user: 提供的汇编代码用C语言表示会是什么样子:

```asm
<os_server_start_2>:
  endbr64
  push   %r14
  push   %r13
  push   %r12
  push   %rbp
  push   %rbx
  sub    $0x10,%rsp
  mov    %rdi,%rbx
  mov    %fs:0x28,%rax
  mov    %rax,0x8(%rsp)
  xor    %eax,%eax
  test   %esi,%esi
  je     L124
  mov    D14(%rip),%rax
  mov    (%rax),%rdi
  test   %rdi,%rdi
  je     L84
  call   <fflush@plt>
  mov    D14(%rip),%rax
  mov    (%rax),%rdi
  call   <fclose@plt>
L64:
  mov    D60(%rip),%rax
  mov    (%rax),%rdi
  test   %rdi,%rdi
  je     L93
  call   <fflush@plt>
  mov    D60(%rip),%rax
  mov    (%rax),%rdi
  call   <fclose@plt>
L40:
  mov    D15(%rip),%rax
  mov    (%rax),%rdi
  test   %rdi,%rdi
  je     L147
  call   <fclose@plt>
L87:
  mov    $0x14,%esi
  mov    $0x3,%edi
  call   <os_close_files@plt>
L124:
  mov    $0x0,%eax
  test   %rbx,%rbx
  je     L14
  cmpb   $0x0,(%rbx)
  jne    L74
L14:
  mov    0x8(%rsp),%rdx
  sub    %fs:0x28,%rdx
  jne    L94
  add    $0x10,%rsp
  pop    %rbx
  pop    %rbp
  pop    %r12
  pop    %r13
  pop    %r14
  ret
L84:
  mov    $0x2,%edi
  call   <fsync@plt>
  mov    $0x2,%edi
  call   <close@plt>
  jmp    L64
L93:
  mov    $0x1,%edi
  call   <fsync@plt>
  mov    $0x1,%edi
  call   <close@plt>
  jmp    L40
L147:
  mov    $0x0,%edi
  call   <close@plt>
  jmp    L87
L74:
  mov    %rsp,%rsi
  lea    0x4(%rsp),%rdi
  mov    %rbx,%rdx
  call   <os_uinfo@plt>
  test   %rax,%rax
  jne    L57
  call   <geteuid@plt>
  mov    %eax,%r12d
  mov    $0x0,%eax
  call   <getuid@plt>
  mov    $0x0,%eax
  call   <getegid@plt>
  mov    %eax,%ebp
  mov    $0x0,%eax
  call   <getgid@plt>
  mov    %eax,%ebx
  mov    $0x0,%edi
  call   <seteuid@plt>
  mov    (%rsp),%edi
  call   <setgid@plt>
  test   %rax,%rax
  jne    L154
  mov    0x4(%rsp),%edi
  call   <setuid@plt>
  test   %rax,%rax
  jne    L142
  mov    $0x0,%eax
  jmp    L14
L154:
  mov    D25(%rip),%rbx
  mov    (%rbx),%ebp
  mov    %r12d,%edi
  call   <seteuid@plt>
  mov    %ebp,(%rbx)
  mov    $0xffffffff,%eax
  jmp    L14
L142:
  mov    D25(%rip),%r14
  mov    (%r14),%r13d
  mov    %r12d,%edi
  call   <seteuid@plt>
  mov    %ebx,%edi
  call   <setgid@plt>
  mov    %ebp,%edi
  call   <setegid@plt>
  mov    %r13d,(%r14)
  mov    $0xffffffff,%eax
  jmp    L14
L57:
  mov    $0xffffffff,%eax
  jmp    L14
L94:
  call   <__stack_chk_fail@plt>
```

assistant: ```c
int os_server_start_2(const char *user, int close_stdfiles) {
  int istat;
  gid_t gid, old_egid, old_gid;
  uid_t uid, old_euid, old_uid;
  if (close_stdfiles) {
    if (stderr) {
      fflush(stderr);
      fclose(stderr);
    } else {
      fsync(2);
      close(2);
    }
    if (stdout) {
      fflush(stdout);
      fclose(stdout);
    } else {
      fsync(1);
      close(1);
    }
    if (stdin)
      fclose(stdin);
    else
      close(0);
    os_close_files(3, 20);
  }
  if (user && *user) {
    if (os_uinfo(&uid, &gid, user))
      return (-1);
    old_euid = geteuid();
    old_uid = getuid();
    old_egid = getegid();
    old_gid = getgid();
    seteuid(0);
    if (setgid(gid)) {
      int save_errno = errno;
      seteuid(old_euid);
      errno = save_errno;
      return (-1);
    }
    if (setuid(uid)) {
      int save_errno = errno;
      seteuid(old_euid);
      setgid(old_gid);
      setegid(old_egid);
      errno = save_errno;
      return (-1);
    }
  }
  return (0);
}
```