How to clear input buffer after fgets overflow?

Question

I'm facing a little problem with fgets when the input string exceeds its predefined limit.

Taking the example below:

    for(index = 0; index < max; index++)
    {printf(" Enter the %d string : ",index+1)
                if(fgets(input,MAXLEN,stdin))
                {
                    printf(" The string and size of the string is %s and %d \n",input,strlen(input) + 1);
                    removeNewLine(input);
                    if(strcmp(input,"end") != 0)
                   { //Do something with input
                   }
                }

Now when I exceed the length MAXLEN and enter a string, I know that the input will append a '\0' at MAXLEN -1 and that would be it. The problem happens when I try to enter the 2nd string which is not asked for i.e

Output :
Enter the first string : Aaaaaaaaaaaaaaaaaaaa //Exceeds limit
Enter the second string : Enter the third string : ....Waits input

So, I thought I should clear the buffer the standard way as in C. It waits until I enter

return

two times, The first time it being appended to the string and the next time,expecting more input with another return. 1. Is there any method by which I can clear the buffer without entering the extra return? 2. How do I implement error handling for the same? Because the fgets return value will be Non-null and strlen(input) gives me the accepted size of the string by fgets, what should be done?

Thanks a lot

If you keep on editing you post, others answers will not make any sense in future. SO doesn't aim at solving specific issues, but also helps others to search for answers. — P0W
– P0W, Commented Aug 21, 2013 at 7:35
@IDK please do not revert , right now question seems what you required... — Gangadhar
– Gangadhar, Commented Aug 23, 2013 at 8:53

P0W · Accepted Answer · 2013-08-21 06:25:11Z

4

If I understood correctly, looks like you want to avoid twice enter press, when entered input is within range.

A work around would be

for(index = 0; index < max; index++)
{
    printf(" Enter the %d th string :",index);
    // if (strlen(input) >=MAXLEN )

    if(fgets(input,MAXLEN,stdin))
    {

        removeNewLine(input);

        if(strcmp(input,"end") != 0)
        // Do something with input 
          ;
    }
    if (strlen(input) == MAXLEN-1 )
      while((ch = getchar())!='\n'  && ch != EOF  );

 }

With a limitation that it will again ask for two times enter when entered characters are exactly MAXLEN-2.

Or else you can simply form your input using character by character input.

edited Aug 21, 2013 at 6:25

answered Aug 21, 2013 at 6:14

P0W

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4 Comments

Hooli Over a year ago

Worked for me!Can you please explain the limitation part?

P0W Over a year ago

@IDK, well say your MAXLEN is 10 , if you enter abcdefgh and ENTER it will again wait for another ENTER

Hooli Over a year ago

It is not and why should it? I entered a string and if it's string length is equal to 9 excluding the null character ofcourse,it's possible that the string overflowed. Any code dependant issue here related to '\n' which is why its not waiting? Refer to the code above. Thanks!

P0W Over a year ago

Well, for your case fflush(stdin) should just work fine. But its use is highly discouraged as it may lead to UB on some C implementation. while((ch = getchar())!='\n' && ch != EOF ); is used instead.

R.. GitHub STOP HELPING ICE · Accepted Answer · 2013-08-21 04:44:28Z

3

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

or:

scanf("%*[^\n]%*c");

answered Aug 21, 2013 at 4:44

R.. GitHub STOP HELPING ICE

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3 Comments

Grijesh Chauhan Over a year ago

For @OP: For first choice read "How to Flush the input buffer?"

P0W Over a year ago

I think OP has already tried this "//An attempt to clear buffer" And he wants to avoid twice enter press, when entered input is not overflowing

R.. GitHub STOP HELPING ICE Over a year ago

This code should just be move to after the call to fgets, not before it, and should only be called if the buffer filled by fgets did not end in '\n'.

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