What is fflush(stdout) doing in this strcmp example program

[poggyQ]

So, I was trying to understnad strcmp listed on this site.
http://www.cplusplus.com/reference/cstring/strcmp/

What is the purpose for using fflush(stdout) in the example program provided on the above site?

I remove that line, and the program seems to work just fine without it.
I realize that I should probably tell you how I'm compiling it:
Using:
Windows 10 cmd.exe command prompt
Added a path variable to the Cygwin folder that contains g++ and gcc executables
I typically use g++ to compile it, but it seems this program can use either gcc or g++ to compile.

Now, I have changed the 80 in buffer[80] to different numbers
I have also changed the 79 in scanf ("%79s",buffer) to different numbers.

I have discovered that if you put an answer that contains more characters than the number shown originally as %79s then when you press enter, the question is printed more than once. I shortened that to %10s. Then the question repeats itself once for every over 10 characters used in the answer. For example, if I use 11 to 20 characters, it prints the question twice. If I use 41 to 50 characters, it prints the question 5 times.

This behavior happens whether or not I use the fflush or comment that line out.

Likewise, I discovered that there are ways to input characters so that it breaks the program so that even if I put "apple," which is the correct answer, it still doesn't read it. This also seems to happen irrespective of whether I have the fflush line included when compiling.

I have also discovered that so long as the number in buffer[80] remains higher than the number in the %79s, then when you cause the repeat, you can type the correct answer right at the character that will cause the next repeat and you'll get the question repeated, but when it reaches that correct answer repeat, it will return the correct answer prompt and close the program. It also does this irrespective of whether the fflush line is included.

So, I cannot for the life of me figure out what the fflush line is doing. I cannot figure out how or even if it's contributing anything to the program.

I have copy-pasted the example program below:

#include <stdio.h>
#include <string.h>

int main ()
{
  char key[] = "apple";
  char buffer[80];
  do {
     printf ("Guess my favorite fruit? ");
     fflush (stdout);
     scanf ("%79s",buffer);
  } while (strcmp (key,buffer) != 0);
  puts ("Correct answer!");
  return 0;
}

[jlb]

Now, I have changed the 80 in buffer[80] to different numbers
I have also changed the 79 in scanf ("%79s",buffer) to different numbers.

Do you realize that these two values are related? The 80 is the size of the array, the value used in the scanf() call should be one less than the size of the array. This is to insure that scanf() doesn't overflow the bounds of the array.

As far as the fflush() goes it is really not needed in normal operation, the scanf() call will normally cause a flush of the stdout stream.

[poggyQ]

As far as the fflush() goes it is really not needed in normal operation, the scanf() call will normally cause a flush of the stdout stream.

What happens if the flush of the stdout stream didn't happen?

Do you realize that these two values are related? The 80 is the size of the array, the value used in the scanf() call should be one less than the size of the array. This is to insure that scanf() doesn't overflow the bounds of the array.

What seemed to happen when the number in buffer[] is lower than the number in %79s is that it allows me to break the program. It causes a situation where I can make it so that I cannot input the correct answer and have it execute the code to tell me that I have a correct guess.

So, the reason may be what you said. I don't think that I ever broke the program whenever the buffer[] number was the higher number of the two.

[jlb]

What happens if the flush of the stdout stream didn't happen?

You won't see the previous message, in this case "Guess my favorite fruit? ".

What seemed to happen when the number in buffer[] is lower than the number in %79s is that it allows me to break the program.

Yes it will allow you to overflow your array. That "79" needs to be changed as well, it should always be one less than the size of the array.

I don't think that I ever broke the program whenever the buffer[] number was the higher number of the two.

No, but you will only be able to enter the number of characters equal to the width specifier.

Did you try finding and reading the documentation for the scanf() function? The documentation should help you to understand how you should be using that function.

https://linux.die.net/man/3/scanf

s

Matches a sequence of non-white-space characters; the next pointer must be a pointer to character array that is long enough to hold the input sequence and the terminating null byte ('\0'), which is added automatically. The input string stops at white space or at the maximum field width, whichever occurs first.