Get stdout of a shell command using boost process

Question

I am trying to implement a function in C++ that runs a shell command and returns the exit code, stdout and stderr. I am using the Boost process library

std::vector<std::string> read_outline(std::string & file)
{
    bp::ipstream is; //reading pipe-stream
    bp::child c(bp::search_path("nm"), file, bp::std_out > is);

    std::vector<std::string> data;
    std::string line;

    while (c.running() && std::getline(is, line) && !line.empty())
        data.push_back(line);

    c.wait();

    return data;
}

In the above example from boost's website, in the while loop the condition c.running() is checked. What if the process finishes executing before the while loop is reached? In that case I won't be able to store the child process's stdout to data. Boost's documentation also mentions the following

[Warning] Warning The pipe will cause a deadlock if you try to read after nm exited

Hence it seems that the check for c.running() should be there in the while loop.

How do I get the stdout (and stderr) from the processes that finish running before the program reaches the while loop?

Answer 1

I believe that whats the wait call is there for. The child process isn't actually gone in whatever OS before that (it merely changes state after it's not in running state).