如何在C ++程序中打开进程并将结果读入ifstream？

Question

I am writing a C++11 program that will run in a Unix environment (portability isn't a concern). Currently, I have a Makefile that invokes two processes, one which writes to a file and the second which reads from that file.

target: mkfifo myfile other_program > myfile & my_program myfile

For various reasons, I want invoke all of this from within my_program. It looks like popen is promising, as it invokes an external process and provides a FILE* I can read. However, the existing file processing code I've already written uses ifstream:

std::ifstream stream1(argv[1]);

Is there a decent way to connect popen 's FILE* to an ifstream? Is there something else I should use instead of popen ?

Answer 1

You can create a stream buffer which reads from a FILE* . Clearly, you may need to change your code to use std::istream in case you use std::ifstream in other places than creating the stream but this should be a straight forward change. Here is a simple demo showing how to create a corresponding stream buffer and how to use it:

#include <cstdio>
#include <iostream>

struct FILEbuf
    : std::streambuf {
    FILEbuf(FILE* fp): fp_(fp) {}
    int underflow() {
        if (this->gptr() == this->egptr()) {
            int size = fread(this->buffer_, 1, int(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];
};

int main()
{
    FILEbuf      sbuf(popen("ls -l", "r"));
    std::istream in(&sbuf);
    for (std::string line; std::getline(in, line); ) {
        std::cout << line << '\n';
    }
}

In the past I have been told off for using popen() or system() because these calls are considered to be unsafe: both of these calls spawn a shell which can be used to hijack their behavior. The alternative is to create a stream buffer using file descriptors and using pipe() , dup() (or one of its siblings), close() , fork() , and execl() (or one of its siblings) to build the pipe directly.