Question

It seems that when I run the following code:

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char* argv)
{
    int i=0;

    setvbuf(stdout, NULL, _IOLBF,0);

    while (1)
            printf("%d ",i++);

    return 0;
}

it prints in chunks of 1024 chars, no matter the size I define for setvbuf().
The question is is if size affects somehow in this case and where is the definition for 1024 chars is coming from.

Answer 1

I don’t know how you identified 1024 but it’s probably BUFSIZ. BUFSIZ is defined in stdio.h.

If buf is NULL, then the stdio library
automatically allocates a buffer for
use with stream (unless we select
unbuffered I/O).

EDIT

Here’s something glibc says:

Macro: int BUFSIZ
The value of this macro is an integer constant expression that is
good to use for the size argument to
setvbuf. This value is guaranteed to
be at least 256.

The value of BUFSIZ is chosen on each system so as to make stream I/O
efficient. So it is a good idea to use
BUFSIZ as the size for the buffer when
you call setvbuf.

EDIT 2

@larsmans is right. I looked at how setvbuf is implemented and it ignores a call when asking for line buffering and presenting a NULL buf. Now, stdout is no ordinary file, it’s attached to a terminal. So, heading over to pixelbeat

• Buffer size only directly affects buffered mode
• The default size like the kernel is based on the page size (4096 bytes on
  my system)
• if stdin/stdout are connected to a terminal then default size = 1024;
  else size = 4096