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The following program doesn’t "seem" to print "hello-out".  What is
the reason behind it?

#include <stdio.h>
#include <unistd.h>
int main()
{
    while(1)
    {
        fprintf(stdout,"hello-out");
        fprintf(stderr,"hello-err");
        sleep(1);
    }
    return 0;
}

When you observe, it looks as if the program prints only "hello-err".
But then, all of sudden, it prints "hello-out" in a bulk!  Why the
program should behave like that when it should print "hello-out" and
"hello-err" in sequence?

If you understand the concept of buffering, you will come know to why
the above program behaves like this.

Streams are of two types:  buffered and unbuffered.  Streams also can
also be categorized as text streams and binary streams.  However, on a
system like UNIX, these are identical.  Text streams that are buffered
are flushed when any of the following conditions are met:

   - The buffer is full
   - When ‘\n’ is encounterd, if the stream is line buffered
   - When a function to read from stdin is invoked
   - When the program exits
   - If the default flushing behaviour is modified by the setvbuf(3)
     library function.

Whereas, unbufferd stream are flushed as soon as the data arrive.
stderr is unbuffered by default, because this stream is used for error
reporting, and error reporting should not be delayed by buffering.

Run the above program without change and see that after some time
"hello-out" is printed in bulk.  stdout is line buffered, so if you
replace the above

    fprintf(stdout,"hello-out");

with

    fprintf(stdout,"hello-out\n");

"hello-out" is printed in iteration.

To know more on streams, refer K&R-II, section B.1.

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Related Blog Items

• The Structure and Interpretation of Computer Programs
• Operating Systems and Systems Programming
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• Machine structures lectures
• Binary Streams Utility Classes

This entry was written by Vijay Zanvar and posted on March 30, 2007 at 3:59 pm and filed under C Tidbits, I/O. Bookmark the permalink. Follow any comments here with the RSS feed for this post. Post a comment or leave a trackback: Trackback URL.