strncmp doesn't return 0 on equal strings

Question

Shouldn't the strncmp("end",input,3) == 0 return 0 if the input is end? It returns a number > 0 though.

#include <stdio.h>

int main(void) {
  char *strArray[100];
  int strLengths[100];
  char input[100];
  int flag = 0;
  do {
    scanf("%c",&input);
      if(strncmp("end",input,3) == 0) {
        printf("end\n");
      }
      printf("%d\n",strncmp("end",input,3));
    } while(flag !=0);
  return 0;
}

The `"%c"` format is to read a *single character*. It will be written into the first position of an array that is full of *indeterminate data* (which will seem random) without the ending null-terminator needed by all string functions. — Some programmer dude, Nov 07 '18 at 13:59
Yes you were right @MathieuBunel it should be %s, thank you! — Stelios Papamichail, Nov 07 '18 at 14:00
@Someprogrammerdude *The "%c" format is to read a single character.* Maybe. By passing the address of an array for a `%c` format specifier, undefined behavior is invoked. It *probably* does read a single `char` into the first element of the array. — Andrew Henle, Nov 07 '18 at 14:02

score 3 · Accepted Answer · answered Nov 07 '18 at 14:01

This

scanf("%c",&input);

reads just a single char - maybe. It's wrong - pay attention to the errors and warnings you get from your compiler.

The format specifier is not correct - %c means scanf() will attempt to read a char, but you're passing the address of a char[100] array. That's undefined behavior, so anything might happen.

You're also not checking the return value to see if scanf() worked at all, so you don't really know what's in input.

score 0 · Answer 2 · answered Nov 07 '18 at 14:03

0

In your case, the strings compared with strncmp("end",input,3) == 0 will never match as input will contains only one character.

The call scanf("%c",&input); will read only the 1st character entered, and will store it in input

answered Nov 07 '18 at 14:03

CimCim

48
5

Accurate diagnosis. Pity there’s no explicit prescription. – Jonathan Leffler Nov 07 '18 at 14:41

Capie · Answer 3 · 2018-11-09T14:36:39.400

-1

#include <stdio.h>

int main(void) {
  char *strArray[100];
  int strLengths[100];
  char input[100];
  int flag = 0;
  do {
    scanf("%s",input);  //%c is for a single char and %s is for strings.
      if(strncmp("end",input,3) == 0) {
        printf("end\n");
      }
      printf("%d\n",strncmp("end",input,3));
    } while(flag !=0);
  return 0;
}

Next time print the input before continuing to confirm that you are doing it correct.

edited Nov 09 '18 at 14:36

answered Nov 07 '18 at 14:01

Capie

976
1
8
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3

`&input` is still incorrect. You need to use just `input` – Madhusoodan P Nov 07 '18 at 14:12

strncmp doesn't return 0 on equal strings

3 Answers3