Print (work with) substrings in a C program

Question

How do I work with a string if I only want to print part of it?

For example, print "bcd" from "abcdef"?

I know how to work with a pointer to the beginning of a string, but I don't know how to determine the end.

void vypis(char *retezec)
{   
    printf("%s", retezec);
}

int main (void)
{
    char *str = NULL;
    size_t capacity;
        
    getline(&str, &capacity, stdin);    
    vypis(str);
    vypis(str+1);       
}

Answer 1

I know how to work with a pointer to the beginning of a string, but I don't know how to determine the end.

A pointer to the last character you'd like to print is a possible solution:

void vypis(const char *retezec, const char *end)
{   
    while (retezec <= end && *retezec != '\0')
    {
        putchar(*retezec);
        retezec++;
    }
    putchar('\n');
}

int main (void)
{
    char *str = NULL;
    size_t capacity = 0;
        
    getline(&str, &capacity, stdin);    
    vypis(str, str + 5);      //prints from str[0] to str[5]
    vypis(str + 1, str + 3);  //prints from str[1] to str[3]     
}