user: 提供的汇编代码用C语言表示会是什么样子：

```asm
<envvar_subst>:
  endbr64
  push   %rbp
  mov    %rsp,%rbp
  sub    $0xa0,%rsp
  mov    %rdi,-0x98(%rbp)
  mov    %rsi,-0xa0(%rbp)
  mov    %fs:0x28,%rax
  mov    %rax,-0x8(%rbp)
  xor    %eax,%eax
  movl   $0x0,-0x84(%rbp)
  mov    -0x84(%rbp),%eax
  mov    %eax,-0x88(%rbp)
  jmp    L75
L129:
  mov    -0x88(%rbp),%eax
  movslq %eax,%rdx
  mov    -0xa0(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  cmp    $0x24,%al
  jne    L57
  mov    -0x88(%rbp),%eax
  mov    %eax,-0x7c(%rbp)
  addl   $0x1,-0x88(%rbp)
  movl   $0x0,-0x80(%rbp)
  jmp    L53
L68:
  mov    -0x88(%rbp),%eax
  movslq %eax,%rdx
  mov    -0xa0(%rbp),%rax
  lea    (%rdx,%rax,1),%rcx
  mov    -0x80(%rbp),%eax
  lea    0x1(%rax),%edx
  mov    %edx,-0x80(%rbp)
  movzbl (%rcx),%edx
  cltq
  mov    %dl,-0x70(%rbp,%rax,1)
  addl   $0x1,-0x88(%rbp)
L53:
  mov    -0x88(%rbp),%eax
  movslq %eax,%rdx
  mov    -0xa0(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  test   %al,%al
  je     L2
  mov    -0x88(%rbp),%eax
  movslq %eax,%rdx
  mov    -0xa0(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  cmp    $0x20,%al
  je     L2
  mov    -0x88(%rbp),%eax
  movslq %eax,%rdx
  mov    -0xa0(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  cmp    $0x5c,%al
  je     L2
  mov    -0x88(%rbp),%eax
  movslq %eax,%rdx
  mov    -0xa0(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  cmp    $0x3a,%al
  je     L2
  mov    -0x88(%rbp),%eax
  movslq %eax,%rdx
  mov    -0xa0(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  cmp    $0x2f,%al
  jne    L68
L2:
  mov    -0x80(%rbp),%eax
  cltq
  movb   $0x0,-0x70(%rbp,%rax,1)
  movzbl -0x70(%rbp),%eax
  test   %al,%al
  je     L74
  lea    -0x70(%rbp),%rax
  mov    %rax,%rdi
  call   <getenv@plt>
  mov    %rax,-0x78(%rbp)
  cmpq   $0x0,-0x78(%rbp)
  je     L74
  mov    -0x84(%rbp),%eax
  movslq %eax,%rdx
  mov    -0x98(%rbp),%rax
  add    %rdx,%rax
  movb   $0x0,(%rax)
  mov    -0x78(%rbp),%rdx
  mov    -0x98(%rbp),%rax
  mov    %rdx,%rsi
  mov    %rax,%rdi
  call   <strcat@plt>
  mov    -0x98(%rbp),%rax
  mov    %rax,%rdi
  call   <strlen@plt>
  mov    %eax,-0x84(%rbp)
  subl   $0x1,-0x88(%rbp)
  jmp    L21
L74:
  mov    -0x7c(%rbp),%eax
  mov    %eax,-0x88(%rbp)
L57:
  mov    -0x88(%rbp),%eax
  movslq %eax,%rdx
  mov    -0xa0(%rbp),%rax
  lea    (%rdx,%rax,1),%rcx
  mov    -0x84(%rbp),%eax
  lea    0x1(%rax),%edx
  mov    %edx,-0x84(%rbp)
  movslq %eax,%rdx
  mov    -0x98(%rbp),%rax
  add    %rax,%rdx
  movzbl (%rcx),%eax
  mov    %al,(%rdx)
L21:
  addl   $0x1,-0x88(%rbp)
L75:
  mov    -0x88(%rbp),%eax
  movslq %eax,%rdx
  mov    -0xa0(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  test   %al,%al
  jne    L129
  mov    -0x84(%rbp),%eax
  movslq %eax,%rdx
  mov    -0x98(%rbp),%rax
  add    %rdx,%rax
  movb   $0x0,(%rax)
  nop
  mov    -0x8(%rbp),%rax
  sub    %fs:0x28,%rax
  je     L13
  call   <__stack_chk_fail@plt>
L13:
  leave
  ret
```

assistant: ```c
void envvar_subst(char *dest, char *src) {
  int i, i0, j, k;
  char envvar[100];
  char *p;
  for (i = j = 0; src[i] != '\0'; i++) {
    if (src[i] == '$') {
      i0 = i;
      for (i++, k = 0; src[i] != '\0' && src[i] != ' ' && src[i] != '\\' && src[i] != ':' && src[i] != '/'; i++)
        envvar[k++] = src[i];
      envvar[k] = '\0';
      if (envvar[0] != '\0' && (p = getenv(envvar)) != NULL) {
        dest[j] = '\0';
        strcat(dest, p);
        j = strlen(dest);
        i--;
        continue;
      }
      i = i0;
    }
    dest[j++] = src[i];
  }
  dest[j] = '\0';
}
```