Suppose I want to call a subprocess from within my program, and I want to read the output from that subprocess into my program.
Here is a trivial way to do that:
//somefile.cpp
system("sub_process arg1 arg2 -o file.out");
//call the subprocess and have it write to file
FILE *f = std::fopen("file.out", "r");
//.... and so on
We all know that i/o operations are computationally slow. To speed this up, I would like to skip the write-to-file-then-read-from-file step, and instead redirect the output of this sub-process directly into stdin (or some other stream)
How would I do this? How do I skip the i/o operation?
Note: many programs spit out some diagnostic stuff into stdout while they run, and write a clean version of the output to stdout (ex: stdout: "step1...done, step2...done, step3..done" -o file-out: "The magic number is: 47.28"), so ignoring the "-o " argument and trusting that output will be automatically re-directed to stdout isn't necessarily helpful...
Thanks to all in advance.
Using popen
skips the file, and gets you command's output through an in-memory buffer.
#include <iomanip>
#include <iostream>
using namespace std;
const int MAX_BUFFER = 255;
int main() {
string stdout;
char buffer[MAX_BUFFER];
FILE *stream = popen("command", "r");
while ( fgets(buffer, MAX_BUFFER, stream) != NULL )
stdout.append(buffer);
pclose(stream);
cout << endl << "output: " << endl << stdout << endl;
}
If you happen to be on windows :
follow this : http://support.microsoft.com/kb/190351
It describes it better than I ever would. You can redirect everything, everywhere.
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