Creating fstream object from a FILE* pointer

Question

The well known way of creating an fstream object is:

ifstream fobj("myfile.txt");

ie. using a filename.

But I want to create an ifstream object using a file descriptor.

Reason: I want to execute a command using _popen(). _popen() returns the output as a FILE*. So there is a FILE* pointer involved but no filename.

@Joe: Posix file descriptors are yet another thing. Presumably, both C++ iostreams and C I/O are implemented in terms of them on a Posix platform. The present question is more reasonable, though, since both iostreams *and* C I/O are part of the standard library. — Kerrek SB, May 19 '12 at 17:53
this is a duplicate of http://stackoverflow.com/questions/2746168/how-to-construct-a-c-fstream-from-a-posix-file-descriptor given you can use `fileno` on the `FILE*` returned from `popen` — Alnitak, Oct 10 '14 at 14:39

score 8 · Accepted Answer · edited Apr 13 '17 at 12:40

8

You cannot do that just in standard C++, since iostreams and C I/O are entirely separate and unrelated. You could however write your own iostream that's backed by a C FILE stream. I believe that GCC comes with one such stream class as a library extension.

Alternatively, if all you want is an object-y way of wrapping a C FILE stream, you could use a unique pointer for that purpose.

edited Apr 13 '17 at 12:40

Community

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answered May 19 '12 at 17:51

Kerrek SB

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Basically I want to read _popen()'s output into a single string. A little research led me to fstream. Please suggest something to that context. – Tabrez Ahmed May 19 '12 at 18:08
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@ahmedtabrez: Just go with `std::fread` as usual; use an intermediate buffer and append the bytes you read to your result string. Or look into those GCC extensions. – Kerrek SB May 19 '12 at 18:14
Please, can we discuss here: http://chat.stackoverflow.com/rooms/11467/room-for-ahmedtabrez-and-kerrek-sb – Tabrez Ahmed May 19 '12 at 18:20

score 0 · Answer 2 · answered May 10 '22 at 07:34

0

You can try QTextStream.

See in https://doc.qt.io/qt-6/qtextstream.html#QTextStream-2

answered May 10 '22 at 07:34

untitled

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score -3 · Answer 3 · answered Oct 10 '14 at 14:35

You can create a string, and use fread to read and append to it. It's not clean, but you're working with a C interface.

Something like this should work:

FILE * f = popen(...)
const unsigned N=1024;
string total;
while (true) {
    vector<char> buf[N];
    size_t read = fread((void *)&buf[0], 1, N, f);
    if (read) { total.append(buf.begin(), buf.end()); }
    if (read < N) { break; }
}
pclose(f);

Creating fstream object from a FILE* pointer

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