CODEWITHSUNDEEP
c
jhonnna
jhonnna

Reputation: 107

Pointer subtraction strlen

I don't understand the last line of sstrlen, return t-str;.
str points to "my string" and t points to \0 so why does it work?

#include <stdio.h>

size_t sstrlen(char *str){
    char *t = str;
    for(;*t != '\0';t++);
    return t-str; // how does it work?
}

int main()
{
    char *str = "my string";
    printf("%zu",sstrlen(str));     
    return 0;
}

Upvotes: 0

Views: 322

Answers (2)

Joseph
Joseph

Reputation: 97

That is used to know the length of the string... Because of while loop t will go to the end of string or character array, and when you subtract ending address with starting address you will get string length. It's a bit faster then using index.

Upvotes: 0

Soner from The Ottoman Empire
Soner from The Ottoman Empire

Reputation: 20901

sizeof(char) is defined to always be 1. From C99:

When applied to an operand that has type char, unsigned char, or signed char, (or a qualified version thereof) the result is 1.

Assume that your m character of my string is stored at a memory location which is 100. Arrays are always consecutive.

100 -->  m          <-- t and str point to here    
101 -->  y
102 -->  (space)
103 -->  s
104 -->  t
105 -->  r
106 -->  i
107 -->  n
108 -->  g           
109 -->  \0          <-- end of for loop t points to here

Subtraction address 100 pointed to by str from address 109 pointed to by t results in 9. That's how it works.

Upvotes: 5

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