I am writing a simple client/server application and I ran into a very weird issue.
I am trying to send a .zip file to the client and some more data after that.
Sending the .zip works fine, the server writes to the socket, the client reads from the socket, just as intended.
The problem is right after that.
The server runs fine and keeps writing to the socket, but the client just won't read anything. It gets stuck on the very next read() call, no matter what I try to send.
I've checked to see if the socket descriptors are alright, and they are. I also thought that maybe there is not enough data in the socket for the client to read, but there definitely is.
I also tried doing the same write/read before and after sending that .zip file: it works fine before, but client doesn't see it after sending that .zip.
I am out of ideas.
Here's the function I use to send the .zip:
typedef struct thData{
int idThread; //thread ID
int cl; //client descriptor
}thData;
void send_info(struct thData tdL)
{
char file_path[256]="v1.zip";
char sd_buffer[256];
bzero(sd_buffer, 256);
FILE *fd = fopen(file_path, "rb");
if(fd == NULL)
{
printf("ERROR: %s not found.\n", file_path);
exit(1);
}
int read_size;
int write_size;
while((read_size = fread(sd_buffer, sizeof(char), 256, fd)) > 0)
{
if((write_size=write(tdL.cl, sd_buffer, read_size)) < 0)
{
perror("ERROR: writing to client: \n");
break;
}
bzero(sd_buffer, 256);
}
}
And what I use to receive the .zip:
void receive_info(int sd) //sd being the socket descriptor
{
char* file_path = "subject.zip";
char received_buffer[256];
int total_received=0;
int total_wrote=0;
FILE *fd = fopen(file_path, "wb");
if(fd == NULL)
printf("Cannot open %s\n", file_path);
else
{
bzero(received_buffer, 256);
int read_size = 0;
while((read_size = read(sd, received_buffer, 256)) > 0)
{
total_received=total_received+read_size;
int write_size = fwrite(received_buffer, sizeof(char), read_size, fd);
total_wrote=total_wrote+read_size;
if(write_size < read_size)
{
perror("ERROR: \n");
}
bzero(received_buffer, 256);
if (read_size == 0 || read_size != 256)
{
break;
}
}
if(read_size < 0)
{
perror("ERROR: reading: ");
exit(1);
}
fclose(fd);
}
}
Any help would be greatly appreciated.
I think that the problem is that you are reading too much.
In TCP there are no boundaries in the packets sent from one peer to the other. It is just a stream of bytes, and the pieces received from recv/read() bear no relation (in principle) to the pieces sent from send/write().
Now, imagine that your ZIP file is 300 bytes long, and your extra data is 10 bytes long. Your sender code will do:
write 256 bytes (first piece from ZIP).
write 44 bytes (last piece from ZIP).
write 10 bytes (the extra data).
And your receiver code will do:
read 256, get 256 bytes (first piece from ZIP).
read 256, get 54 bytes (last piece from ZIP plus the extra data).
read XXX bytes, wait forever!
If you look carefully to the ZIP files you'll probably see those extra bytes at the end of subject.zip.
The solution, if you don't want to close and open another socket, is to make the protocol a bit more complicated. For example, you could send a structure before the file (a header) that includes the size of the file. That way the receiver will know when to stop reading.
PS: Note that your code has a few risky edges. For example, write() might not write all the given bytes, but you are not checking that; you are not closing the file...
PS2: I find curious that you feel the need to write sizeof(char) instead of just 1 but you write 256 instead of sizeof(sd_buffer).
Related
EDIT: Changed the while loop condition from > 0 to !=0. There are also 2 or more newlines being output on the terminal after printing the content, but in the file there aren't.
I'm doing a file transfer from the client to the server. The contents are transferred successfully, but when receiving it doesn't exit the while loop. Is my while condition somehow wrong? After I force stop the program, i can see that the file has actually been copied successfully, it just does not exit this while loop after writing everything to the file.
This is the code where I'm having trouble:
ssize_t bytes_read = 0;
int error;
while((bytes_read = read(sd, buf, sizeof(buf))) != 0){
printf("writing: %s\n", buf);
if((error = fwrite(buf, 1, bytes_read, fp)) < 0)
{
printf("error");
}
if((bytes_read = read(sd, buf, sizeof(buf))) < 0){
break;
}
}
Tried to keep it as minimal as possible, as I'm sure there's something very simplistically wrong here.
EDIT: I tried it with a larger file, and it seems that its missing some content here and there but not in a consistent pattern.
EDIT: Here is the client side.
else if(strcmp(shortCommand, "put") == 0){
char *tmp = buf + 4;
char filename[MAX_BLOCK_SIZE];
strcpy(filename, "filename ");
strcat(filename, tmp);
FILE *fp;
printf("File name: %s\n", tmp);
fp = fopen(tmp, "rb");
if(fp == NULL){
printf("ERROR: Requested file does not exist.\n");
}
else{
printf("Client sending filename...\n");
if ((nw = write(sd, buf, sizeof(buf))) < 0){ //sending the file name to the client first
printf("Error sending client's filename.\n");
}
printf("Client sending file...\n");
size_t bytes_read = 0;
ssize_t bytes_written = 0;
while((bytes_read = fread(buf, 1, sizeof(buf), fp)) != 0){ //sending the file contents
if ((bytes_written = write(sd, buf, bytes_read)) < 0){
printf("Error sending client file.\n");
}
}
fclose(fp);
}
}
The server code assumes that it will observe read() to return 0 after it reads all the data that the client transfers, but that will not happen just because the client stops sending data. read() returns 0 to indicate that the end of the file has been reached, but "file" in that sense means the read side of its socket. Until the client closes the peer socket, the server will not perceive EOF on its socket. And that's perfectly reasonable because until then, the client might send more data, proving that indeed EOF had not previously been reached. Therefore, if the server tries to perform a (blocking) read on the socket after receiving all the data then it will block until the client sends more or closes the connection, or until it detects an error, is interrupted by a signal, etc..
If the client doesn't actually intend to send any further data after the file content then it should close() that socket after sending all the bytes of the file content, or at least shutdown() the write side.
If, on the other hand, the client is keeping the socket open because it intends to send more data later, then you need a new strategy. In that case the end of the file content does not coincide with the logical end of the stream, and you therefore need a different mechanism to communicate to the server where the end of the file data is. This is where communication protocols such as HTTP come in, but for your purpose, you don't need a protocol nearly as complicated as HTTP.
It might suffice, say, for the client to send data to the server as one or more chunks, each comprising a byte count followed by the specified number of bytes of content. You could insist that the whole file content always go into one chunk, but if you want to allow splitting the file into multiple chunks then you could use a chunk length of zero to signal the end of the data.
I've seen a lot of questions related to what I'm trying to do but I haven't been able to get any solution to work for me as yet.
I'm try to write two socket programs (one client and one server) so that the server is able to send files of any type (by bytes) to the client. I'm not sure how to construct/coordinate the following:
(in the server) the read statement(s) to get data from the file (I'm using fread())
(in the server) the write statement(s) to send the data to the client (I'm using write())
(in the client) the read statement(s) to receive data from the server (I'm using read())
(in the client) the write statement(s) to write the data received from the server to a file (I'm using fwrite())
I want to transmit the data in chunks of bytes. Will one fread() statement and one write() statement in the server be sufficient regardless of how large the file is? (or does that depend on the size of my buffer?)
On the client side, will one call to read() and one call to fwrite() be enough? (or does that depend on the size of my buffer?)
I'm new to Socket Programming. Thanks in advance!
Parts of my server and client below:
Client Side:
/*try to get file from server*/
char receive_buffer[256];
FILE * new_file;
int bytes_received = 0;
new_file = fopen("newfile.txt", "w");
bytes_received = read(conn_s, receive_buffer, 256);
printf("Received %d bytes\n", bytes_received);
while( bytes_received > 0)
{
bytes_received = read(conn_s, receive_buffer, 256);
printf("Received %d bytes\n", bytes_received);
fwrite(receive_buffer, 1, bytes_received, new_file);
printf("writing bytes!\n");
break;
}
if(bytes_received < 0)
{
printf("/nError reading bytes of data/n");
}
Parts of the server code:
FILE * file_to_get = fopen("sample.txt", "rb");
if(file_to_get == NULL)
{
printf("No such file!");
exit(0);
}
while(1)
{
unsigned char buff[256];
int num_read = fread(buff, 1, 256, file_to_get);
printf("read %d bytes\n", num_read);
if(num_read > 0)
{
write(conn_s, buff, num_read);
printf("writing %d bytes\n", num_read);
}
if(num_read < 256)
{
if(feof(file_to_get))
printf("End of file\n");
if(ferror(file_to_get))
printf("Error reading bytes\n");
break;
}
}
When sending to the network, you can use a single call to write(). If there's not enough buffer space in the kernel for the whole thing, write() will block until it's able to process all of it. You can also write it in chunks, that's fine as well. It doesn't really matter.
When reading from the network, you have to call read() in a loop. The buffer size you specify when calling read() is just the maximum it's allowed to return, it won't wait for that much to be received. So it can (and often does) return less than this. Just keep calling read() until it returns 0 to indicate EOF, writing each buffer to the file.
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'm writing a toy client/server app and sending data over a UDP stream. Everything is working well when I send small messages like "test" or something like that. But when I go to transfer files, I seem to be transferring too much data.
The file that I am transferring is a little less than 4k in size (at least according to an ls command). But when my sendto operation completes, my confirmation message shows that I have sent exactly 5000 bytes (which is the max size of my buffer).
I'd rather not break the sendto up if I don't need to since UDP can potentially transfer messages out of order. Anyone see what I am doing wrong? I think that is has to do with my use of sizeof(buffer), but I don't know how to transfer exactly the number of bytes that I want. Thank you!
Here is the relevant part of the code. Please let me know if you need more:
char buffer[5000];
char path[] = "myfile";
FILE* fp;
if ((fp = fopen(path, "rb")) == NULL) {
printf(stdout, "Error opening the file: %s\n", strerror(errno));
exit(1);
}
memset(&buffer[0], 0, sizeof(buffer));
do {
fread(buffer, 1, MAXBUFSIZE, fp);
} while (!feof(fp));
if ( (numbytes = sendto(sockfd, buffer, sizeof(buffer), 0,
(struct sockaddr *)&server_addr, sizeof(server_addr))) < 0)
{
perror("send error");
}
fprintf(stderr, "client sent %d bytes to server\n", numbytes);
As per the sendto command, UDP will send the same amount of data as specified in the send length; the third param is the send length: http://linux.die.net/man/2/sendto . The example is specifying sizeof(buffer) as the send length and sizeof(buffer) is 5000. Accordingly, UDP sends 5000 bytes from the buffer (which is actually the entire buffer). If we want to send the exact number of bytes as present in the file, then we should read the number of bytes in the file, and pass that as the send length.
numbytes = sendto(sockfd, buffer, sizeof(buffer), 0,
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.