Finding out the number of chars read/write reads

Question

I'm fairly new to c so bear with me.

How do I go about finding out the number of chars read/write reads?
Can I be more specific and designate the # of chars read/write reads in an argument? If so, how?

Be more specific, give examples of input you receive and what you expect the output would be, and what you have tried so far. Nobody is going to guess what you're wishing — Inox, Sep 03 '14 at 13:18
How do you do IO? (with or without ``?). On which operating system (Posix, e.g. Linux)? Edit your question to give some code! — Basile Starynkevitch, Sep 03 '14 at 13:25

Drax · Answer 1 · 2014-09-03T13:22:38.513

2

From man(2) read:

If successful, the number of bytes actually read is returned

From man(2) write:

Upon successful completion the number of bytes which were written is returned

Now concerning:

Can I be more specific and designate the # of chars read/write reads in an argument? If so, how?

AFAIK no, but there might be some device/kernel specific ways using for example ioctl(2)

edited Sep 03 '14 at 13:22

answered Sep 03 '14 at 13:15

Drax

12,682
7
45
85

But OP is probably not using *directly* [read(2)](http://man7.org/linux/man-pages/man2/read.2.html) syscall! – Basile Starynkevitch Sep 03 '14 at 13:16
I highly doubt he is using `boost::asio` :) – Drax Sep 03 '14 at 13:17
No, but probably `printf` or `std::cout` ; I said "directly" in my comment!! – Basile Starynkevitch Sep 03 '14 at 13:17

score 0 · Answer 2 · edited May 23 '17 at 12:29

C and C++ has different IO libraries. I guess you are coding in C.

fprintf(3) returns (when successful) the number of printed characters.

scanf(3) returns the number of successfully read items, but also accept the %n specifier:

   n      Nothing is expected; instead, the number of characters
          consumed thus far from the input is stored through the next
          pointer, which must be a pointer to int.

You could also do IO line by line... (getline, snprintf, sscanf, fputs ....)

for Linux and Posix

If you call directly the read(2) or write(2) functions (i.e. syscalls) they return the number of input or output bytes on success.

And you could use the lseek(2) syscall, or the ftell(3) <stdio.h> function, to query the current file offset (which has no meaning so would fail on non-seekable files like pipes, sockets, FIFOs, ...).

Finding out the number of chars read/write reads

2 Answers2

for Linux and Posix