Variadic string concatenation in C with allocation (you most likely don't wanna use it)

alloc_concat_strings.c

#include <stdarg.h>
#include <stdlib.h>
#include <stdio.h>

int get_string_length(char* string) {
  int size = 0;

  while (*string != '\0') {
    size += 1;
    string += 1;
  }

  return size;
}

void copy_string(char* target, char* source) {
  while (*source != '\0') {
    *target = *source;
    target++;
    source++;
  }
}

char* alloc_concat_strings(int arg_count, ...) {
  va_list args;
  va_start(args, arg_count);

  int total_length = 0;
  int* string_lengths = (int*)malloc(arg_count * sizeof(int));
  if (string_lengths == NULL) return NULL;
  char** string_pointers = (char**)malloc(arg_count * sizeof(char*));
  if (string_pointers == NULL) return NULL;

  for (int idx = 0; idx < arg_count; idx++) {
    string_pointers[idx] = va_arg(args, char*);
    string_lengths[idx] = get_string_length(string_pointers[idx]);
    total_length += string_lengths[idx];
  }

  char* new_string = (char*)malloc(total_length * sizeof(char));
  if (new_string == NULL) return NULL;
  char* result = new_string;

  for (int idx = 0; idx < arg_count; idx++) {
    new_string += idx == 0 ? 0 : string_lengths[idx - 1];
    copy_string(new_string, string_pointers[idx]);
  }

  free(string_lengths);
  free(string_pointers);

  return result;
}

int main() {
  char* hello = "He11o";
  char* world = "W0rl9";
  char* separator = " ";
  
  char* hello_world = alloc_concat_strings(3, hello, separator, world);
  if (hello_world == NULL) return 1;

  printf("%s", hello_world);

  return 0;
}

Compiled assembly on Godbolt (best site evuh)