How do you "flush" the write() system call?

Question

I am taking input from the user and then printing it to the standard output using read() and write() system calls. For some reason, it prints a few extra characters in addition to the regular output. The additional characters are from the user input, so they're not random. However, I am assuming that this may be due to the fact that the buffer is not cleared to take in new input. I searched many different things that would be appropriate in this scenario, but I am still confused. I am only using low-level I/O for this, so no C standard libraries will be implemented. The majority of people I've asked have said use "fflush", but I thought that would only be applicable with the stdio.h library, which I will not be using. Thanks in advance for any advice or resources on the matter.

score 2 · Accepted Answer · answered Oct 31 '17 at 06:44

How do you “flush” the write() system call?

You don't. It's a system call. You don't have any application-side buffer to flush.

I am taking input from the user and then printing it to the standard output using read() and write() system calls. For some reason, it prints a few extra characters in addition to the regular output.

You have a bug in your code. It should look like this:

int count;
while ((count = read(inFD, buffer, sizeof buffer)) > 0)
{
    write(outFD, buffer, count);
}

Ten to one you have the third parameter to write wrong.

The majority of people I've asked have said use "fflush", but I thought that would only be applicable with the stdio.h library, which I will not be using.

They are wrong and you are right.

Yes, you were totally right. The third parameter to my write system call was incorrect. It's all fixed now! Thank you for all the clarifications! Have a good night. — HelpMe, Oct 31 '17 at 06:47
`int` usually works, but to be completely correct, both `read()` and `write()` functions return `ssize_t`. — Andrew Henle, Oct 31 '17 at 08:58

score -1 · Answer 2 · edited Oct 31 '17 at 07:19

-1

try this function:

eraseFunction()
{
    std::cin.clear();
    std::cin.ignore(std::cin.rdbuf()->in_avail());
}

so as soon as you read() something you should call this function to erase the buffer

edited Oct 31 '17 at 07:19

Komal12

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answered Oct 31 '17 at 06:34

christian mali

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The OP mentions [system call](https://en.wikipedia.org/wiki/System_call)s, which on Linux are listed in [syscalls(2)](http://man7.org/linux/man-pages/man2/syscalls.2.html). The `read(2)` and `write(2)` system calls are not using C++ streams, it is the other way round. Read http://pages.cs.wisc.edu/~remzi/OSTEP/ for more – Basile Starynkevitch Oct 31 '17 at 07:23

How do you "flush" the write() system call?

2 Answers2