 bytes, rest 251 will wait in socket buffer (assuming buffer is larger, which is true for any non-embedded system) and you can get them on next recv(). With UDP and without MSG_PEEK, rest of a single datagram is lost, but, if MSG_PEEK is specified, next recv() will give the datagram from the very beginning. With SCTP or another "sequential packet" protocol, AFAIK, the same behavior as with UDP is got, but I'm unsure in Windows implementation specifics.
With the following pseudo-Python script for sending data to a local socket:
s = socket.socket(AF_UNIX, SOCK_STREAM)
s.connect("./sock.sock")
s.send("test\n")
s.send("aaa\0")
s.close()
My C program will randomly end up recving the following buffers:
test\n
test\n<random chars>
test\naaa (as expected)
The socket is being recv()'d after select() points that the socket is readable. Question is, how to avoid the first two cases?
And side question: Is it possible to send the following two messages from that script:
asd\0
dsa\0
And have select() to show the socket as readable on each of those sends, or will it only do that if I run the script again (restarting the socket client connection) and sending a message for each connect?
At a guess, the len argument to recv specifies a maximum amount of data to read, not the precise amount to be returned. recv is free to return any amount of data up to len bytes instead.
If you want to read a specific number of bytes, call recv in a loop.
int bytes = 0;
while (bytes < len) {
int remaining = len - bytes;
int read = recv(sockfd, buf+bytes, remaining, 0);
if (read < 0) {
// error
break;
}
bytes += read;
}
As noted by junix, if you'll need to send unpredictable amounts of data, consider defining a simple protocol that either starts each message with a note of its length or ends with a particular byte or sequence of bytes.