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I am creating a server/client TCP in C.
The idea is for the server to send a relatively large amount of information. However, the buffer in the client has a size of only 512 (I don't want to increase this size), and obviously, the information sent by the server is larger than this. Let's imagine 812 bytes.
What I want to do is, in the client, read 512 bytes, print them on the client's console, and then read the remaining bytes, and print them as well.
Here's what should happen:
1) Create server, and block in the read() system call (waiting for the client to write something);
2) Create the client, and write something in the socket, and then blocks on read(), waiting for the server to respond;
3) The server's read() call returns, and now server has to send that large amount of data, using the following code (after creating a new process):
dup2(new_socketfd, STDOUT_FILENO); // Redirect to socket
execlp("/application", "application", NULL); // Application that prints the information to send to the client
Let's imagine "application" printed 812 bytes of data to the socket.
4) Now the client has to read 812 bytes, with a buffer size of 512. That's my problem.
How can I approach this problem? I was wondering if I could make a loop, and read until there's nothing to read, 512 by 512 bytes. But as soon as there's nothing to read, client will block on read().
Any ideas?
recv will block when there is no data in the stream. Any data extracted from the stream, the length is returned from recv.
You can write a simple function to extract the full data just by using an offset variable and checking the return value.
A simple function like this will do.
ssize_t readfull(int descriptor,char* buffer, ssize_t sizetoread){
ssize_t offset = 0;
while (offset <sizetoread) {
ssize_t read = recv(descriptor,buffer+offset,sizetoread-offset,0);
if(read < 1){
return offset;
}
offset+=read;
}
return offset;
}
Also servers typically send some kind of EOF when the data is finished. Either the server might first send the length of the message to be read which is a constant size either four or eight bytes, then it sends the data so you know ahead of time how much to read. Or, in the case of HTTP for example, there is the content-length field as well as the '\r\n' delimeters.
Realistically there is no way to know how much data the server has available to send you, it's impractical. The server has to tell you how much data there is through some kind of indicator.
Since you're writing the server yourself, you can first send a four byte message which can be an int value of how much data the client should read.
So your server can look like this:
int sizetosend = arbitrarysize;
send(descriptor,(char*)&sizetosend,sizeof(int),0);
send(descriptor,buffer,sizetosend,0);
Then on your client side, read four bytes then the buffer.
int sizetoread = 0;
ssize_t read = recv(descriptor,(char*)&sizetoread,sizeof(int),0);
if(read < 4)
return;
//Now just follow the code I posted above
This is for a Linux system, in C. It involves network programming. It is for a file transfer program.
I've been having this problem where this piece of code works unpredictably. It either is completely successful, or the while loop in the client never ends. I discovered that this is because the fileLength variable would sometimes be a huge (negative or positive) value, which I thought was attributed to making some mistake with ntohl. When I put in a print statement, it seemed to work perfectly, without error.
Here is the client code:
//...here includes relevant header files
int main (int argc, char *argv[]) {
//socket file descriptor
int sockfd;
if (argc != 2) {
fprintf (stderr, "usage: client hostname\n");
exit(1);
}
//...creates socket file descriptor, connects to server
//create buffer for filename
char name[256];
//recieve filename into name buffer, bytes recieved stored in numbytes
if((numbytes = recv (sockfd, name, 255 * sizeof (char), 0)) == -1) {
perror ("recv");
exit(1);
}
//Null terminator after the filename
name[numbytes] = '\0';
//length of the file to recieve from server
long fl;
memset(&fl, 0, sizeof fl);
//recieve filelength from server
if((numbytes = recv (sockfd, &fl, sizeof(long), 0)) == -1) {
perror ("recv");
exit(1);
}
//convert filelength to host format
long fileLength = ntohl(fl);
//check to make sure file does not exist, so that the application will not overwrite exisitng files
if (fopen (name, "r") != NULL) {
fprintf (stderr, "file already present in client directory\n");
exit(1);
}
//open file called name in write mode
FILE *filefd = fopen (name, "wb");
//variable stating amount of data recieved
long bytesTransferred = 0;
//Until the file is recieved, keep recieving
while (bytesTransferred < fileLength) {
printf("transferred: %d\ntotal: %d\n", bytesTransferred, fileLength);
//set counter at beginning of unwritten segment
fseek(filefd, bytesTransferred, SEEK_SET);
//buffer of 256 bytes; 1 byte for byte-length of segment, 255 bytes of data
char buf[256];
//recieve segment from server
if ((numbytes = recv (sockfd, buf, sizeof buf, 0)) == -1) {
perror ("recv");
exit(1);
}
//first byte of buffer, stating number of bytes of data in recieved segment
//converting from char to short requires adding 128, since the char ranges from -128 to 127
short bufLength = buf[0] + 128;
//write buffer into file, starting after the first byte of the buffer
fwrite (buf + 1, 1, bufLength * sizeof (char), filefd);
//add number of bytes of data recieved to bytesTransferred
bytesTransferred += bufLength;
}
fclose (filefd);
close (sockfd);
return 0;
}
This is the server code:
//...here includes relevant header files
int main (int argc, char *argv[]) {
if (argc != 2) {
fprintf (stderr, "usage: server filename\n");
exit(1);
}
//socket file descriptor, file descriptor for specific client connections
int sockfd, new_fd;
//...get socket file descriptor for sockfd, bind sockfd to predetermined port, listen for incoming connections
//...reaps zombie processes
printf("awaiting connections...\n");
while(1) {
//...accepts any incoming connections, gets file descriptor and assigns to new_fd
if (!fork()) {
//close socket file discriptor, only need file descriptor for specific client connection
close (sockfd);
//open a file for reading
FILE *filefd = fopen (argv[1], "rb");
//send filename to client
if (send (new_fd, argv[1], strlen (argv[1]) * sizeof(char), 0) == -1)
{ perror ("send"); }
//put counter at end of selected file, and find length
fseek (filefd, 0, SEEK_END);
long fileLength = ftell (filefd);
//convert length to network form and send it to client
long fl = htonl(fileLength);
//Are we sure this is sending all the bytes??? TEST
if (send (new_fd, &fl, sizeof fl, 0) == -1)
{ perror ("send"); }
//variable stating amount of data unsent
long len = fileLength;
//Until file is sent, keep sending
while(len > 0) {
printf("remaining: %d\ntotal: %d\n", len, fileLength);
//set counter at beginning of unread segment
fseek (filefd, fileLength - len, SEEK_SET);
//length of the segment; 255 unless last segment
short bufLength;
if (len > 255) {
len -= 255;
bufLength = 255;
} else {
bufLength = len;
len = 0;
}
//buffer of 256 bytes; 1 byte for byte-length of segment, 255 bytes of data
char buf[256];
//Set first byte of buffer as the length of the segment
//converting short to char requires subtracting 128
buf[0] = bufLength - 128;
//read file into the buffer starting after the first byte of the buffer
fread(buf + 1, 1, bufLength * sizeof(char), filefd);
//Send data too client
if (send (new_fd, buf, sizeof buf, 0) == -1)
{ perror ("send"); }
}
fclose (filefd);
close (new_fd);
exit (0);
}
close (new_fd);
}
return 0;
}
Note: I've simplified the code a bit, to make it clearer I hope.
Anything beginning with //... represents a bunch of code
You seem to be assuming that each send() will either transfer the full number of bytes specified or will error out, and that each one will will pair perfectly with a recv() on the other side, such that the recv() receives exactly the number of bytes sent by the send() (or error out), no more and no less. Those are not safe assumptions.
You don't show the code by which you set up the network connection. If you're using a datagram-based protocol (i.e. UDP) then you're more likely to get the send/receive boundary matching you expect, but you need to account for the possibility that packets will be lost or corrupted. If you're using a stream-based protocol (i.e. TCP) then you don't have to be too concerned with data loss or corruption, but you have no reason at all to expect boundary-matching behavior.
You need at least three things:
An application-level protocol on top of the network-layer. You've got parts of that already, such as in how you transfer the file length first to advise the client about much content to expect, but you need to do similar for all data transferred that are not of pre-determined, fixed length. Alternatively, invent another means to communicate data boundaries.
Every send() / write() that aims to transfer more than one byte must be performed in a loop to accommodate transfers being broken into multiple pieces. The return value tells you how many of the requested bytes were transferred (or at least how many were handed off to the network stack), and if that's fewer than requested you must loop back to try to transfer the rest.
Every recv() / read() that aims to transfer more than one byte must be performed in a loop to accommodate transfers being broken into multiple pieces. I recommend structuring that along the same lines as described for send(), but you also have the option of receiving data until you see a pre-arranged delimiter. The delimiter-based approach is more complicated, however, because it requires additional buffering on the receiving side.
Without those measures, your server and client can easily get out of sync. Among the possible results of that are that the client interprets part of the file name or part of the file content as the file length.
Even though you removed it from that code I'll make an educated guess and assume that you're using TCP or some other stream protocol here. This means that the data that the servers sends is a stream of bytes and the recv calls will not correspond in the amount of data they get with the send calls.
It is equally legal for your first recv call to just get one byte of data, as it is to get the file name, file size and half of the file.
You say
When I put in a print statement,
but you don't say where. I'll make another educated guess here and guess that you did it on the server before sending the file length. And that happened to shake things enough that the data amounts that were sent on the connection just accidentally happened to match what you were expecting on the client.
You need to define a protocol. Maybe start with a length of the filename, then the filename, then the length of the file. Or always send 256 bytes for the filename regardless of how long it is. Or send the file name as a 0-terminated string and try to figure out the data from that. But you can never assume that just because you called send with X bytes that the recv call will get X bytes.
I believe the issue is actually a compound of everything you and others have said. In the server code you send the name of the file like this:
send (new_fd, argv[1], strlen (argv[1]) * sizeof(char), 0);
and receive it in the client like this:
recv (sockfd, name, 255 * sizeof (char), 0);
This will cause an issue when the filename length is anything less than 255. Since TCP is a stream protocol (as mentioned by #Art), there are no real boundaries between the sends and recvs, which can cause you to receive data in odd places where you are not expecting them.
My recommendation would be to first send the length of the filename, eg:
// server
long namelen = htonl(strlen(argv[1]));
send (new_fd, &namelen, 4, 0);
send (new_fd, argv[1], strlen (argv[1]) * sizeof(char), 0);
// client
long namelen;
recv (sockfd, &namelen, 4, 0);
namelen = ntohl(namelen);
recv (sockfd, name, namelen * sizeof (char), 0);
This will ensure that you are always aware of exactly how long your filename is and makes sure that you aren't accidentally reading your file length from somewhere in the middle of your file (which is what I expect is happening currently).
edit.
Also, be cautious when you are sending sized numbers. If you use the sizeof call on them, you may be sending and receiving different sizes. This is why I hard-coded the sizes in the send and recv for the name length so that there is no confusion on either side.
Well, after some testing, I discovered that the issue causing the problem did have something to do with htonl(), though I had still read the data incorrectly in the beginning. It wasn't that htonl() wasn't working at all, but that I didn't realize a 'long' has different lengths depending on system architecture (thanks #tofro). That is to say the length of a 'long' integer on 32-bit and 64-bit operating systems is 4 bytes and 8 bytes, respectively. And the htonl() function (from arpa/inet.h) for 4-byte integers. I was using a 64-bit OS, which explains why the value was being fudged. I fixed the issue by using the int32_t variable (from stdint.h) to store the file length. So the main issue in this case was not that it was becoming out of sync (I think). But as for everyone's advice towards developing an actual protocol, I think I know what exactly you mean, I definitely understand why it's important, and I'm currently working towards it. Thank you all for all your help.
EDIT: Well now that it has been several years, and I know a little more, I know that this explanation doesn't make sense. All that would result from long being larger than I expected (8 bytes rather than 4) is that there's some implicit casting going on. I used sizeof(long) in the original code rather than hardcoding it to assume 4 bytes, so that particular (faulty) assumption of mine shouldn't have produced the bug I saw.
The problem is almost certainly what everyone else said: one call to recv was not getting all of the bytes representing the file length. At the time I doubted this was the real cause of the behaviour I saw, because the file name (of arbitrary length) I was sending through was never partially sent (i.e. the client always created a file of the correct filename). Only the file length was messed up. My hypothesis at the time was that recv mostly respected message boundaries, and while recv can possibly only send part of the data, it was more likely that it was sending it all and there was another bug in my code. I now know this isn't true at all, and TCP doesn't care.
I'm a little curious as to why I didn't see other unexpected behaviour as well (e.g. the file name being wrong on the receiving end), and I wanted to investigate further, but despite managing to find the files, I can't seem to reproduce the problem now. I suppose I'll never know, but at least I understand the main issue here.
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 am running into problems when I try to transfer a file between the simple server and client applications that I have written. The file gets transferred successfully, but the file size is different at the receiving side (server side).
I open the file on the client side, use fseek() to find the size of the file. Then I use fread() to to read it into a buffer of char type. I send this buffer using sendto() as I have to use UDP sockets.
On the server side, I use recvfrom() to store this and use fwrite() to write it into another file. But when I check the size of the file, it is much bigger. Also I am not able to open it even though it is supposed to be a text file.
Can you give me some pointers as to where I might be going wrong? Also is this the best way to send files over sockets? Are there better methods to send files?
Thanks
Code for client side
//Writing code to open file and copy it into buffer
fseek(fp, 0, SEEK_END);
size_t file_size = ftell(fp);
fseek(fp, 0, SEEK_SET);
if(fread(file_buffer, file_size, 1, fp)<=0)
{
printf("Unable to copy file into buffer! \n");
exit(1);
}
//Sending file buffer
if(sendto(sock, file_buffer, strlen(file_buffer), 0, (struct sockaddr *) &serv_addr, serv_len)<0)
{
printf("Error sending the file! \n");
exit(1);
}
bzero(file_buffer, sizeof(file_buffer));
Code on the server side to receive the file
//Receiving file from client
char file_buffer[BUFSIZE];
if(recvfrom(sock, file_buffer, BUFSIZE, 0, (struct sockaddr *) &client_addr, &client_addr_size)<0)
{
printf("Error receiving file.");
exit(1);
}
char new_file_name[] = "copied_";
strcat(new_file_name,file_name);
FILE *fp;
fp = fopen(new_file_name,"w+");
if(fwrite(file_buffer, 1, sizeof(file_buffer), fp)<0)
{
printf("Error writing file! \n");
exit(1);
}
There are several problems with your code.
In the sender:
How do you allocate file_buffer? What if the file is bigger than the buffer? (Probably this is not causing the problem at hand.)
You only check to see if the return value from fread is <= 0. In fact, if an error or EOF occurs, the return value can be any value less than the full size of the file. (Probably this is not causing the problem at hand.)
You pass strlen(file_buffer) instead of file_size to the sendto system call. strlen looks for NUL bytes which the file may or may not contain. It probably doesn't contain any, since you say it's a text file.
If the file contain at least one NUL byte the packet will be truncated before the first NUL byte and you will not transmit the full contents of the file.
If the file contains no NUL bytes, strlen will scan beyond the end of the file as read into the buffer. Either your program will crash because strlen scans into unallocated memory addresses, or you will send additional garbage past the end of the file.
In the receiver:
You ignore the return value from recvfrom which is the length of the payload of the packet that was received. After that, there is no way for you to know how much data you received.
When you fwrite the result, you pass sizeof(file_buffer) as the size instead of the actual amount of data that was received. This is a fixed value (BUFSIZE) which is probably bigger than your file. The file written on disk will contain garbage beyond the end of the file.
Hi i have written a server application which accepts a name from the client which usually is a file name.It opens the file ,reads the contents into a buffer and then transmits the buffer over the ethernet using send().But the problem arises in the client side where all the bytes are not received successfully.I receive only a part of what i send.
For your reference ,here's the code snippet for the server side:
Server:
fp = fopen(filename,"r+");
strcpy(str,"");
fseek(fp, 0L, SEEK_END);
size = ftell(fp);
fseek(fp, 0L, SEEK_SET);
fread(str, size, 1,fp);
fclose(fp);
printf("Size of the file is : %d\n",size);
sprintf(filename, "%d", size);
n = send(nsd, filename, strlen(filename), 0);
while(size > 0){
n = send(nsd, str, strlen(str), 0);
printf("%d bytes sent successfully\n",n);
if(n == 0) break;
sentbytes = sentbytes + n;
size = size - sentbytes;
}
Please help me with writing the client app.I am currently confused about how to go about writing it.Shall i place the recv() part in a while(1) loop so that the client keeps running until all the bytes have been received successfully?
EDITED
For starters, you could both read from the file and write to the socket in chunks at the same time.
Since, you are transferring data over TCP, remember that data is transferred reliably as a stream and not as messages. So, don't make assumptions about how the data is recv'd except for the order.
Here is how it could be written:
open socket
open file
size_of_file = read_file_size(file);
send(socket, &size_of_file, sizeof(int), ...)
while (all is written)
read fixed chunk from file
write as much was read to the socket
cleanup // close file, socket
As for the recv part, I think it is best you send the file size over as an integer and keep reading in a while loop until you have recv'd as many bytes as you are sending from the server.
It's like this:
recv(socket, &size_of_msg, sizeof(int), ...)
while(all is read)
read fixed chunk from the socket
cleanup
Well I see atleast some issue with the way you are sending message over socket.
First from the man page of fread:
The function fread() reads nmemb elements of data, each size bytes
long, from the stream pointed to by stream, storing them at the loca-
tion given by ptr.
and what you are trying is this:
fread(str, size, 1,fp);
I assume what you meant was
fread(str, 1,size,fp);
Though it shold not casue the issue.
But the problem lies here:
n = send(nsd, str, strlen(str), 0);
printf("%d bytes sent successfully\n",n);
if(n == 0) break;
sentbytes = sentbytes + n;
size = size - sentbytes;
Though you are decreasing 'size' by decreasing by number of bytes successfully send, where are you extending str to point to new buffer location where data will be send.This will only resend initial bytes of the buffer repeatedly.
str += n; //Assuming str is char*
will solve your issue.
Using strlen doesn't seem appropriate. You've read the file, you know how long it is, so why do strlen? Either you'll just get the same result (so it's redundant) or you'll get something else (so it's a bug).
"Shall i place the recv() part in a while(1) loop so that the client keeps running until all the bytes have been received successfully?"
Something like that. Never presume that a recv() call got everything that was sent -- tcp/ip breaks messages into packets at a lower level, and recv() will return after reading whatever amount of data has actually been received at whatever point. You don't have to worry about that directly, except in so far as you do need to use some kind of protocol to indicate how long a message is so the receiver knows how much to read, then eg.:
char buffer[4096];
int msgsz = 600, // see below
sofar = 0,
cur;
while (sofar < msgsz) {
cur = recv (
socket_fd,
&buffer[sofar],
msgsz - sofar,
0
);
if (cur == -1) {
// error
break;
} else if (cur == 0) {
// disconnected
break;
}
sofar += cur;
}
WRT msgsz, you would include this somewhere in a fixed length header, which is read first. A simple version of that might be just 4 bytes containing a uint32_t, ie, an int with the length. You could also use a null terminated string with a number in it, but that means reading until '\0' is found.