Why does scanf() read only after specified text?

Question

I was doing a CTF reversing challenge when I came across this C code in Ghidra:

int main(void)
{
  int iVar1;
  char input[32];
  
  fwrite("Password: ",1,10,stdout);
  __isoc99_scanf("DoYouEven%sCTF",input);
  iVar1 = strcmp(input,"__dso_handle");
  if ((-1 < iVar1) && (iVar1 = strcmp(input,"__dso_handle"), iVar1 < 1)) {
    printf("Try again!");
    return 0;
  }
  iVar1 = strcmp(input,"_init");
  if (iVar1 == 0) {
    printf("Correct!");
  }
  else {
    printf("Try again!");
  }
  return 0;
}

And upon making my own similar code, I noticed that the program only saves in input when my input starts with DoYouEven and only saves whatever comes after it. And I am trying to understand the reasoning for this from the source code of scanf.c and vfscanf.c, but I am unable to understand the actual logic behind this.

My snippet:

int main()
{ 
    char input[32];
    
    printf("Input: ");
    int a = scanf("DoYouEven%sCTF",input);
    printf("Input was: %s\n", input);
    
    int iVar = strcmp(input,"__dsohandle");
    printf("iVar: %d\n", iVar);
    printf("strcmp __dsohandle > -1: %d\n", (-1 < iVar));    
    printf("strcmp __dso_handle: %d\n", iVar);
    printf("strcmp _init: %d\n", strcmp(input,"_init"));
    printf("%d\n",a);
    return 0;
}

Output:

Enter your input: DoYouEvenByee
input is: Byee
iVar: -29
strcmp __dsohandle > -1: 0
strcmp __dso_handle: -29
strcmp _init: -29
1

Can anybody help me understand this through source code? I am not a master at understanding C libraries.

Answer 1

In short, this is how scanf works.

Characters in the format string that are not either whitespace or part of a format specifier are expected to be matched exactly. If at any point reading the input there is a mismatch, it stops reading. So given this call:

scanf("DoYouEven%sCTF",input);

The string "DoYouEven" is expected to be read first. Assuming those characters are read in, the next sequence of characters up to the next whitespace will be stored in input.