clearing buffer after reading string w/ scanf()

Why do I have to clear the input buffer after reading a string with scanf()?
Why do I have to do this to keep may console screen from disappearing:

Code:

---

#include <stdio.h>

int main()
{
char string_ch[50];
printf("please enter a string: ");
scanf("%s", string_ch);
while (getchar() != '\n');
printf("please press enter");
getchar();
return 0;
}

---

instead of this?

Code:

---

#include <stdio.h>

int main()
{
char string_ch[50];
printf("please enter a string: ");
scanf("%s", string_ch);
printf("please press enter");
getchar();
return 0;
}

---

Because scanf() with %s doesn't read a newline, so it's just left there for the next input function to deal with.

Yes, getchar() is taking the left over '\n' that scanf doesn't use.
And reading %s with scanf is vulnerable to buffer overflows. You shoul use

Code:

---


int main()
{
  char string_ch[50];
  printf("please enter a string: ");
  fgets(string_ch,sizeof(string_ch),stdin);
  printf("please press enter");
  getchar();
  return 0;
}

---

And you may need to get rid of the '\n' on the end of the string that fgets() adds on.

fgets doesn't add a '\n'. it simply leaves it. That way you know if you got the entire file (one of the shortcomings of it fgets)

sounds like one has to clear the input buffer manually for both fgets and scanf

> fgets doesn't add a '\n'. it simply leaves it.
It only leaves it if there isn't room in the buffer you provide.

it leaves it if there isn't room? What?

Try it - nice simple program with a buffer size of say 10 bytes, then type in a nice long line and see how many fgets calls you have to make before one of them has a newline in it.

Yes i'm aware of how fgets works, your statement did not make sense though. How would it have a '\n' if there is no room for it?

Code:

---

#include <stdio.h>
#include <string.h>
int main ( void ) {
  char buff[10];
  printf("Type in some long lines\n");
  while ( fgets(buff,sizeof buff,stdin) != NULL ) {
    int hasNewline = strchr(buff,'\n') != NULL;
    printf( "%d %d %s\n", strlen(buff), hasNewline, buff );
  }
  return 0;
}


./a.out
Type in some long lines
hello world, how are you today
9 0 hello wor
9 0 ld, how a
9 0 re you to
4 1 day

---

See - if the buffer fills before the \n, then there is no newline appended (but there's always a \0)
If there is a newline, and it fits in the buffer, then fgets() returns at that point as well.

I think there's just a miscommunication going on. One person is talking about leaving the newline in the buffer passed to fgets() (instead of "adding" it there) and another person is talking about leaving it in the internal input buffer.

Just to make sure it's clear, scanf() leaves the newline in the internal input buffer. fgets() grabs the newline (if there's room) and places it in the buffer you pass to it. If the buffer you pass to fgets() is too small to accomodate the entire line, it will only store as much as it can and leave the rest (including the newline) in the internal input buffer. fgets() will never add a newline where it didn't already exist.