I have the following code which ends up forever reading '/proc/cpuinfo' as it keeps getting the same result every read. Why doesn't the file pointer get advanced and reach eof ever? Seems this special file has different semantics.
const int bufSize = 4096;
char buf[bufSize + 1];
const string cpuInfo = "/proc/cpuinfo";
int cpuFD = ::open(cpuInfo.c_str(), O_RDONLY);
if (cpuFD == -1) {
logOutputStream << "Failed attempt to open '" << cpuInfo << "': "
<< strerror(errno) << endl;
} else {
assert(bufSize <= SSIZE_MAX);
logOutputStream << "Contents of: '" << cpuInfo << "'.\n";
for (int nRead = ::read(cpuFD, buf, bufSize); nRead != 0;) {
if (nRead == -1) {
logOutputStream << "Failed attempt to read '" << cpuInfo << "': "
<< strerror(errno) << endl;
break;
} else {
buf[nRead] = '\0';
logOutputStream << buf;
}
}
if (::close(cpuFD) == -1) {
logOutputStream << "Failed attempt to close '" << cpuInfo << "': "
<< strerror(errno) << endl;
}
}
for (int nRead = ::read(cpuFD, buf, bufSize); nRead != 0;) {
is wrong. You're using read as an initializer, so read is only being called once, not once per loop. After that, you're just looping forever printing it out (because nothing is changing nRead).
What happens if you try dumping the content into an actual text file with something like
cat /proc/cpuinfo > cpuinfo.txt
and then reading that file ?
Related
Another try with getting parallel processes to work. Please excuse the amount of code but every attempt to shorten it makes the error vanish.
What I tested so far:
sending int from parent to child, from child to parent, and from parent to child and then back: works
processing a list of int: send from parent to child, modify and back to parent: works
more data: int + string, from parent to child, modify and back to parent: works
a list of data the same way: works
But when I run the same function that works a second time it always fail.
This is the function that creates the child process:
//parent sends binary data from list to child which sends back modified data
bool processParallel6(std::vector<std::pair<int, std::string>> & data)
{
//define pipe
int parent2Child[2];
int child2Parent[2];
//create pipe
pipe(parent2Child);
pipe(child2Parent);
//fork
pid_t child = fork();
if(child == 0) //child process
{
//close not needed end of pipe
close(parent2Child[1]);
close(child2Parent[0]);
for(;;)
{
struct pollfd pfd;
pfd.fd = parent2Child[0];
pfd.events = POLLIN;
//wait until data is available at the pipe
cout << "c: poll ..." << endl;
if(poll(&pfd, 1, -1) < 0)
{
cout << "c: poll: " << strerror(errno) << endl;
exit(-1);
}
cout << "c: poll says there are data" << endl;
if((pfd.revents&POLLIN) == POLLIN)
{
int data;
std::string text;
if(!readData3(parent2Child[0], data, text))
exit(-2);
cout << "c: data received: " << data << " " << text << endl;
if(data == -1)
break;
if(!writeData3(child2Parent[1], data * 2, text + text))
exit(-3);
cout << "c: sent data to parent: " << 2 * data << " " << text + text << endl;
}
}
close(parent2Child[0]);
close(child2Parent[1]);
exit(0);
}
else //parent process
{
//close not needed end of pipe
close(parent2Child[0]);
close(child2Parent[1]);
//send data to child
if(!writeData3(parent2Child[1], data.back().first, data.back().second))
return false;
cout << "p: wrote data: " << data.back().first << " " << data.back().second << endl;
data.pop_back();
//read result from child
for(;;)
{
struct pollfd pfd;
pfd.fd = child2Parent[0];
pfd.events = POLLIN;
//wait until data is available at the pipe
cout << "p: poll ..." << endl;
if(poll(&pfd, 1, -1) < 0)
{
cout << "p poll: " << strerror(errno) << endl;
return false;
}
cout << "p: poll says there are data" << endl;
if((pfd.revents&POLLIN) == POLLIN)
{
int data;
std::string text;
if(!readData3(child2Parent[0], data, text))
return false;
cout << "p: data received: " << data << " " << text << endl;
}
if(data.empty())
break;
if(!writeData3(parent2Child[1], data.back().first, data.back().second))
return false;
cout << "p: wrote data: " << data.back().first << " " << data.back().second << endl;
data.pop_back();
}
//send stop data
if(!writeData3(parent2Child[1], -1, "notext"))
return false;
cout << "p: sent stop data " << endl;
//wait for child to end
wait(nullptr);
//close all pipes
close(parent2Child[1]);
close(child2Parent[0]);
}
return true;
}
For reading and writing data I use this two functions:
bool readData3(int fd, int & number, std::string & text)
{
char numberBuf[sizeof(int)];
int bytesRead = read(fd, numberBuf, sizeof(int));
if(bytesRead > 0)
{
number = *(int *)numberBuf;
}
else if(bytesRead < 0)
{
cout << "readData3: " << strerror(errno) << endl;
return false;
}
char sizeBuf[sizeof(int)];
int size = -1;
bytesRead = read(fd, sizeBuf, sizeof(int));
if(bytesRead > 0)
{
size = *(int *)sizeBuf;
}
else if(bytesRead < 0)
{
cout << "readData3: " << strerror(errno) << endl;
return false;
}
char textBuf[size];
bytesRead = read(fd, textBuf, size);
if(bytesRead > 0)
{
text = std::string(textBuf);
}
else if(bytesRead < 0)
{
cout << "readData3: " << strerror(errno) << endl;
return false;
}
return true;
}
bool writeData3(int fd, const int number, const std::string text)
{
int bytesWritten = write(fd, &number, sizeof(int));
if(bytesWritten < 0)
{
cout << "writeData3: " << strerror(errno) << endl;
return false;
}
int size = text.size() + 1;
bytesWritten = write(fd, &size, sizeof(int));
if(bytesWritten < 0)
{
cout << "writeData3: " << strerror(errno) << endl;
return false;
}
bytesWritten = write(fd, text.c_str(), size);
if(bytesWritten < 0)
{
cout << "writeData3: " << strerror(errno) << endl;
return false;
}
return true;
}
Finally I run it like this:
#include <iostream>
#include <vector>
#include <unistd.h>
#include <sys/wait.h>
#include <fcntl.h>
#include <bitset>
#include <memory>
#include <poll.h>
#include <cstring>
using namespace std;
int main(int /*argc*/, char* /*argv*/[])
{
std::vector<std::pair<int, std::string>> data;
data.push_back(std::make_pair(1, "one"));
data.push_back(std::make_pair(2, "two"));
cout << "6a ########################################################" << endl << flush;
processParallel6(data);
cout << "6b ########################################################" << endl << flush;
processParallel6(data);
return 0;
}
This is the output:
6a ###############################################
p: wrote data: 2 two
p: poll ...
c: poll ...
c: poll says there are data
c: data received: 2 two
p: poll says there are data
p: data received: 4 twotwo
p: wrote data: 1 one
p: poll ...
c: sent data to parent: 4 twotwo
c: poll ...
c: poll says there are data
c: data received: 1 one
p: poll says there are data
p: data received: 2 oneone
p: sent stop data
c: sent data to parent: 2 oneone
c: poll ...
c: poll says there are data
c: data received: -1 notext
6b ###################################################
c: poll ...
terminate called after throwing an instance of 'std::logic_error'
what(): basic_string::_M_construct null not valid
c: poll says there are data
c: poll ...
c: poll says there are data
c: poll ...
The last 4 lines are repeated a thousands of times. This output comes most of the times, but sometimes I have seen a std::bad_alloc error. When I try strace it crashes too, but when it runs I have seen directly after the second run of processParallel6() a line with mmap, ENOEM and 'Cannot allocate memory'
What happens here? Why is it working the first time, but not the second time?
You attempting to copy an invalid std::string reference.
std::terminate is getting called in the constructor of std::string. The constructor is implicitly called in processParallel6 when calling writeData3:
bool writeData3(int fd, const int number, const std::string text)
...
//send data to child
if(!writeData3(parent2Child[1], data.back().first, data.back().second))
return false;
You are expecting that data.back().second is a valid string reference, but nothing in the code ensures that is the case.
You construct data and place two entries in it:
data.push_back(std::make_pair(1, "one"));
data.push_back(std::make_pair(2, "two"));
In the first call to processParallel6 you run the following block of code twice:
if(!writeData3(parent2Child[1], data.back().first, data.back().second))
return false;
cout << "p: wrote data: " << data.back().first << " " << data.back().second << endl;
data.pop_back();
At this point data is empty. You cannot make another call to processParallel6 because it expects that data contains at least one element.
When using a FIFO on a single process, it looks like after both ends have been opened and one is then closed, it is not possible to reuse the FIFO. Any attempt to reopen the closed end fails or the returned file descriptor is useless.
Is it possible to work around this behavior, or do we have to keep both ends of the FIFO open until we are absolutely sure we don't need it anymore?
Here is some test code that shows and attempt to reopen a closed write end of a FIFO:
#include <iostream>
#include <cstdio>
#include <errno.h>
#include <fcntl.h>
#include <limits.h>
#include <stdio.h>
#include <string.h>
#include <unistd.h>
using namespace std;
int main(int argc, const char **argv)
{
cout << "Creating an instance of a named pipe..." << endl;
mode_t prevMask = umask(0);
if (mknod("my_pipe", S_IFIFO | 0666, 0))
return -1;
umask(prevMask);
cout << "Opening Read end..." << endl;
int fdM = open("my_pipe", O_RDONLY | O_NONBLOCK);
if (fdM == -1)
return -1;
cout << "Opening Write end..." << endl;
int fdS = open("my_pipe", O_WRONLY | O_NONBLOCK);
if (fdS == -1)
return -1;
cout << "Sending data to pipe..." << endl;
const char *data = "Hello my friend!";
ssize_t NbOfBytesWritten = write(fdS, data, strlen(data));
if (NbOfBytesWritten < 0)
return -1;
cout << "Number of bytes sent: " << NbOfBytesWritten << endl;
cout << "Closing Write end..." << endl;
if (close(fdS))
return -1;
cout << "Reopening Write end..." << endl;
fdS = open("my_pipe", O_WRONLY | O_NONBLOCK);
if (fdS == -1)
{
cout << "open() - failed("<< errno << "): " << strerror(errno) << '.';
remove("my_pipe");
return -1;
}
cout << "Sending some more data to pipe..." << endl;
data = "What's up?";
NbOfBytesWritten = write(fdS, data, strlen(data));
if (NbOfBytesWritten < 0)
return -1;
cout << "Number of bytes sent: " << NbOfBytesWritten << endl;
cout << "Reading data from pipe..." << endl;
char buff[128];
ssize_t numBytesRead = read(fdM, buff, 127);
if (NbOfBytesWritten < 0)
return -1;
buff[numBytesRead] = '\0'; // null terminate the string
cout << "Number of bytes read: " << numBytesRead << endl;
cout << "Message: " << buff << endl;
cout << "Closing Write end..." << endl;
if (close(fdS))
return -1;
cout << "Closing Read end..." << endl;
if (close(fdM))
return -1;
cout << "Deleting pipe..." << endl;
if (remove("my_pipe"))
return -1;
return 0;
}
Here is the output:
Creating an instance of a named pipe...
Opening Read end...
Opening Write end...
Sending data to pipe...
Number of bytes sent: 16
Closing Write end...
Reopening Write end...
open() - failed(6): No such device or address.
I also tested similar code trying to reopen a closed read end (While the write end was kept open). In that case the open() function succeed, but the read() function using the file descriptor returned by open() fails with:
Communication error on send. (70)
EDIT:
I'm using CYGWIN.
You code works fine on Linux. I think the issue you are running into is that named pipes on CYGWIN don't work very well and fail to follow POSIX semantics. See FIFO (named pipe) is broken on Cygwin. Likely the same problem you have.
I have a function that uses zlib to unzip files from a zip file to memory, and then stores the resulting file content in a string object. The problem is that the string is now composed of the binary representation of the files, and I don't know how to read that string in blocks like with
ifstream::read():
file.read((char*)result, sizeof(int));
It's necessary that I read the data in block by block, but I haven't found any process to do that from a string variable.
This is the function that returns the file content as a string (I can change the function if there's a better approach):
string externalresourcemanager::getFileInsideZip(string zipFile, string fileInZip) {
int err = UNZ_OK; // error status
uInt size_buf = WRITEBUFFERSIZE; // byte size of buffer to store raw csv data
void* buf; // the buffer
string sout; // output strings
char filename_inzip[256]; // for unzGetCurrentFileInfo
unz_file_info file_info; // for unzGetCurrentFileInfo
unzFile uf = unzOpen(zipFile.c_str()); // open zipfile stream
if (uf==NULL) {
cerr << "Cannot open " << zipFile << endl;
return sout;
} // file is open
if ( unzLocateFile(uf,fileInZip.c_str(),1) ) { // try to locate file inside zip
// second argument of unzLocateFile: 1 = case sensitive, 0 = case-insensitive
cerr << "File " << fileInZip << " not found in " << zipFile << endl;
return sout;
} // file inside zip found
if (unzGetCurrentFileInfo(uf,&file_info,filename_inzip,sizeof(filename_inzip),NULL,0,NULL,0)) {
cerr << "Error " << err << " with zipfile " << zipFile << " in unzGetCurrentFileInfo." << endl;
return sout;
} // obtained the necessary details about file inside zip
buf = (void*)malloc(size_buf); // setup buffer
if (buf==NULL) {
cerr << "Error allocating memory for read buffer" << endl;
return sout;
} // buffer ready
err = unzOpenCurrentFilePassword(uf,NULL); // Open the file inside the zip (password = NULL)
if (err!=UNZ_OK) {
cerr << "Error " << err << " with zipfile " << zipFile << " in unzOpenCurrentFilePassword." << endl;
return sout;
} // file inside the zip is open
// Copy contents of the file inside the zip to the buffer
cout << "Extracting: " << filename_inzip << " from " << zipFile << endl;
do {
err = unzReadCurrentFile(uf,buf,size_buf);
if (err<0) {
cerr << "Error " << err << " with zipfile " << zipFile << " in unzReadCurrentFile" << endl;
sout = ""; // empty output string
break;
}
// copy the buffer to a string
if (err>0) for (int i = 0; i < (int) err; i++) sout.push_back( *(((char*)buf)+i) );
} while (err>0);
err = unzCloseCurrentFile (uf); // close the zipfile
if (err!=UNZ_OK) {
cerr << "Error " << err << " with zipfile " << zipFile << " in unzCloseCurrentFile" << endl;
sout = ""; // empty output string
}
free(buf); // free up buffer memory
return sout;
}
Thank you!
EDIT:
im trying your solution (john) but i get an empty result. Here is an example code to explain the problem:
int store = 1024;
string result = "";
ofstream file("test.txt", std::ios::binary);
//save a few data
file.write(reinterpret_cast<const char*>(&store), sizeof(int));
file.write(reinterpret_cast<const char*>(&store), sizeof(int));
file.close();
//remember that i receive the "string binary data" from the getFileInsideZip() function (here i simulating the problem and store the binary data in a string only for explanation purpose)
ifstream newFile("test.txt", std::ios::binary);
newFile.seekg(0, std::ios::end);
int length = newFile.tellg();
newFile.seekg(0, std::ios::beg);
char* begin = &*result.begin();
//here i have a binary file in a string variable
newFile.read(begin, length);
newFile.close();
cout << "result:" << result << "\n";
//now im trying to get the first int from this binary data
int stored = 0;
std::istringstream bin(result);
bin.read((char*)&stored, sizeof(int));
cout << "stored: " << stored << "\n";
//but the output is:
//result -> empty (initial value)
//stored: 0 -> (initial value)
any suggestion?
Ok I did some research and I couldn't turn up anything useful. I am trying to write a program that will receive input from iwconfig (on a linux machine). It will then sort through the input, do some calculations and output to a database. Sorting through the input and outputting isn't an issue (or so I really hope it not to be) but what I am struggling with is reading input from another command line program. What I have right now as a base Hello World program is:
#include <iostream>
#include <cstdlib>
using namespace std;
int main() {
int numbr = 0;
cout << "Hello world!" << endl;
cin >> numbr;
cout << "number is " << numbr;
cout << system("iwconfig");
return 0;
}
However upon running the program, all it does is output hello world, ask for my random input and output it again. It does not output iwconfig (I also ran the line as just system("iwconfig"); without the output statement). Would someone be kind enough to explain how I could run a program like iwconfig and capture it's output?
"Would someone be kind enough to explain how I could run a program like iwconfig and capture it's output?"
Check the int system( const char *command ); documentation. It certainly doesn't provide to return the value, you want to output with your cout statement.
You probably want to have pipes established between your main and the iwconfig program, as described here, to control the input and output streams used by the child process.
To replicate the mentioned answer adapted:
int main() {
int fd_p2c[2], fd_c2p[2], bytes_read;
pid_t childpid;
char readbuffer[80];
string program_name = "iwconfig";
string receive_output = "";
if (pipe(fd_p2c) != 0 || pipe(fd_c2p) != 0) {
cerr << "Failed to pipe\n";
exit(1);
}
childpid = fork();
if (childpid < 0) {
cout << "Fork failed" << endl;
exit(-1);
}
else if (childpid == 0) {
if (dup2(fd_p2c[0], 0) != 0 ||
close(fd_p2c[0]) != 0 ||
close(fd_p2c[1]) != 0) {
cerr << "Child: failed to set up standard input\n";
exit(1);
}
if (dup2(fd_c2p[1], 1) != 1 ||
close(fd_c2p[1]) != 0 ||
close(fd_c2p[0]) != 0) {
cerr << "Child: failed to set up standard output\n";
exit(1);
}
execl(program_name.c_str(), program_name.c_str(), (char *) 0);
cerr << "Failed to execute " << program_name << endl;
exit(1);
}
else {
close(fd_p2c[0]);
close(fd_c2p[1]);
cout << "Writing to child: <<" << gulp_command << ">>" << endl;
int nbytes = gulp_command.length();
if (write(fd_p2c[1], gulp_command.c_str(), nbytes) != nbytes) {
cerr << "Parent: short write to child\n";
exit(1);
}
close(fd_p2c[1]);
while (1) {
bytes_read = read(fd_c2p[0], readbuffer, sizeof(readbuffer)-1);
if (bytes_read <= 0) break;
readbuffer[bytes_read] = '\0';
receive_output += readbuffer;
}
close(fd_c2p[0]);
cout << "From child: <<" << receive_output << ">>" << endl;
}
return 0;
}
I have the following code:
char*
Sender::PrepareData(char* filename, unsigned long long int bytesToTransfer)
{
FILE* dataFile = fopen(filename, "rb");
if (dataFile==NULL) {fputs ("File error",stderr); exit (1);}
cout << "File Open: " << filename << endl;
char* theData;
size_t bytesRead = fread(&theData, 1, bytesToTransfer, dataFile);
if (bytesRead != bytesToTransfer) {fputs ("Reading error",stderr); exit (3);}
cout << "Data Read -- Num Bytes: " << bytesRead << endl;
cout << "Data to Send: " << *theData << endl;
return theData;
}
When this method gets hit, my output is:
File Open: t.bin
Data Read -- Num Bytes: 10
Segmentation fault (core dumped)
My t.bin file contains the following:
This is a test.
98172398172837129837
alsjdf89u32ijofiou2
TEST TEST...
!!## TESTING TEST!! ###(DLKAJ)
When I run through gdb, the segfault output is:
File Open: t.bin Data Read -- Num Bytes: 10
Program received signal SIGSEGV, Segmentation fault. 0x00000000004015e2 in Sender::PrepareData (this=0x603010, filename=0x7fffffffe363 "t.bin", bytesToTransfer=10)
at sender.cpp:98 98 cout << "Data to Send: " << *theData << endl;
Can someone tell me what I'm doing wrong?
You need a buffer, theData is just a pointer.
Something like
char theData[1000];
size_t bytesRead = fread(theData, 1, bytesToTransfer, dataFile);
might work depending on the maximum value of bytesToTransfer
If you are sure you are reading a string you might also need to terminate theData before writing to cout,
theData[bytesRead] = '\0';
You'll need to allocate your buffer on the heap if you want to return it.
It'd be a lot easier to do something like
std::vector<char>
PrepareData(char* filename, unsigned long long int bytesToTransfer)
{
std::ifstream file(filename, file.binary);
if (!file) {
std::cerr << "File error";
exit(1);
}
std::cout << "File Open: " << filename << '\n';
std::vector<char> data(bytesToTransfer + 1);
file.read(data.data(), bytesToTransfer);
data.back() = '\0';
if (!file) {
std::cerr << "Reading error";
exit(3);
}
std::cout << "Data Read -- Num Bytes: " << bytesToTransfer << '\n';
std::cout << "Data to Send: " << data.data() << '\n';
return data;
}
Then again if all you are doing is reading chars from a file you should probably consider using strings.