Thread: flushall()

Let me explain what's happening here.

When you ask scanf() to read a number, it will discard any spaces it finds, and then read in as many digits as it finds, and then return. (Assuming no errors occur.) This leaves any spaces and newlines (whitespace) after the number still in the input stream. If you want to read the next line, you have to discard any spaces on the current line first.

You're doing this with fgets() and gets(), but since you don't really care what you're discarding, you can just read one character at a time until you get a newline.

Code:

while(getchar() != '\n');

That's assuming I've understood your problem correctly.

Or maybe you're stumbling across the fact that fgets() will not read an entire line if there are more characters in the line than will fit in the buffer you passed it. If you're worried about this, you could examine the last character in the string fgets() read -- if it's not a newline, and EOF was not encountered (check fgets()'s return value), then the whole line couldn't fit into your buffer.

I suggest you read this thread, especially my responses. http://cboard.cprogramming.com/showthread.php?t=97080

Hi Guys,
I wasn't expecting so many replys on this one:
I will post my question again to focus on my problem.

I am looking for an efficient way to flush stdin (IN C) to avoid the following known problem:

Code:

scanf("%d",&num);
fgets(buf,80,stdin);
scanf() will take the number and leave newline in the stream so fgets() will not wait for the user input.

"The function fflush forces a write of all user-space buffered data for the given OUTPUT or update stream via the stream's underlying write function. The open status of the stream is unaffected. If the stream argument is NULL, fflush flushes all open output streams."
The behaviour of fflush() for stdin is undefined! and should not be used for input.
So this one is not an option.

The second option I could use is:

Code:

while ((c = getchar()) != '\n' && c != EOF);

This will clear the buffer, but if there is no data in the input stream, we will have to wait until there is, which is again undesirable, so this is not an option too.

My 3rd option was not to mix fgets and scanf, so I came up with the following code:

Code:

int main(void)
{
  char cNum[10];
  int iNum=0;
  char cStr[20];
 
  puts("Enter a number:");
  fgets(cNum,10,stdin);
  sscanf(cNum,"%d",&iNum);
  puts("Enter a string:");
  fgets(cStr,20,stdin);
  
  printf("Num=%d\n",iNum);
  puts(cStr);
  return 0;
}

Everything works fine until the size of the input is bigger than the size of the array. In this case the results are the same as:

Code:

scanf("%d",&num);
fgets(buf,80,stdin);

So this one is not an option eigher.
There must be an ANSI solution for this, it is impossible that C ANSI don't give this basic option and forcing me to hack the most basic operation of user input.
Please guys guide me here, I am running out of options and it is driving me CRAZY!

Thanks a million
Salvador

Originally Posted by salvadoravi

Hi Guys,
I wasn't expecting so many replys on this one:
I will post my question again to focus on my problem.

I am looking for an efficient way to flush stdin (IN C) to avoid the following known problem:

Code:

scanf("%d",&num);
fgets(buf,80,stdin);
scanf() will take the number and leave newline in the stream so fgets() will not wait for the user input.

"The function fflush forces a write of all user-space buffered data for the given OUTPUT or update stream via the stream's underlying write function. The open status of the stream is unaffected. If the stream argument is NULL, fflush flushes all open output streams."
The behaviour of fflush() for stdin is undefined! and should not be used for input.
So this one is not an option.

That is correct - fflush is not an option of input files.

The second option I could use is:

Code:

while ((c = getchar()) != '\n' && c != EOF);

This will clear the buffer, but if there is no data in the input stream, we will have to wait until there is, which is again undesirable, so this is not an option too.

But as I stated, if you use this in conjunction with scanf, it's fine - because scanf _IS_ guaranteed to leave at least a newline ('\n') in the input stream. So there will be data in the input stream after scanf().

My 3rd option was not to mix fgets and scanf, so I came up with the following code:

Code:

int main(void)
{
  char cNum[10];
  int iNum=0;
  char cStr[20];
 
  puts("Enter a number:");
  fgets(cNum,10,stdin);
  sscanf(cNum,"%d",&iNum);
  puts("Enter a string:");
  fgets(cStr,20,stdin);
  
  printf("Num=%d\n",iNum);
  puts(cStr);
  return 0;
}

Everything works fine until the size of the input is bigger than the size of the array. In this case the results are the same as:

Code:

scanf("%d",&num);
fgets(buf,80,stdin);

So this one is not an option eigher.
There must be an ANSI solution for this, it is impossible that C ANSI don't give this basic option and forcing me to hack the most basic operation of user input.
Please guys guide me here, I am running out of options and it is driving me CRAZY!

Thanks a million
Salvador

Well, if you use fgets() to read a number, and you can't rely on the user to enter a "short" number, then you may want to extend the size of the buffer. In a modern machine, a 200 byte buffer is "a drop in the ocean", with regards to memory usage. So if you make your input buffer "long enough that anyone except the most malicious user" will never fill the buffer, then you should be fine.

As dwks points out, you CAN check if the input was terminated by "out of buffer or newline" - by the fact that the input buffer contains a newline if it got there before the buffer was filled. If you don't get a newline, you will again need to read until a newline, so the solution is the same. But if there is no newline in the input buffer [and we didn't hit end-of-file conditions] there is a newline somewhere else in the input stream - that is guaranteed, because there is only two conditions that the data is made available to the actual application:
1. The input was terminated by a newline.
2. The input was terminated by end-of-file.

The input is held by the OS until one of the two above conditions apply [unless you go messing with very specific settings in the input system of your OS - and you would know if this is happening].

--
Mats