Question

I need to read the output from another program from my program. To do that, I can use popen(). This way, I get a FILE* to the program’s output that can be parsed in my program. How can I read and parse the content of the return value of popen() from my program? Alternatively, is there a more C++-like version of popen somewhere?

Answer 1

You can do either create a stream buffer doing the moral equivalent of popen() or you can create a stream buffer wrapping FILE*. Once you got a stream buffer, you can use it with std::istream which seems to be what you intend to do. A simple (and untested) stream buffer reading from a FILE* would look something like this:

struct FILEbuf
    : std::streambuf {
    FILEbuf(FILE* fp): fp_(fp) {}
    int underflow() {
        if (this->gptr() == this->egptr()) {
            int size = fread(this->buffer_, 1, s_size, this->fp_);
            if (0 < size) {
                this->setg(this->buffer_, this->buffer_, this->buffer_ + size);
            }
        }
        return this->gptr() == this->egptr()
            ? traits_type::eof()
            : traits_type::to_int_type(*gptr());
     }
     FILE* fp_;
     enum { s_size = 1024 };
     char  buffer_[s_size];
};

I reckon I mistyped something but roughly the idea does work: Just read from the FILE* into a stream buffer. Alternatively, create a stream reading from a spawned process. This take a couple of platform specific calls to fork(), exec..(), pipe(), close(), dup2() (probably forgot something here, too) but can be done as well.

A stream buffer like the one above could be used, e.g., like this:

int main()
{
    FILEbuf      sbuf(fopen("pstream.cpp", "r"));
    std::istream in(&sbuf);
    for (std::string line; std::getline(in, line); ) {
        std::cout << line << '\n';
    }
}