strncmp() function vs !(strncmp()) function

Question

I'm programming a lock where to unlock it you have to insert a PIN code in a Keypad. I have the following variables:

char password_init[4] = {'1', '2', '3', '4'}; //initial password
char password[4];

When the user press a key in the keypad, that digit will be stored in the variable password and after the user press 4 digits both variables will be compared in order to give, or not, acess to the lock.

I found that one solution to do this would be using strncmp() function as:

if (!(strncmp(password, password_init, 4))){
Serial.println("PIN Code correct");
}

This works but I don't understand why I should use !(strncmo()) instead of strncmo(). If I use if (strncmp(password, password_init, 4)) the outcome will be an incorrect PIN code.

The strncmp() function compares two strings, character by character, so can someone explain me why I have to use it in a negative way in orther to the initial password and the passaword pressed by the user in the keypad match?

Some people write `if (strncmp(...) == 0)`. There are many function that return `0` on success, so eyes get used to seeing `== 0`. — KamilCuk, Dec 11 '20 at 15:33

score 5 · Answer 1 · answered Dec 11 '20 at 15:32

5

strncmp() function returns zero when the two strings are same and non-zero when they are different.

Using ! (logical not) operator, the result will be true (1) when the strings are same and false (0) when they are different.

answered Dec 11 '20 at 15:32

MikeCAT

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score 3 · Accepted Answer · answered Dec 11 '20 at 17:09

int strncmp(const char *s1, const char *s2, size_t n); not only compares for equality, it also check for order. So it needs at least 3 different return values.

The strncmp function returns an integer greater than, equal to, or less than zero, accordingly as the possibly null-terminated array pointed to by s1 is greater than, equal to, or less than the possibly null-terminated array pointed to by s2. C17dr § 7.24.4.4 3

Return some positive int: s1 is "greater than" s2
Return some negative int: s1 is "less than" s2
Return 0: s1 is "equal to" s2

!(strncmp(password, password_init, 4)) implies they are equal (up to the first 4 characters of the strings).

I find the below easier to read as a test for string equality.

if (strncmp(password, password_init, 4) == 0) {

strncmp() function vs !(strncmp()) function

2 Answers2