pipe command make standard input broken - linux

I have two processes: t1.cpp and t2.cpp.
t1.cpp and t2.cpp are simplified ,I want to describe the problem easily.
//t1.cpp
#include <iostream>
using namespace std;
int main()
{
cout << "hello\n"
<< "world\n"
<< "ok ok\n";
return 0;
}
//t2.cpp
#include <iostream>
#include <limits>
using namespace std;
int main()
{
string str;
while(getline(cin,str)){
cout << str <<endl;
}
//cin.clear();
//flush the cin
//cin.ignore(std::numeric_limits<std::streamsize>::max(),'\n');
char x;
cin >> x;
return 0;
}
After compiling t1.cpp and t2.cpp. I execute them in this way ./t1 | ./t2.
Problems occur! cin >> x; in t2.cpp failed! I have no chance to type from the keyboard.
It seems the pipe command implements by redirecting the STDIN_FILENO. Does it forbid the standard input simultaneously?
My harsh requirements is obtain data from the output of t1 with shell command |,in addition,I want interact with users in t2.For example,I would display Sure to del?[y/n],and wait users's anwser.


Actually, there is something you can do in the C++ code. t1 has to read from stdin and echo that to stdout, which is redirected. You can do something like this at various places:
T value;
std::cin >> value;
std::cout << value;
Or to emulate the behaviour of the shell command in the comments (append stdin after t1 has finished writing its data to stdout):
std::copy(std::istreambuf_iterator<char>(std::cin),
std::istreambuf_iterator<char>(),
std::ostreambuf_iterator<char>(std::cout));


Finally,we deal this problem using "/dev/tty"
FILE *file = fopen("/dev/tty","r");
if(NULL == file){
/*error*/
}
int ch = fgetc(file);
if('y' == ch || 'Y' == ch){
/*balabala*/
}
when stdin or stdout was redirected , we also can read or write from the /dev/tty.

Related

How to extract numbers correctly from an input file in command line argument

I am attempting use ifstream to extract two numbers from a file in argv[1], named "inputFile", and the extracting operator seems to be extracting the bits of code rather than the numbers needed.
inputFile.txt was put into the command line operator by right clicking the project, going to properties -> debugging -> command arguments -> typing inputFile.txt into command arguments in visual studio 2017.
The file inputFile.txt is as below:
1 2
#include <iostream>
#include <fstream>
#include <string>
#include <iomanip>
using namespace std;
int main(int argc, char *argv[])
{
//Test opening file
cout << "Input file: " << argv[1] << endl;
ifstream in(argv[1]);
if (!in)
{
cerr << "Unable to open " << argv[1] << " for input";
return 1;
}
//extract numbers
int num1;
int num2;
in >> num1 >> num2;
cout << num1 << endl << num2 << endl;
in.close();
return 0;
}
I expect the int num1 to hold 1, and the int num2 to hold 2, but instead each variable holds the number -858993460.

Are you sure the file you are reading has got the data you expect? This code works fine for me, compiling with Visual Studio 2005. But, If I change the contents of the file, writing not numbers (for example if I write: a b), num1 and num2 ends with -858993460.

How to determine / set the length of a string from user input (std::cin)

I've try this case but I got stuck in a trouble. I just want to to determine length of a string from user input or from console. But it doesn't work.
#include <iostream>
#include <string>
using namespace std;
int main(){
int N;
string S;
char vocal;
vocal = {'a'||'i'||'u'||'e'||'o'};
cout << "Length of Spaghetti Name: ";
cin >> N;
fflush(stdin);
cout << "Spaghetti Name: ";
cin >> S;
if (S[0] != vocal){
S.erase(0,1);
} else if (S[N] != vocal){
S.erase(N,1);
}
cout << S << endl;
cin.get();
return 0;
}
expected input
9
Carbonara
expected output
arbonara
This program will delete non vocal character in spaghetti names.

squaring an int and showing the last digit - upgrade for a code

Could anyone help me to upgrade my code ?
I bet the function could be done in shorter and more elegant way, could you show me the proper direction or examples how to write that?
Thanks for the replies, here's the code ( written in Codeblocks 16.01):
#include <iostream>
#include <string>
#include <stdlib.h>
#include <stdio.h>
using namespace std;
int D,a,b,bb,liczba,dlugosc;
int main()
{
cout << "liczba testow: ";
cin >> D;
while(D<11 && D>0){
for (int j=0;j<D;j++){
cin >> a >> b;
if(a<0 | a>1000000000 | b<0 | b>1000000000){
break;
}
liczba =a;
for(int i=1;i<b;i++){
liczba = liczba*a;
}
char aa[10] = {0};
itoa(liczba,aa,10);
string bb = aa;
int dlugosc = bb.length();
cout << bb << endl;
cout << bb[dlugosc-1] << endl;
a = 0;
}
return 0;
}
return 0;
}

C++ : Run Code Error with EOF ( End-Of-File )

I'm using Visual Studio Community 2017 to run the following code but i get an error. Can anyone tell me what is the problem ?
C++ Code :
#include <iostream>
using namespace std;
int main()
{
int a, b;
while ((cin >> a >> b) != EOF)
{
cout << "Sum is: " << a + b << endl;
}
return 0;
}

Most likely issue is the EOF - the >> operator returns a reference to a stream object - not return an integer like EOF
just using tis may work ok - it will continue until a fail bit is set.
while (cin >> a >> b)

Reading with cat: Stop when not receiving data

Is there any way to tell the cat command to stop reading when not receiving any data? maybe with some "timeout" that specifies for how long no data is incoming.
Any ideas?

There is a timeout(1) command. Example:
timeout 5s cat /dev/random
Dependening on your circumstances. E.g. you run bash with -e and care normally for the exit code.
timeout 5s cat /dev/random || true

cat itself, no. It reads the input stream until told it's the end of the file, blocking for input if necessary.
There's nothing to stop you writing your own cat equivalent which will use select on standard input to timeout if nothing is forthcoming fast enough, and exit under those conditions.
In fact, I once wrote a snail program (because a snail is slower than a cat) which took an extra argument of characters per second to slowly output a file (a).
So snail 10 myprog.c would output myprog.c at ten characters per second. For the life of me, I can't remember why I did this - I suspect I was just mucking about, waiting for some real work to show up.
Since you're having troubles with it, here's a version of dog.c (based on my afore-mentioned snail program) that will do what you want:
#include <stdio.h>
#include <unistd.h>
#include <errno.h>
#include <sys/select.h>
static int dofile (FILE *fin) {
int ch = ~EOF, rc;
fd_set fds;
struct timeval tv;
while (ch != EOF) {
// Set up for fin file, 5 second timeout.
FD_ZERO (&fds); FD_SET (fileno (fin), &fds);
tv.tv_sec = 5; tv.tv_usec = 0;
rc = select (fileno(fin)+1, &fds, NULL, NULL, &tv);
if (rc < 0) {
fprintf (stderr, "*** Error on select (%d)\n", errno);
return 1;
}
if (rc == 0) {
fprintf (stderr, "*** Timeout on select\n");
break;
}
// Data available, so it will not block.
if ((ch = fgetc (fin)) != EOF) putchar (ch);
}
return 0;
}
int main (int argc, char *argv[]) {
int argp, rc;
FILE *fin;
if (argc == 1)
rc = dofile (stdin);
else {
argp = 1;
while (argp < argc) {
if ((fin = fopen (argv[argp], "rb")) == NULL) {
fprintf (stderr, "*** Cannot open input file [%s] (%d)\n",
argv[argp], errno);
return 1;
}
rc = dofile (fin);
fclose (fin);
if (rc != 0)
break;
argp++;
}
}
return rc;
}
Then, you can simply run dog without arguments (so it will use standard input) and, after five seconds with no activity, it will output:
*** Timeout on select
(a) Actually, it was called slowcat but snail is much nicer and I'm not above a bit of minor revisionism if it makes the story sound better :-)

mbuffer, with its -W option, works for me.
I needed to sink stdin to a file, but with an idle timeout:
I did not need to actually concatenate multiple sources (but perhaps there are ways to use mbuffer for this.)
I did not need any of cat's possible output-formatting options.
I did not mind the progress bar that mbuffer brings to the table.
I did need to add -A /bin/false to suppress a warning, based on a suggestion in the linked man page. My invocation for copying stdin to a file with 10 second idle timeout ended up looking like
mbuffer -A /bin/false -W 10 -o ./the-output-file

Here is the code for timeout-cat:
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
void timeout(int sig) {
exit(EXIT_FAILURE);
}
int main(int argc, char* argv[]) {
int sec = 0; /* seconds to timeout (0 = no timeout) */
int c;
if (argc > 1) {
sec = atoi(argv[1]);
signal(SIGALRM, timeout);
alarm(sec);
}
while((c = getchar()) != EOF) {
alarm(0);
putchar(c);
alarm(sec);
}
return EXIT_SUCCESS;
}
It does basically the same as paxdiablo's dog.
It works as a cat without an argument - catting the stdin. As a first argument provide timeout seconds.
One limitation (applies to dog as well) - lines are line-buffered, so you have n-seconds to provide a line (not any character) to reset the timeout alarm. This is because of readline.
usage:
instead of potentially endless:
cat < some_input > some_output
you can do compile code above to timeout_cat and:
./timeout_cat 5 < some_input > some_output

Try to consider tail -f --pid
I am assuming that you are reading some file and when the producer is finished (gone?) you stop.
Example that will process /var/log/messages until watcher.sh finishes.
./watcher.sh&
tail -f /var/log/messages --pid $! | ... do something with the output

I faced same issue of cat command blocking while reading on tty port via adb shell but did not find any solution (timeout command was also not working). Below is the final command I used in my python script (running on ubuntu) to make it non-blocking. Hope this will help someone.
bash_command = "adb shell \"echo -en 'ATI0\\r\\n' > /dev/ttyUSB0 && cat /dev/ttyUSB0\" & sleep 1; kill $!"
response = subprocess.check_output(['bash', '-c', bash_command])

Simply cat then kill the cat after 5 sec.
cat xyz & sleep 5; kill $!
Get the cat output as a reply after 5 seconds
reply="`cat xyz & sleep 5; kill $!`"
echo "reply=$reply"

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