scanf() function doesn't work?

Question

This may be a simple question, but i searched a lot and still didn't figure it out. I compiles below snip code by gcc and run program from terminal. In correct, It allow to enter an int and a char but it doesn't. It doesn't wait to enter the char??

Anyone here can help me will be kind. thanks in advance!

#include <stdio.h>

int main()
{

  char c;
  int i;

  // a
  printf("i: ");
  fflush(stdin); scanf("%d", &i);

  // b
  printf("c: ");
  fflush(stdin); scanf("%c", &c);

  return 0;

}

fflush is not defined on the input stream. Too bad some C books actually encourage it. — digital_revenant, Aug 25 '13 at 03:35
What does it give? The code snippet ends with scanf(), so nothing is being done with the variable `c`. Actually, the same for `i`. Note that scanf() parsing depends a lot on the `stty` settings of the terminal, if the input is a terminal. — ash, Aug 25 '13 at 03:58

score 8 · Answer 1 · edited May 23 '17 at 11:57

8

%d will read consecutive digits until it encounters a non-digit. %c reads one character. Probably what's happening is that you're giving it a number (several digits) followed by a new line. %c then reads in that new line. You were probably intending for the fflush(stdin); to discard anything that hadn't yet been read, but unfortunately, that's undefined behavior.

The solution is to discard all whitespace before reading the character:

scanf(" %c", &c);

Note the space at the start. That means to discard all whitespace.

edited May 23 '17 at 11:57

Community

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answered Aug 25 '13 at 03:20

icktoofay

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It is exactly what I did: enter a number and followed by a new line. It works now! Thanks so much! – dangnam2910 Aug 25 '13 at 03:29

Rahul Tripathi · Answer 2 · 2013-08-25T03:29:27.000

2

You can use the getchar() to achieve what you want.

or consume the extra newline by using:-

 scanf(" %c", &c);
  ^^^   <------------Note the space

Reason:- Your next scanf for reading the character just reads/consumes the newline and hence never waits for user input

edited Aug 25 '13 at 03:29

answered Aug 25 '13 at 03:19

Rahul Tripathi

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Replacing `scanf("%c", &c);` with `c = getchar();` shows the same behavior for me. – icktoofay Aug 25 '13 at 03:21
I know, but I want to figure out how it doesn't work in this case. Futher more, when I changes the order //a and //b then it works. – dangnam2910 Aug 25 '13 at 03:23
Your next scanf for reading the character just reads/consumes the newline and hence never waits for user input – Rahul Tripathi Aug 25 '13 at 03:25
@dangnam2910: updated my answer with the reason. Hope that makes it clear!!! – Rahul Tripathi Aug 25 '13 at 03:30

Gangadhar · Answer 3 · 2013-08-25T03:33:34.847

2

Instead of fflush(stdin); scanf("%c", &c);

1.use scanf with extra space

scanf(" %c",&c);

or

2.use getchar() two times , first time reads '\n' which is entered after giving integer input and second time call ask you for give input as c:

getchar();
c=getchar();

would help you.

edited Aug 25 '13 at 03:33

answered Aug 25 '13 at 03:22

Gangadhar

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score 1 · Answer 4 · answered Aug 25 '13 at 03:36

First of all, scanf works when used as directed. I think the following code does what you want. Stdout is flushed so that user is prompted to enter an integer or a character. Using %1s allows white space like \n.

int main()
{

  char c[2];
  int i;

  printf("i: ");
  fflush(stdout);
  scanf("%d", &i);

  printf("c: ");
  fflush(stdout);
  scanf("%1s", &c);

  printf("\ni = %d, c = %c", i, c[0]);

  return 0;

}

This code was tested/run on an Eclipse/Microsoft C compiler.

Mike Housky · Answer 5 · 2022-09-07T04:37:31.413

That fflush() is not guaranteed to do anything, and gcc/g++ doesn't. Not on Linux, anyway.

I thought I invented the following way to flush the rest of a line...until I saw it as an example in the ISO C spec (90 or 99...forgot which, but it's been there a long time either way...and I'll bet most readers here have seen it before.)

scanf("%*[^\n]%*c"); /* discard everything up to and including the next newline */

You can put that in your own "flush" function to save typing or pasting that all over the place.

You should still follow the suggestions to to put a space in scanf(" %c", &c);.

That will patiently wait for a non-whitespace character in case of a leading space or a double hit of the enter key.

scanf() function doesn't work?

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