Can I use a FILE* to initialize a C++ ostream object?

Question

Can I use an elsewhere opened

FILE* f = ...

thing to initialize some std::ostream instance? Like this (pseudocode):

FILE *f = ...;
std::ostream os;
os./*bind_to_f*/( f );    // HOW?
os << "Hello world" << std::endl;

possible duplicate of [Wrapping FILE* with custom std::ostream](http://stackoverflow.com/questions/4151504/wrapping-file-with-custom-stdostream) — GSerg, Jul 18 '11 at 15:25
@GSerg: Ben Voigt's answer certainly seems to answer this question too. — Jørgen Fogh, Jul 18 '11 at 15:38
Not an exact dup, but informational: http://stackoverflow.com/q/109449/14065 — Martin York, Jul 18 '11 at 15:45
Related to http://stackoverflow.com/questions/2746168/how-to-construct-a-c-fstream-from-a-posix-file-descriptor — Hugues, May 06 '16 at 16:15

score 4 · Answer 1 · answered Jul 18 '11 at 15:23

FILE* and streams are C and C++ style I/O capabilities respectively. Since the C++ library doesn't always know about the implementation of the underlying C library there's no way to do this. Additionally consider, what would the ownership semantics be if you could do this?

orlp · Accepted Answer · 2011-07-18T15:32:01.833

2

With GCC you can just pass the existing file descriptor to the constructor (ref):

FILE *f = ...;
std::ofstream os(fileno(f));

Constructor: ofstream::ofstream (int fd)

Make an ofstream for writing to a file that was already open, using file descriptor fd.

edited Jul 18 '11 at 15:32

answered Jul 18 '11 at 15:24

orlp

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`FILE*` isn't a file descriptor. – Šimon Tóth Jul 18 '11 at 15:29
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And using a file descriptor isn't standard, so I would advise against anything like this. – John Jul 18 '11 at 15:32
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@Let_Me_Be: Woops, but `fileno(FILE*)` returns the fd from the file. – orlp Jul 18 '11 at 15:32
@John: This is GCC only, so no need to worry about the regular standard. Though I would prefer an all-standard solution too. – orlp Jul 18 '11 at 15:34
2

actually, I think I want to try this. I like portable code as well, yes, but in this case it might save me a couple of hundred lines. How bad can it be? http://xkcd.com/292/ ;-) – towi Jul 18 '11 at 16:45
How do you close this properly? You have two objects (`FILE *` and `ofstream`) referring to the same file descriptor. Although you might just want to ignore the error. But an `fclose(f)` is necessary to free memory and will interact with the `ofstream`. A call to `dup()` might help. – sstn Jul 05 '13 at 08:29

score 2 · Answer 3 · answered Jul 18 '11 at 15:29

2

If you were to create a streambuf class that used a FILE* internally, you could do it. For example, HP OpenVMS seems to have a class like that.

answered Jul 18 '11 at 15:29

Benjamin Lindley

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Can I use a FILE* to initialize a C++ ostream object?

3 Answers3