using getchar() in C to send input to a char array

Question

edit I change my for loops into a function called input, and initialized the value of c in said function. Also changed the branching condition inside the loop, so that it inserts '\0' to the fifth element if necessary. My original problem of string1 spilling into string2 still persists. I've gotten rid of fflush( stdin ) because I'm unsure if it is supported by Mac OSX El Capitan.

I have a program that is suppose to concatenate two strings, but I want to make sure that the user doesn't overfill the size of the array

Right now, if the first string is too long, it will write characters into the next string, which is undesirable - my intention is that any extra characters beyond the space of the array will simply be ignored.

void input( char *s1, int size ){
  for( int i = 0, c = 0; ( i < size ) && ( c != '\n' ); i++ ){
      c = getchar();

      if( i == size - 1 || c == '\n' )
          s1[i] = '\0';
      else
          s1[i] = c;
  }
}

int main(){
  const int SIZE = 5;
  char string1[ SIZE ]; // create a char array
  char string2[ SIZE ]; // and another one

  printf( "Enter two strings: " );

  input( string1, SIZE );
  printf("String1: %s\n", string1);
  input( string2, SIZE );
  printf("String2: %s\n", string2);
}

Example output...

Enter two strings: foobarr
String1: foob
String2: rr

How can I change this so that 'arr' is entirely ignored, and

getchar()

in the second function call waits for new input?

There are two (unrelated) problems: If there is no newline among the five characters you read you will have an unterminated string. Also, and more related to your problem, calling `fflush` with an input-only stream like `stdin` is technically *undefined behavior*. You might also want to consider *initializing* `c` before you use it in the loop condition. — Some programmer dude, Feb 12 '17 at 18:24
As for what you ask about, how about just reading until you *get* a newline? — Some programmer dude, Feb 12 '17 at 18:26
Note that only some platforms support [Using `fflush(stdin)`](http://stackoverflow.com/questions/2979209/using-fflushstdin). — Jonathan Leffler, Feb 12 '17 at 18:26
You don't ensure that your character strings are null-terminated. The `string1` and `string2` arrays can only hold at most 4 characters plus terminating null byte. You should also be using a function to read the data rather than repeating the loop for the two different variables. — Jonathan Leffler, Feb 12 '17 at 18:28
BTW you're not checking for `getchar` returning `EOF` (just try ctrl+d in Unix or ctrl+z + enter in windows and it should get pretty wild). — Antti Haapala, Feb 12 '17 at 19:43

score 1 · Accepted Answer · edited May 23 '17 at 11:46

Okay this seems to work, I got clear_buffer() function from How to clear input buffer in C?.

void input( char *s1, int size ){

  for( int i = 0, c = 0; ( i < size ) && ( c != '\n' ); i++ ){
      c = getchar();

      // if we're out of space or the user is done with this string 
      if( i == size - 1 || c == '\n' ){

          // add null char to the array
          s1[i] = '\0';

          // if we added null char because we ran out of space
          if( i == size - 1 )
              clear_buffer();
      }
      //Otherwise add a char to the array
      else
          s1[i] = c;
  }
}

void clear_buffer(){
  char c = '\0';
  while (( c = getchar()) != '\n' && c != EOF) { }
}



int main(){
  const int SIZE = 5;

  printf( "Enter two strings: " );

  char string1[ SIZE ] = { '\0' }; // create a char array
  input( string1, SIZE );
  printf("String1: %s\n", string1);

  char string2[ SIZE ] = { '\0' }; // create another char array
  input( string2, SIZE );
  printf("String2: %s\n", string2);

  return 0;
}

using getchar() in C to send input to a char array

1 Answers1