user: 这段汇编代码用C语言表示是什么样的:

```asm
<string_join>:
  endbr64
  push   %rbp
  mov    %rsp,%rbp
  push   %r15
  push   %r14
  push   %r13
  push   %r12
  push   %rbx
  sub    $0x38,%rsp
  mov    %rdi,%r13
  mov    %rsi,%rbx
  mov    %rsi,-0x60(%rbp)
  mov    %fs:0x28,%rax
  mov    %rax,-0x38(%rbp)
  xor    %eax,%eax
  test   %rdi,%rdi
  setne  %dil
  movzbl %dil,%edi
  call   <assert@plt>
  test   %rbx,%rbx
  setne  %dil
  movzbl %dil,%edi
  call   <assert@plt>
  mov    0x0(%r13),%r14
  test   %r14,%r14
  je     L51
  mov    $0x0,%eax
L142:
  mov    %rax,%r12
  add    $0x1,%rax
  lea    0x0(,%rax,8),%rdx
  mov    0x0(%r13,%rax,8),%r14
  test   %r14,%r14
  jne    L142
  test   %rax,%rax
  je     L51
  add    $0xf,%rdx
  mov    %rdx,%rcx
  and    $0xfffffffffffffff0,%rcx
  and    $0xfffffffffffff000,%rdx
  mov    %rsp,%rax
  sub    %rdx,%rax
L131:
  cmp    %rax,%rsp
  je     L91
  sub    $0x1000,%rsp
  orq    $0x0,0xff8(%rsp)
  jmp    L131
L91:
  mov    %rcx,%rax
  and    $0xfff,%eax
  sub    %rax,%rsp
  test   %rax,%rax
  je     L52
  orq    $0x0,-0x8(%rsp,%rax,1)
L52:
  mov    %rsp,%r15
  mov    -0x60(%rbp),%rdi
  call   <strlen@plt>
  mov    %rax,-0x58(%rbp)
  mov    $0x0,%r14d
  mov    $0x0,%ebx
  jmp    L12
L74:
  lea    0x1(%r14),%rax
  cmp    %r14,%r12
  je     L53
  mov    %rax,%r14
L12:
  mov    0x0(%r13,%r14,8),%rdi
  test   %rdi,%rdi
  je     L74
  call   <strlen@plt>
  mov    %rax,(%r15,%r14,8)
  add    %rax,%rbx
  mov    -0x58(%rbp),%rax
  add    %rbx,%rax
  cmp    %r14,%r12
  cmova  %rax,%rbx
  jmp    L74
L53:
  lea    0x1(%rbx),%rdi
  call   <malloc@plt>
  mov    %rax,%r14
  test   %rax,%rax
  je     L51
  movb   $0x0,(%rax,%rbx,1)
  mov    %rax,-0x48(%rbp)
  mov    $0x0,%ebx
  jmp    L59
L44:
  lea    0x1(%rbx),%rax
  cmp    %rbx,%r12
  je     L51
  mov    %rax,%rbx
L59:
  mov    0x0(%r13,%rbx,8),%rsi
  test   %rsi,%rsi
  je     L44
  mov    (%r15,%rbx,8),%rax
  mov    %rax,-0x50(%rbp)
  mov    %rax,%rdx
  mov    -0x48(%rbp),%rdi
  call   <strncpy@plt>
  mov    -0x50(%rbp),%rcx
  add    %rcx,-0x48(%rbp)
  cmp    %rbx,%r12
  jbe    L44
  mov    -0x58(%rbp),%rdx
  mov    -0x60(%rbp),%rsi
  mov    -0x48(%rbp),%rdi
  call   <strncpy@plt>
  add    -0x58(%rbp),%rax
  mov    %rax,-0x48(%rbp)
  jmp    L44
L51:
  mov    -0x38(%rbp),%rax
  sub    %fs:0x28,%rax
  jne    L165
  mov    %r14,%rax
  lea    -0x28(%rbp),%rsp
  pop    %rbx
  pop    %r12
  pop    %r13
  pop    %r14
  pop    %r15
  pop    %rbp
  ret
L165:
  call   <__stack_chk_fail@plt>
```

assistant: ```c
char *string_join(const char **strings, const char *sep) {
  assert(strings != NULL);
  assert(sep != NULL);
  size_t strings_count;
  for (strings_count = 0; strings[strings_count]; strings_count++)
    ;
  if (strings_count == 0)
    return NULL;
  size_t lengths[strings_count];
  size_t sep_len = strlen(sep);
  size_t len = 0;
  for (size_t i = 0; i < strings_count; i++) {
    if (!strings[i])
      continue;
    lengths[i] = strlen(strings[i]);
    len += lengths[i];
    if (i < strings_count - 1)
      len += sep_len;
  }
  char *result = malloc(len + 1);
  if (!result)
    return NULL;
  result[len] = 0;
  char *dst = result;
  for (size_t i = 0; i < strings_count; i++) {
    if (!strings[i])
      continue;
    strncpy(dst, strings[i], lengths[i]);
    dst += lengths[i];
    if (i < strings_count - 1) {
      dst = strncpy(dst, sep, sep_len);
      dst += sep_len;
    }
  }
  return result;
}
```