strlen library in c

1#include <stdio.h>
2#include <string.h>
3int main()
4{
5    char a[20]="Program";
6    char b[20]={'P','r','o','g','r','a','m','\0'};
7
8    // using the %zu format specifier to print size_t
9    printf("Length of string a = %zu \n",strlen(a));
10    printf("Length of string b = %zu \n",strlen(b));
11
12    return 0;
13}

1int my_strlen(char *str) {
2  int i = -1;
3  
4  while (str && str[++i]);
5  return (i);
6}