Thread: my fflush() and other questions~

hi all,
I am a novice of C. i've read almost all articles about the topic of flushing the stdin,
I mimic and try to make my own flush function ( myfflush() ).

Code:

#include "stdio.h"
#include "string.h"

char str[5];

void main()
{
                //char str[5];                /*why can't here?*/
	void myfflush();

	fgets ( str, sizeof( str ), stdin );

	//flushall();                  /* these 3 funcs,only one available each time*/
	//myfflush();
	//fflush ( stdin );
	printf ( "str is:%s\n", str );
	printf( "Stdin is:%s\n", stdin );

	//fflush ( stdout );          /* usage of ffush(stdout)?*/
	printf ( "Stdout is:%s\n", stdout );
}

void myfflush()
{
     //extern char str[5];                 /* why can't work when defined in main()*/
	 char eat;
	 if ( !strchr( str, '\n') )
	 {
		 while ( (eat = getchar() != '\n' ) && (eat != '\0') )
		 ;
	 }
}

then I compare it with fflushall(), fflush(stdin)--not caring the portability.

when I do this in tc++, it works well, here is the input and outputs:
with any one of the 3 functions: (remove "//" before the function when using it, and add it back when using another)
input: abcdefgh
output: str is:abcd
Stdin is:
Stdout is:

it indicates that all the 3 functions worked well and had the same result.
***********************

when I do this in vc++6.0, the returned results are the same,but they have a little different with that of tc,
Stdin and Stdout have contents this time.

with any one of the 3 functions:
input: abcdefgh
output: str is:abcd
Stdin is:(some confusing characters)
Stdout is:(some other confusing characters)
***********************

then I run it in VC++2005, my function( myfflush() ) seems to work differently with the other two functions,as the
Stdin is different...:

1.with fflush(stdin) or flushall();
input: abcdefgh
output: str is:abcd
Stdin is:(some confusing characters--let's call it cstring1.)
Stdout is:(some other confusing characters--let's call it cstring2.)
2.with myfflush();
input: abcdefgh
output: str is:abcd
Stdin is:(some confusing characters--different with cstring1)
Stdout is:(some other confusing characters--the same with cstring2.)

Here are my questions:
1.Does it mean that my function( myfflush() ) doesn't have the same function with the other two?
if it does, where?
2.what does fflush( stdout ) do as it doesn't clean the screem~...? does printf() reads the string
from str[5] to a buffer of stdout, and fflush( stdout ) clears this buffer? if so, why that buffer
left uncleaned after performing printf();
3.i try to define str[5] inside main() and declare it in myfflush() with extern char str[5], but it
doesn't work , why?


many thanks! :)

getchar() returns an int, not a char, and char < int, so your getchar() won't fit into you char, possible loss of data there.
NEVER fflush( stdin ), fflush is only for output streams (read the FAQ).

2. fflush clears the internal output buffer, for example printf(string) might load 50 chars into the internal input buffer but it doesn't want to output them yet, until it gets 120... Of course this is a trvial example. So fflush() forces the flushing of the buffer, to the stream open'd (screen or file).

3. Variables declared in the global scope are in scope everywhere, its not like PHP where you must say so.

Code:

fflush clears the internal output buffer, for example printf(string) might load 50 chars into the 
internal input buffer but it doesn't want to output them yet, until it gets 120... Of course this
 is a trvial example. So fflush() forces the flushing of the buffer, to the stream open'd (screen or file).

what is internal output buffer? is it created for printf() automatically?
see...

Code:

 
......
char string[15] = "string buf";
printf (string);

Does printf() read data from string[15] and store it in its own buffer(internal out buffer) first before
putting it to screem?

Code:

for example printf(string) might load 50 chars into the internal input buffer but it doesn't want to 
output them yet, until it gets 120...

printf() can wait until it has enough data? how does it happen?


could i have the explanations? thank you

fflush clears the internal output buffer, for example printf(string) might load 50 chars into the internal input buffer

what is internal output buffer?

For now let's assume that we're working with files here, instead of the screen.

Instead of just writing every character to a file every time you want it to, the computer queues up the data you "wrote" to the file with your program. When the buffer is full, then the hard drive actually writes the data to the disk. (Hard drives and other magnetic disk drives are more efficient that way.)

But this means that, if you print something to a file, it won't actually appear unless you print lots of stuff after it. An alternative to printing lots of stuff to the file just to get the data to appear is to flush the file, that is, tell the operating system that you want the data in the buffer written to the file, even though the buffer may not be full. (Closing a file also has the same effect, because you can't write anything else to it.)

Thus, fflush() flushes an output file, causing data in the buffer to be written to the file. It does not do anything to input files. Its behaviour is undefined for input files. Do not use it on input files. See this FAQ: http://faq.cprogramming.com/cgi-bin/...&id=1043284351

The screen is a little different: it's relatively efficient to print small amounts of data to the screen, so the screen is often line buffered (the buffer holds one line) or not buffered at all. But of course you might not be writing to the screen but rather to a file, if your program's output is redirected. It's best to flush stdout if any data you write to it does not end with a newline.

So how does one clear the input buffer? http://faq.cprogramming.com/cgi-bin/...&id=1043284392