Problematic code:
#include <array>
#include <iostream>
#include <cstring>
int main(int argc, char *argv[])
{
using namespace std;
cout << "Read from file:" << endl;
while (!cin.eof())
{
array<char, 16> l_array;
cin.read(l_array.data(), l_array.size());
cout.write(l_array.data(), cin.gcount());
}
cout << endl;
cout << "Read from keyboard:" << endl;
cin.rdbuf(cout.rdbuf());
while (!cin.eof())
{
array<char, 64> l_array;
memset(l_array.data(), 0, l_array.size());
cin.read(l_array.data(), l_array.size());
cout << "===== DATA =====" << endl;
cout << l_array.data() << endl;
cout << "================" << endl;
}
}
This is how i run my program:
./application < file.txt
I can read data from pipe without problems but when i want to read it again it is still asociated with pipe. I have no idea how to switch it back. I have found 'rdbuf' function which can change it, but I have no idea how to use it.
I only found examples when you stard with keyboard switch to file and back to keyboard.
Like here: http://www.cplusplus.com/reference/iostream/ios/rdbuf/
But i don't have streambuf remembered so I can't do it like they did. I want to write program which can read most of data from file, and ask only when something is missing or just to ask user in runtime about permision or something. All inside console under linux.
#EDIT
Thank you for help, I post solution
class RedirectCinToConsole
{
protected:
std::ifstream m_console;
std::streambuf *m_oldCin;
bool m_success;
public:
RedirectCinToConsole() :
m_oldCin(0),
m_success(false)
{
m_console.open("/dev/tty");
if (m_console.is_open())
{
m_success = true;
m_oldCin = std::cin.rdbuf(m_console.rdbuf());
}
}
virtual ~RedirectCinToConsole()
{
if (m_oldCin)
{
std::cin.rdbuf(m_oldCin);
}
m_console.close();
}
operator bool () const { return m_success; }
};
int main()
{
RedirectCinToConsole l_redirect;
if (l_redirect)
{
std::string l_helloWorld;
std::cin >> l_helloWorld;
std::cin.ignore();
std::cout << l_helloWorld;
}
return 0;
}
It occurs to me that, regardless of the proposed solutions, the easiest
solution (and probably the best) would be to do things the opposite:
don't redirect the input, but pass the filename to the program, and let
it open an std::ifstream to read it, keeping std::cin free for
interactive input.
Ben Voigt has suggested the standard Unix solution, but on thinking
about it, it seems the above is more natural; it is certainly easier and
more portable.
Perhaps you should use fstream to create your own stream and either ask for a file name or take the file name as a command-line parameter. This will leave cin available for other input operations.
Try opening /dev/tty. This will be your process's associated console, if there is any. If your process was started from a daemon, it could fail.
Related
#include <cstddef>
#include <iostream>
#include <fstream>
#include <string>
using namespace std;
int main() {
//read the lines from the piped file using cin
string response;
int i = 0;
while (getline(cin, response)) {
//if the response is empty, stop
if (response.empty()) {
break;
}
//Write each odd line (1,3, etc..) to stderr (cerr)
//Write each even line (2,4. etc..) to stdout (cout)
if (i % 2 != 1) { //send odd to stderr
cerr << "err: " << response << endl;
}
else { //send even to stdout
cout << "out: " << response << endl;
}
i++;
}
return 0;
}
I want to redirect stderr to /dev/null, how would I go about doing so? I'm new to C++ and trying to learn by practicing, however, I'm not easily able to find an answer that fits my existing program.
Besides the excellent commentary above, it is pretty easy to make a “null” streambuf sink in C++.
#include <iostream>
struct null_streambuf: public std::streambuf
{
using int_type = std::streambuf::int_type;
using traits = std::streambuf::traits_type;
virtual int_type overflow( int_type value ) override
{
return value;
}
};
To use it, just set the rdbuf:
int main()
{
std::cerr.rdbuf( new null_streambuf );
std::cerr << "Does not print!\n";
}
If you wish to be able to turn it off and on, you will have to remember the original and restore it, not forgetting to delete the new null_streambuf.
int main()
{
std::cerr << "Prints!\n";
auto original_cerr_streambuf = std::cerr.rdbuf( new null_streambuf );
std::cerr << "Does not print.\n";
delete std::cerr.rdbuf( original_cerr_streambuf );
std::cerr << "Prints again!\n";
}
This does have the objective effect of being compiled to code, which I suspect is the advantage you are looking for: the ability to dynamically enable and disable diagnostic output.
This is, however, the usual function of a debug build, where you use the DEBUG macro to decide whether or not to compile something (such as error output operations) into the final executable.
Keep in mind that this does not disable output on standard error via other means, but only through cerr.
This small bit of code is designed to look through a text file and identify account numbers that have already been written so that later on in my program, you can find the correct account without the error of two accounts with the same account number(id). But no matter what i do, whether its using double backslashes, forward slashes, or double forward slashes in the location for the ifstream object; i always get "cannot find file" as the output.
#include <iostream>
#include <fstream>
using namespace std;
int main() {
ifstream accountsread("G:/Coding/Test/test/test/accounts.txt");
if (accountsread.is_open()) {
int tempAccno;
std::string tempname;
char tempchar;
int accountsfound = 0;
int input;
std::cout << "Enter the ID of the account \n";
cin >> x;
while (!accountsread.eof()) {
accountsread >> tempAccno;
if (tempAccno == input) {
accountsfound++;
}
else {}
}
if (accountsfound > 0) {
cout << "number found";
}
else {
cout << "number not found";
}
}
else {
cout << "cannot find file";
}
}
in windows, the location of the text file is G:\Coding\Test\test\test\accounts.txt
std::ifstream can use relative paths as well as absolute ones. For your problem, I'd recommend looking into the <filesystem> header from the STL if you really need an absolute path to your file. However, if it's in the same directory as your working directory, you don't need to use absolute paths. Here's how I'd accomplish your task
#include <iostream>
#include <fstream>
#include <string> // Should include since you're using std::string
// Note that I am NOT "using namespace std;"
int main()
{
std::ifstream accountsRead("accounts.txt");
if (accountsRead.is_open())
{
int account_id;
bool account_found = false;
std::cout << "Enter the ID of the account: ";
while (!(std::cin >> account_id))
{ // This loop handles if the user inputs non-number
std::cout << "Please enter a NUMBER below!\n";
std::cout << "Enter: ";
std::cin.ignore(10000, '\n');
std::cin.clear();
}
int tmpAccNum;
while (accountsRead >> tmpAccNum)
{ // This loop reads the file, line by line, into tmpAccNum
if (tmpAccNum == account_id)
{
account_found = true;
break;
}
}
if (account_found)
{
std::cout << "Number found!" << std::endl;
}
else
{
std::cout << "Number not found..." << std::endl;
}
}
else
{ // Error opening file
std::cout << "File not found or is corrupted" << std::endl;
}
}
A few things about your code stylistically speaking. First, you should never be using namespace std, and (if you are for some reason) there isn't a reason to mix and match specifying the std namespace on only some std members. Second, you don't need to specify an else for every if-statement, and you probably shouldn't unless there actually are commands to execute if the else case is reached.
Edit
If you still need an absolute path, here is how you can do that:
#include <filesystem>
int main()
{
// Create path object for the file path
std::filesystem::path file_path("G:\Coding\Test\test\test\accounts.txt");
// The '.string()' method for a 'std::path' object returns the string
// version of the path, so you can use it with an 'std::ifstream'
std::ifstream accounts(file_path.string()); // Opens file via 'ifstream'
/* And so on... */
}
I'm doing a C++ assingment for a class and I haven't used C++ in a decade so this might be something I'm missing that is simple; however ,I can't seem to figure it out.
I have a class I defined with a function that is producing no output; it looks like it's not even running and I don't have a clue why. Could someone point out my problem to me?
Issue: cout from the function getwords of the class readwords doesn't display any results.
Here is my class:
class readwords {
private:
char c;
//string aword;
public:
void getwords(std::istream& file) {
cout << "I got here" << std::flush;
/*while(file.good()) {
cout << "I got here\n";
c = file.get();
if(isspace(c)) cout << "\n"; //continue;
if(isalnum(c)) {
cout << c; //aword.insert(aword.end(),c);
}
}
*/
}
};
Which is being called from my main:
#include <fstream>
#include <stdlib.h>
#include "lab1.h"
using namespace std;
readwords wordsinfile;
words wordslist;
int main ( int argc, char *argv[] )
{
if ( argc != 2 ) {
// Looks like we have no arguments and need do something about it
// Lets tell the user
cout << "Usage: " << argv[0] <<" <filename>\n";
} else {
// Yeah we have arguements so lets make sure the file exists and it is readable
ifstream ourfile(argv[1]);
if (!ourfile.is_open()) {
// Then we have a problem opening the file
// Lets tell the user and exit
cout << "Error: " << argv[0] << " could not open the file. Exiting\n";
exit (1);
}
// Do we have a ASCII file?
if (isasciifile(ourfile)) {
cout << "Error: " << argv[0] << " only can handle ASCII or non empty files. Exiting\n";
exit(1);
}
// Let ensure we are at the start of the file
ourfile.seekg (0, ios::beg);
// Now lets close it up
ourfile.close();
}
// Ok looks like we have past our tests
// Time to go to work on the file
ifstream ourfile2(argv[1]);
wordsinfile.getwords(ourfile2);
}
Thank you for any help you can provide.
Try to use a debugger. Most IDEs (NetBeans, Code::Blocks, etc) provide an interactive interface with gdb.
I just compiled and ran your code, but nothing wrong with the code itself,
except that I needed to include to use the 'cout' method.
"I got here" has been successfully displayed in my ubuntu machine.
What is your execution environment? You should check it first.
The problem appears to be redefining my own class. When actually coding the function I needed to use:
in readwords::countwords(std::istream& file) {
....
}
Once doing this output produced fine.
I'm trying to redirect some of the standard output to a text file, and some other to the command prompt.
I'm currently outputting all of it to a file, but I'd like to output some to the command prompt, so I can know at least (get some hits), on what's been recorded (since it takes like 10 minutes to run this code)
This is what I'm doing;
FILE *stream ;
std::stringstream ss;
ss << "K_file.txt";
if((stream = freopen(ss.str().c_str(), "w", stdout)) == NULL)
exit(-1);
std::cout<<"blah blah blah...";
Edit based on comment;
'some' is part of the code where I would like to explicitly specify, example;
for(int i = 0; i<1000; i++)
{
std::cout<<"I would like this to go to the file - since it's detailed";
}
std::cout<<"loop finished - I would like this to go to the command prompt";
This might not be the best example but I hope you get the point.
You could "abuse" standard output and standard error stream for that. For example:
#include <iostream>
void main() {
std::cout << "standard output";
std::cerr << "standard error";
}
Now, if you redirect just the standard error to file...
your_program.exe 2> file.txt
...you'll get "standard output" in console window and "standard error" in file.txt.
(NOTE: This is Windows redirection syntax - I'm sure you'll have no trouble doing redirection on other OSes if you need to.)
I think this might help:
#include <fstream>
#include <iostream>
class stream_redirector {
public:
stream_redirector(std::ostream& dst, std::ostream& src)
: src(src), sbuf(src.rdbuf())
{
src.rdbuf(dst.rdbuf());
}
~stream_redirector() {
src.rdbuf(sbuf);
}
private:
std::ostream& src;
std::streambuf* const sbuf;
};
int main() {
std::ofstream log("log.txt");
std::cout << "Written to console." << std::endl;
{
// We redirect std::cout to log.
stream_redirector redirect(log, std::cout);
std::cout << "Written to log file" << std::endl;
// When this scope ends, the destructor will undo the redirection.
}
std::cout << "Also written to console." << std::endl;
}
I'm trying to use the ofstream class to write some stuff to a file, but all that happens is that the file gets created, and then nothing. I have some simply code here:
#include <iostream>
#include <fstream>
#include <cstring>
#include <cerrno>
#include <time.h>
using namespace std;
int main(int argc, char* argv[])
{
ofstream file;
file.open("test.txt");
if (!file) {
cout << strerror(errno) << endl;
} else {
cout << "All is well!" << endl;
}
for (int i = 0; i < 10; i++) {
file << i << "\t" << time(NULL) << endl;
}
file.flush();
file.close();
return 0;
}
When I create a console application, everything works fine, so I'm afraid this code is not completely representative. However, I am using code like this in a much larger project that - to be honest - I don't fully understand (Neurostim). I'm supposed to write some class that is compiled to a dll which can be loaded by Neurostim.
When the code is run, "test.txt" is created and then "No error!" is printed, as this is apparently the output from strerror. Obviously this is wrong however. The application runs perfectly otherwise, and is not phased by the fact that I'm trying to write to a corrupted stream. It just doesn't do it. It seems to me like there is no problem with permissions, because the file is in fact created.
Does anyone have any ideas what kind of things might cause this odd behavior? (I'm on WinXP Pro SP3 and use Visual C++ 2008 Express Edition)
Thanks!
Just a thought :- in your real code are you re-using your stream object?
If so, you need to ensure that you call clear() on the stream before re-using the object otherwise, if there was a previous error state, it won't work. As I recall, not calling clear() on such a stream would result in an empty file that couldn't be written to, as you describe in your question.
ofstream file;
file.open("test.txt");
Just a nit: you can combine that into a single line. ofstream file("test.txt");
if (file) {
cout << strerror(errno) << endl;
} else {
cout << "All is well!" << endl;
}
Your test is backwards. If file is true, it's open and ready for writing.
Also, I wouldn't count on strerror() working correctly on Windows. Most Windows APIs don't use errno to signal errors. If your failure is happening outside the C/C++ run-time library, this may not tell you anything interesting.
UPDATE Thinking more about this, failing to open a file via fstreams is not guaranteed to set errno. It's possible that errno ends up set on some platforms (espeically if those platforms implement fstream operations with FILE* or file descriptors, or some other library that sets errno) but that is not guaranteed. The official way to check for failure is via exceptions, std::io_state or helper methods on std::fstream (like fail or bad). Unfortunately you can't get as much information out of std::streams as you can from errno.
You've got the if statement wrong. operator void* returns NULL (a.k.a. false) if the file is not writable. It returns non-zero (a.k.a. true) if the file is writeable. So you want:
if (!file) {
cout << strerror(errno) << endl;
} else {
cout << "All is well!" << endl;
}
Or:
if (!file.good()) {
cout << strerror(errno) << endl;
} else {
cout << "All is well!" << endl;
}