永久获取C ++中运行的Shell命令的输出

Question

I want to get the output from a shell command (my_program). Here is a example taken from this site:

#include <iostream>
#include <stdio.h>

using namespace std;

int main() {
   FILE *in;
   char buff[512];

   if(!(in = popen("my_program", "r"))){
      return 1;
   }

   while(fgets(buff, sizeof(buff), in)!=NULL){
      cout << buff;
   }
   pclose(in);

   return 0;
}

The problem:

I want to run my program permanently and get the output while the program is still running, NOT only get the output from the program once.

Any idea?

Thanks in advanced.

Answer 1

Your question is somewhat unclear but I suspect that the problem is buffer and the answer flushing: The way I understand is the program my_program produced outputs occassionally and the program you show should kick into action when my_program prints something. Further I suspect that this program doesn't produce a lot of output in total, eg less than 4k.

The problem is that most implementations buffer standard output differently depending of whether this is directed to a "console" or something else: when writing to the console output tends to be line-buffer while other destinations buffer substantially bigger buffers. However, the buffer can be explicitly flushed, reducing the performance of the output substantially. Of course, in the scenario above you'd be more interest in timely outputs rather than output performance.

A simple approach to flush an output buffer in C++ us to use out << std::flush . If you quickly want to verify that this the problem you can set up the stream to flush after every output operation using out << std::unitbuf . There are a number of other approaches to automatically flush but these are a bit more involved.