Understanding code of function strlen in C Programming-CodePudding

I was given a solution to the problem of recreating the function strlen in C Programming and it was the following, I need help understanding what the code is actually doing.

void    *ft_memset(void *b, int c, size_t len)
{
    size_t i;

    i = 0;
    while (i < len)
    {
        ((unsigned char *)b)[i] = c;
        i  ;
    }
    return (b);
}

My biggest doubts are the meaning of ((unsigned char *)b)[i] = c, specially because I don't understand the ( *)b, and the need for a return b in a void function.

Thank you very much for your help, sorry if I did something wrong its my first post in stack overflow.

CodePudding user response：

Let's break it down, albeit my C skills are somewhat rusty. Edits are welcome :)

void    *ft_memset(void *b, int c, size_t len)
{
    size_t i;

    i = 0;
    while (i < len)
    {
        ((unsigned char *)b)[i] = c;
        i  ;
    }
    return (b);
}

First, you have declared a function called ft_memset which returns void *.

void * means a pointer to any object. It's just a number, pointing to a memory address. But we do not know what's in it because it's not properly annotated.

Your function takes three arguments: b which is a pointer to anything (or void pointer if you will) c which is an int 32-bit signed integer number. len which is a size_t which is usually an alias to an unsigned integer. Read more about size_t over here

Your function iterates through the first len bytes and sets the c's value to those.

We're basically overwriting b's contents for the first len bytes.

Now, what does ((unsigned char*)b) mean. This statement is casting your void * to an unsigned char* (byte pointer), this is just telling the compiler that we're gonna deal with the contents pointed by b as if they were just a unsigned char.

Later we're just indexing on the i-th position and setting the value c in it.

CodePudding user response：

Understanding code of function strlen

void *ft_memset(void *b, int c, size_t len) is like void *memset(void *s, int c, size_t n). They both assign values to the memory pointed to by b. These functions are quite different form strlen().

size_t strlen(const char *s) does not alter the memory pointed to by s. It iterates through that memory looking for a null character.

size_t strlen(const char *s) {
  const char *end = s;
  while (*end != '\0') {
    end  ;
  }
  return (size_t)(end - s); 
}

// or

size_t strlen(const char *s) {
  size_t i = 0;
  while (s[i] != '\0') {
    i  ;
  }
  return i; 
}