scanf reading "Enter" key

Question

Why scanf doesn't work when I type "Enter" in the code below?

#include <stdlib.h>
#include <stdio.h>
#include <string.h>

int main(int argc, char**argv)
{
 char *msg = malloc(100*sizeof(char));
 do{
        scanf("%s",msg);
        printf("%s\n",msg);
 } while(strcmp(msg,"")!=0);
}

Answer 1

The "%s" in scanf("%s",... skips over leading whitespace (including "Enter" or \n) and so patiently waits for some non-whitespace text.

Best to take in a \n, use fgets().

char msg[100];
if (fgets(msg, sizeof msg, stdin)) {
  // success

If you need to use scanf()

int result = scanf("%99[^\n]%*c", msg);
if (result != 1) handle_rump_line_or_end_of_file_or_IOError();

This will scan in 1 to 99 non-\n chars and then append a \0. It will then continue to scan 1 more char (presumably the \n) but not save it due to the *. If the first character is a '\n', msg is not changed and the '\n' remains in stdin.

---

Edit (2016): To cope with lines that begin with '\n', separate the scan that looks for the trailing '\n'.

msg[0] = '\0';
int result = scanf("%99[^\n]", msg);
scanf("%*1[\n]");
if (result == EOF) handle_end_of_file_or_IOError();