I have this code:
while (1) {
char buffer[4096];
memset(buffer,0,4096);
int bytes_read = recv(client_fd, buffer, sizeof(buffer),0);
if (bytes_read == 0)
break;
if (bytes_read < 0) {
std::cout<< "Error "<<endl;
}
void *p = buffer;
int dest;
dest=open("/root/hello.txt",O_WRONLY);
while (bytes_read > 0) {
int bytes_written = send(dest, buffer, bytes_read,0);
if (bytes_written <= 0) {
std::cout<< "Error2 "<<endl;
}
bytes_read -= bytes_written;
p += bytes_written;
}
}
I receive the file correctly.
The program goes in loop where the cout is "Error2", so the send returns -1.
The problem is to write the file in a new file descriptor, in this case the variable called dest.
How can I solve this problem?
According to the manual-page for send(2), it can only write to a socket, not to a regular file; if you had checked errno (using e.g. perror), you would have seen that it gets set to ENOTSOCK. You should always see what error is being set, otherwise debugging is just shooting in the dark.
Also, since this is C++, I really think you should use C++-style I/O (std::ofstream and so on) to write to a regular file.
Check the return value of
dest=open("/root/hello.txt",O_WRONLY);
It should be a positive integer; everything else indicates an error. My guess is that the file can't be opened for whatever reason.
Related
I have a server that sends raw binary data to print a "map" that a user must traverse through, however, I am having trouble clearing out my buffer after each line read and thus keep getting residual data printed at the end of the shorter lines. In the screenshot below you can see my output on the left, and what the output should be on the right. What is the best way to solve this? I feel like I am missing something but cant seem to find a solution.
And the code that is reading/printing this is below:
char* mapData = NULL;
string command = "command> ";
size_t dataSize = 0;
while(mapData != command.c_str()) {
unsigned char* buffer = (unsigned char*) &dataSize;
connection = read(mySocket, buffer, 8);
if(connection == -1 || connection < 0) {
cerr << "**Error: could not read text size" << endl;
return 1;
}
mapData = (char*)malloc(dataSize);
buffer = (unsigned char*) mapData;
while((connection = read(mySocket, buffer, dataSize)) != -1) {
if(connection == -1 || connection < 0) {
cerr << "**Error: could not read text size" << endl;
return 1;
}
if(dataSize != 1) {
cout << buffer;
}
free(buffer);
buffer = NULL;
}
}
You are ignoring the return value of read() to know how many bytes are in the buffer.
read() returns the actual number of bytes that were read, which may be fewer than you requested. So you need to call read() in a loop until you have read all of the bytes you are expecting, eg:
int readAll(int sock, void *buffer, size_t buflen)
{
unsigned char* pbuf = reinterpret_cast<unsigned char*>(buffer);
while (buflen > 0) {
int numRead = read(sock, pbuf, buflen);
if (numRead < 0) return -1;
if (numRead == 0) return 0;
pbuf += numRead;
buflen -= numRead;
}
return 1;
}
Also, after reading the buffer, you are treating it as if it were null-terminated, but it is not, which is why you get extra garbage in your output.
More importantly, mapData != command.c_str() will ALWAYS be true, so your while loop iterates indefinitely (until a socket error occurs), which is not what you want. You want the loop to end when you receive a "command> " string instead.
mapData is initially NULL, and c_str() NEVER returns NULL, so the loop ALWAYS iterates at least once.
Then you allocate and free mapData but don't reset it to NULL, so it is left pointing at invalid memory. Which doesn't really matter, since your while loop is just comparing pointers. c_str() will NEVER return a pointer to memory that mapData ever points to.
To end your loop correctly, you need to compare the contents of mapData after reading, not compare its memory address.
Try this instead:
char *mapData = NULL;
uint64_t dataSize = 0;
const string command = "command> ";
bool keepLooping = true;
do {
if (readAll(mySocket, &dataSize, sizeof(dataSize)) <= 0) {
cerr << "**Error: could not read text size" << endl;
return 1;
}
if (dataSize == 0)
continue;
mapData = new char[dataSize];
if (readAll(mySocket, mapData, dataSize) <= 0) {
cerr << "**Error: could not read text" << endl;
delete[] mapData;
return 1;
}
cout.write(mapData, dataSize);
keepLooping = (dataSize != command.size()) || (strncmp(mapData, command.c_str(), command.size()) != 0);
delete[] mapData;
}
while (keepLooping);
Alternatively:
string mapData;
uint64_t dataSize = 0;
const string command = "command> ";
do {
if (readAll(mySocket, &dataSize, sizeof(dataSize)) <= 0) {
cerr << "**Error: could not read text size" << endl;
return 1;
}
mapData.resize(dataSize);
if (dataSize > 0) {
if (readAll(mySocket, &mapData[0], dataSize) <= 0) {
cerr << "**Error: could not read text" << endl;
return 1;
}
cout << mapData;
}
}
while (mapData != command);
like #eozd pointed out, calling malloc and free in your loop is a bad idea since you use return statements. Your code may leak memory. You should ensure you call free before returns. Even better, you could declare your buffer outside of while loop, and use break instead of return, and call free if there was en error
Looking at your solution, it seems that the communication protocol involves sending data size first, followed by the actual data. How is data size written to the wire? You may need to convert it from network byte order.
To debug, you could print out the value of dataSize before every read to make sure that it is what you expect
You should clear the buffer too. Add:
memset(mapData, 0, dataSize);
after the malloc.
I am trying to read a large binary file thought input redirection (stdin) at runtime, and stdin is mandatory.
./a.out < input.bin
So far I have used fgets. But fgets skips blanks and newline. I want to include both. My currentBuffersize could dynamically vary.
FILE * inputFileStream = stdin;
int currentPos = INIT_BUFFER_SIZE;
int currentBufferSize = 24; // opt
unsigned short int count = 0; // As Max number of packets 30,000/65,536
while (!feof(inputFileStream)) {
char buf[INIT_BUFFER_SIZE]; // size of byte
fgets(buf, sizeof(buf), inputFileStream);
cout<<buf;
cout<<endl;
}
Thanks in advance.
If it were me I would probably do something similar to this:
const std::size_t INIT_BUFFER_SIZE = 1024;
int main()
{
try
{
// on some systems you may need to reopen stdin in binary mode
// this is supposed to be reasonably portable
std::freopen(nullptr, "rb", stdin);
if(std::ferror(stdin))
throw std::runtime_error(std::strerror(errno));
std::size_t len;
std::array<char, INIT_BUFFER_SIZE> buf;
// somewhere to store the data
std::vector<char> input;
// use std::fread and remember to only use as many bytes as are returned
// according to len
while((len = std::fread(buf.data(), sizeof(buf[0]), buf.size(), stdin)) > 0)
{
// whoopsie
if(std::ferror(stdin) && !std::feof(stdin))
throw std::runtime_error(std::strerror(errno));
// use {buf.data(), buf.data() + len} here
input.insert(input.end(), buf.data(), buf.data() + len); // append to vector
}
// use input vector here
}
catch(std::exception const& e)
{
std::cerr << e.what() << '\n';
return EXIT_FAILURE;
}
return EXIT_SUCCESS;
}
Note you may need to re-open stdin in binary mode not sure how portable that is but various documentation suggests is reasonably well supported across systems.
I'm sending a request to get an image from a webpage.
I'm using C++ with winsocket and tcp http get request.
I receive all the info in my char buffer but when I stream it to a file or string it's very short because there are string terminators in it.
What's the best/most efficient way to deal with the escape characters?
Thanks in advance.
EDIT:
ofstream out("temp.jpg");
//m_Received.reserve(STRINGBUFFERSIZE);
char rBuffer[BUFFERSIZE];
int readSize = 0;
int totalSize = 0;
do
{
PRINT("Reset buffer");
ZeroMemory(rBuffer, sizeof(rBuffer));
PRINT("Receiving...");
readSize = recv(socket, rBuffer, sizeof(rBuffer), 0);
PRINT("Received " << readSize << " bytes...");
if (readSize > 0)
{
totalSize += readSize;
//m_Received.append(rBuffer);
for (int i = 0; i < readSize; ++i)
{
out << rBuffer[i];
}
if (readSize < BUFFERSIZE)
{
PRINT("Stopping receiving...");
break;
}
}
else if (readSize == -1)
throw SocketError("ErrorReceiving", readSize);
} while (readSize > 0);
Even when putting each character in individually I get a small difference with the original image which leads to corruption.
Alright, so I fixed it by simply writing the char buffer binary to my file. Should have thought of this earlier.
I'm developing a server-client application using Winsock in c++ and have a problem.
For getting the message from the client by the server I use the code below.
int result;
char buffer[200];
while (true)
{
result = recv(client, buffer, 200, NULL);
if (result > 0)
cout << "\n\tMessage from client: \n\n\t" << message << ";";
}
I send the message "Hello" from the client to the server. However the buffer is actually this:
HelloÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌÌ
What am I missing?
Since recv might not receive as many bytes as you told it, you typically use a function
like this to receive specified number of bytes. Modified from here
int receiveall(int s, char *buf, int *len)
{
int total = 0; // how many bytes we've received
int bytesleft = *len; // how many we have left to receive
int n = -1;
while(total < *len) {
n = recv(s, buf+total, bytesleft, 0);
if (n <= 0) { break; }
total += n;
bytesleft -= n;
}
*len = total; // return number actually received here
return (n<=0)?-1:0; // return -1 on failure, 0 on success
}
It's up to you to null terminate the string if you receive string which is not null terminated.
The result tells you how many bytes were received. recv doesn't add a terminator since, in general, network data is binary data which might not be usable as a C-style string.
You can add a terminator yourself, if you know the message won't contain the termination character:
buffer[result] = 0; // make sure the buffer is large enough
or make a string (or vector, or whatever) from it:
std::string message_str(message, result);
Note that what you receive might not be a single "message", especially if you're uses a stream protocol like TCP. It might contain more than one message, or just the start of one.
memset(&receive[0], 0, sizeof(receive));
To clear the buffer
You didn't initialize your buffer
char buffer[200] = {0};
while (true)
{
result = recv(client, buffer, 200, NULL);
if (result > 0)
cout << "\n\tMessage from client: \n\n\t" << message << ";";
memset(buffer, 0, 200);
}
I've got the following two programs, one acting as a reader and the other as a writer. The writer seems to only send about 3/4 of the data correctly to be read by the reader. Is there any way to guarantee that all the data is being sent? I think I've got it set up so that it reads and writes reliably, but it still seems to miss 1/4 of the data.
Heres the source of the writer
#define pipe "/tmp/testPipe"
using namespace std;
queue<string> sproutFeed;
ssize_t r_write(int fd, char *buf, size_t size) {
char *bufp;
size_t bytestowrite;
ssize_t byteswritten;
size_t totalbytes;
for (bufp = buf, bytestowrite = size, totalbytes = 0;
bytestowrite > 0;
bufp += byteswritten, bytestowrite -= byteswritten) {
byteswritten = write(fd, bufp, bytestowrite);
if(errno == EPIPE)
{
signal(SIGPIPE,SIG_IGN);
}
if ((byteswritten) == -1 && (errno != EINTR))
return -1;
if (byteswritten == -1)
byteswritten = 0;
totalbytes += byteswritten;
}
return totalbytes;
}
void* sendData(void *thread_arg)
{
int fd, ret_val, count, numread;
string word;
char bufpipe[5];
ret_val = mkfifo(pipe, 0777); //make the sprout pipe
if (( ret_val == -1) && (errno != EEXIST))
{
perror("Error creating named pipe");
exit(1);
}
while(1)
{
if(!sproutFeed.empty())
{
string s;
s.clear();
s = sproutFeed.front();
int sizeOfData = s.length();
snprintf(bufpipe, 5, "%04d\0", sizeOfData);
char stringToSend[strlen(bufpipe) + sizeOfData +1];
bzero(stringToSend, sizeof(stringToSend));
strncpy(stringToSend,bufpipe, strlen(bufpipe));
strncat(stringToSend,s.c_str(),strlen(s.c_str()));
strncat(stringToSend, "\0", strlen("\0"));
int fullSize = strlen(stringToSend);
signal(SIGPIPE,SIG_IGN);
fd = open(pipe,O_WRONLY);
int numWrite = r_write(fd, stringToSend, strlen(stringToSend) );
cout << errno << endl;
if(errno == EPIPE)
{
signal(SIGPIPE,SIG_IGN);
}
if(numWrite != fullSize )
{
signal(SIGPIPE,SIG_IGN);
bzero(bufpipe, strlen(bufpipe));
bzero(stringToSend, strlen(stringToSend));
close(fd);
}
else
{
signal(SIGPIPE,SIG_IGN);
sproutFeed.pop();
close(fd);
bzero(bufpipe, strlen(bufpipe));
bzero(stringToSend, strlen(stringToSend));
}
}
else
{
if(usleep(.0002) == -1)
{
perror("sleeping error\n");
}
}
}
}
int main(int argc, char *argv[])
{
signal(SIGPIPE,SIG_IGN);
int x;
for(x = 0; x < 100; x++)
{
sproutFeed.push("All ships in the sea sink except for that blue one over there, that one never sinks. Most likley because it\'s blue and thats the mightiest colour of ship. Interesting huh?");
}
int rc, i , status;
pthread_t threads[1];
printf("Starting Threads...\n");
pthread_create(&threads[0], NULL, sendData, NULL);
rc = pthread_join(threads[0], (void **) &status);
}
Heres the source of the reader
#define pipe "/tmp/testPipe"
char dataString[50000];
using namespace std;
char *getSproutItem();
void* readItem(void *thread_arg)
{
while(1)
{
x++;
char *s = getSproutItem();
if(s != NULL)
{
cout << "READ IN: " << s << endl;
}
}
}
ssize_t r_read(int fd, char *buf, size_t size) {
ssize_t retval;
while (retval = read(fd, buf, size), retval == -1 && errno == EINTR) ;
return retval;
}
char * getSproutItem()
{
cout << "Getting item" << endl;
char stringSize[4];
bzero(stringSize, sizeof(stringSize));
int fd = open(pipe,O_RDONLY);
cout << "Reading" << endl;
int numread = r_read(fd,stringSize, sizeof(stringSize));
if(errno == EPIPE)
{
signal(SIGPIPE,SIG_IGN);
}
cout << "Read Complete" << endl;
if(numread > 1)
{
stringSize[numread] = '\0';
int length = atoi(stringSize);
char recievedString[length];
bzero(recievedString, sizeof(recievedString));
int numread1 = r_read(fd, recievedString, sizeof(recievedString));
if(errno == EPIPE)
{
signal(SIGPIPE,SIG_IGN);
}
if(numread1 > 1)
{
recievedString[numread1] = '\0';
cout << "DATA RECIEVED: " << recievedString << endl;
bzero(dataString, sizeof(dataString));
strncpy(dataString, recievedString, strlen(recievedString));
strncat(dataString, "\0", strlen("\0"));
close(fd);
return dataString;
}
else
{
return NULL;
}
}
else
{
return NULL;
}
close(fd);
}
int main(int argc, char *argv[])
{
int rc, i , status;
pthread_t threads[1];
printf("Starting Threads...\n");
pthread_create(&threads[0], NULL, readItem, NULL);
rc = pthread_join(threads[0], (void **) &status);
}
You are definitely using signals the wrong way. Threads are completely unnecessary here - at least in the code provided. String calculations are just weird. Get this book and do not touch the keyboard until you finished reading :)
The general method used to send data through named pipes is to tack on a header with the length of the payload. Then you read(fd, header_len); read(rd, data_len); Note the latter read() will need to be done in a loop until data_len is read or eof. Note also if you've multiple writers to a named pipe then the writes are atomic (as long as a reasonable size) I.E. multiple writers will not case partial messages in the kernel buffers.
It's difficult to say what is going on here. Maybe you are getting an error returned from one of your system calls? Are you sure that you are successfully sending all of the data?
You also appear to have some invalid code here:
int length = atoi(stringSize);
char recievedString[length];
This is a syntax error, since you cannot create an array on the stack using a non-constanct expression for the size. Maybe you are using different code in your real version?
Do you need to read the data in a loop? Sometimes a function will return a portion of the available data and require you to call it repeatedly until all of the data is gone.
Some system calls in Unix can also return EAGAIN if the system call is interrupted - you are not handling this case by the looks of things.
You are possibly getting bitten by POSIX thread signal handling semantics in your reader main thread.
The POSIX standard allows for a POSIX thread to receive the signal, not necessarily the thread you expect. Block signals where not wanted.
signal(SIG_PIPE,SIG_IGN) is your friend. Add one to reader main.
POSIX thread handling semantics, putting the POS into POSIX. ( but it does make it easier to implement POSIX threads.)
Examine the pipe in /tmp with ls ? is it not empty ?