How to copy the output of linux command to a C++ variable - c++

Question

I'm calling a LINUX command from within a C++ programme which creates the following output. I need to copy the first column of the output to a C++ variable (say a long int). How can I do it?? If that is not possible how can I copy this result into a .txt file with which I can work with?
Edit
0 +0
2361294848 [phone]
2411626496 [phone]
2545844224 [phone]
2713616384 [phone]
I have this stored as a file, file.txt and I'm using the following code to
extract the left column with out the 0 to store it at integers
string stringy="";
int can_can=0;
for(i=begin;i<length;i++)
{
if (buffer[i]==' ' && can_can ==1) //**buffer** is the whole text file read in char*
{
num=atoi(stringy.c_str());
array[univ]=num; // This where I store the values.
univ+=1;
can_can=1;
}
else if (buffer[i]==' ' && can_can ==0)
{
stringy="";
}
else if (buffer[i]=='+')
{can_can=0;}
else{stringy.append(buffer[i]);}
}
I'm getting a segmentation error for this. What can be done ?
Thanks in advance.

Answer 1

Just create a simple streambuf wrapper around popen()
#include <iostream>
#include <stdio.h>
struct SimpleBuffer: public std::streambuf
{
typedef std::streambuf::traits_type traits;
typedef traits::int_type int_type;
SimpleBuffer(std::string const& command)
: stream(popen(command.c_str(), "r"))
{
this->setg(&c[0], &c[0], &c[0]);
this->setp(0, 0);
}
~SimpleBuffer()
{
if (stream != NULL)
{
fclose(stream);
}
}
virtual int_type underflow()
{
std::size_t size = fread(c, 1, 100, stream);
this->setg(&c[0], &c[0], &c[size]);
return size == 0 ? EOF : *c;
}
private:
FILE* stream;
char c[100];
};
Usage:
int main()
{
SimpleBuffer buffer("echo 55 hi there Loki");
std::istream command(&buffer);
int value;
command >> value;
std::string line;
std::getline(command, line);
std::cout << "Got int(" << value << ") String (" << line << ")\n";
}
Result:
> ./a.out
Got int(55) String ( hi there Loki)

Answer 2

It is popen you're probably looking for. Try
man popen
.
Or see this little example:
#include <iostream>
#include <stdio.h>
using namespace std;
int main()
{
FILE *in;
char buff[512];
if(!(in = popen("my_script_from_command_line", "r"))){
return 1;
}
while(fgets(buff, sizeof(buff), in)!=NULL){
cout << buff; // here you have each line
// of the output of your script in buff
}
pclose(in);
return 0;
}

Answer 3

Unfortunately, it’s not easy since the platform API is written for C. The following is a simple working example:
#include <cstdio>
#include <iostream>
int main() {
char const* command = "ls -l";
FILE* fpipe = popen(command, "r");
if (not fpipe) {
std::cerr << "Unable to execute commmand\n";
return EXIT_FAILURE;
}
char buffer[256];
while (std::fgets(buffer, sizeof buffer, fpipe)) {
std::cout << buffer;
}
pclose(fpipe);
}
However, I’d suggest wrapping the FILE* handle in a RAII class to take care of resource management.

Answer 4

You probably want to use popen to execute the command. This will give you a FILE * that you can read its output from. From there, you can parse out the first number with (for example) something like:
fscanf(inpipe, "%d %*d", &first_num);
which, just like when reading from a file, you'll normally repeat until you receive an end of file indication, such as:
long total = 0;
while (1 == fscanf(inpipe, "%l %*d", &first_num))
total = first_num;
printf("%l\n", total);