Why can't c++ ifstreams read from devices?

Question

I knew I should never have started using c++ io, the whole "type safety" argument is a red herring (does anyone really find that it's one of their most pressing problems?). Anyhow, I did, and discovered a strange difference between ifstreams and FILE*s and plain old file descriptors: ifstreams cannot read from a device. Can anyone think of a reason why?

const char* path = "/dev/disk3";
char  b;

// this works
FILE* f= fopen(path, "rb");
int i = fread(&b, 1, 1, f);  // returns 1, success!

// this does not work
ifstream cf(path, ios::binary);
cf.read(&b, 1);
bool  good = cf.good(); // returns false, failure.

Answer 1

I have found random issues like this in C++ I/O for years. It has always seemed like a backwards step to me.

I haven't tried it, but you might want to look at what Boost has to offer:

http://www.boost.org/doc/libs/1_36_0/libs/iostreams/doc/index.html

Answer 2

Works fine for me, its not a problem inherent to c++ stream file I/O as you seem to think.

Maybe try adding ios::in to the openmode flags. According to 27.8.1.6 of the standard, it is required to or with ios::in so this probably won't do anything.

Try checking which bit is actually set - instead of calling good(), try checking bad(), eof(), and fail() individually. This might give a clue as to what the error was.

Answer 3

The device is unbuffered and must be read from in 512 byte multiples. ifstream does it's own buffering and strangely decided to read 1023 bytes ahead, which fails with "Invalid argument". Interestingly, this ifstream is implemented on top of a FILE*. However, FILE* left to its own devices was reading ahead using a nicer, rounder number of bytes.

Thanks to dtrace for vital clues. I guess we'll never know if the folk who thought they knew answer but didn't want to say were right.