c - Disable buffering for stdin and stdout using setvbuf()

Question

When I was reading about the usage of setvbuf() , I came across the _IONBF(no buffering) mode. So I was curious how stdin and stdout will be affected if I try to disable the buffering. Below is an example code :

The Code :

#include <stdio.h>

int main(void)
{

   int num;
   char a;
   setvbuf(stdin, NULL, _IONBF, 0); //turn off buffering
   scanf("%d", &num);
   a = getchar();
   printf("%d %c
", num , a);

       return 0;
}

The Question :

1.) From the above code, the sample input I've given to the program (123a and etc) yield the same output even if I didn't include setvbuf().

2.) I understand that buffer is an intermediate storage in which a chunk of data can be filled into it and all those data will be send to the input or output stream either when the buffer is full or a newline is given.

3.)So what does the effect of disabling buffer? Is it in terms of performance?

Answer 1

It is partly performance and partly control over how stream library functions (fread, fgets, fprintf, etc.) relate to actual I/O to a device/file.

For example, stream output to a character device (e. g. your terminal) are, by default, line buffered. The effect of this is that the following code,

printf("start ");
sleep(10);
printf("stop
");

will wait 10 seconds and then print start stop[NL]. The first print was buffered because there was no new-line to flush the buffer. To get start to print, then sleep 10 seconds,you could either add a fflush call before the sleep call, or turn off buffering on stdout with setvbuf.

Stream output to a block device or disk file is, by default, fully buffered. This means that the buffer won't flush until either you overflow the buffer or do an fflush. This could be a problem with files, for example, if you want to monitor the output in real-time with tail -f. If you know that this monitoring may be done, you could switch the stream to line-buffering so that every time a new-line is printed, the buffer is flushed to the file. This would be at the cost of increased overhead as disk blocks are written several times as new-lines are printed. (Note: this overhead depends on how the file system is mounted. A fixed drive, mounted write-back cache, will have less overhead as the OS buffers writes to the disk, vs. a removable drive mounted write-though. In the latter case, the OS will try to do the partial writes to improve the chances of avoiding data loss if the drive is removed without dismounting.)