What does space in scanf mean? [duplicate]

Question

#include <stdio.h>
int main(int argc, char* argv[]) {
  char c;
  scanf(" %c", &c);
  printf("%c\n", c);
  return 0;
}

[root@test]# ./scanf 
a
a

[root@test]# ./scanf 
 h
h

It seems always matching whether space exists,why?

Answer 1

The %c will not skip whitespace char as numeric format specifiers do. So if you use :

#include<stdio.h>
int main(int argc, char* argv[]){
  char c;
  scanf("%c", &c);
  printf("%c\n", c);
  scanf("%c", &c); // Try running with and without space
  printf("%c\n", c);
  return 0;
}

It is very likely that the previous whitespace character in the input buffer will be taken in the second scanf, and you will not get a chance to type. The space before %c will make scanf skip whatever whitespace character is there in the input buffer so that you will be able to enter your input properly. Sometimes to get the same effect people write:

fflush(stdin);
scanf("%c" &c);

But this is considered very bad programming as C Standard specifies the behavior of fflush(stdin) is undefined. So always use space in the format string unless you have a specific reason to capture whitespacesas well.