Question

I use setbuf in order to redirect stdout to char buffer
But I get some side effect after it ,when I want to write to the stdout only the new data

As explained in the following code:

#define bufSize  100
int main()
{
    char buf[bufSize];
    setbuf(stdout, buf);
    printf("123");             //123 is written to the buffer

    setbuf(stdout,NULL);       //123 is written to the stdout(unwanted side effect)
    printf("456");             //123456 appears in the stdout
}

How can I solve the problem?

Other question regarding this – will this code work for unix/linux/mac os?

Can anyone propose the solution for redirection?

Answer 1

I don’t think you can redirect stdout to a buffer like this. It ends up in stdout anyway. You only specify a buffer which will be used by your stream.

If you use setbuf(stdout, NULL), it will create a new buffer behind the scenes. And it will be unbuffered writing.. i.e. no flush needed.

So there is no side effect here, just normal behaviour.

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If you want to redirect stdout look here. The first two answers describe two techniques doing that.