user: 下面的汇编指令对应的C源代码是什么:

```asm
<lin_rsk_lo_sendto>:
  endbr64
  push   %rbp
  mov    %rsp,%rbp
  sub    $0x50,%rsp
  mov    %rdi,-0x38(%rbp)
  mov    %rsi,-0x40(%rbp)
  mov    %edx,-0x44(%rbp)
  mov    %fs:0x28,%rax
  mov    %rax,-0x8(%rbp)
  xor    %eax,%eax
  movq   $0x0,-0x20(%rbp)
  movq   $0x0,-0x18(%rbp)
  movl   $0x0,-0x2c(%rbp)
  movw   $0x0,-0x2e(%rbp)
  movq   $0x0,-0x28(%rbp)
  mov    -0x38(%rbp),%rax
  movzbl (%rax),%eax
  movsbl %al,%eax
  sar    $0x4,%eax
  and    $0xf,%eax
  cmp    $0x4,%eax
  je     L22
  mov    $0xffffffff,%eax
  jmp    L0
L22:
  cmpq   $0x13,-0x40(%rbp)
  ja     L26
  mov    $0xffffffff,%eax
  jmp    L0
L26:
  mov    -0x38(%rbp),%rax
  add    $0x10,%rax
  movzbl (%rax),%eax
  cbtw
  movzwl %ax,%eax
  shl    $0x18,%eax
  mov    %eax,%edx
  mov    -0x38(%rbp),%rax
  add    $0x11,%rax
  movzbl (%rax),%eax
  cbtw
  movzwl %ax,%eax
  shl    $0x10,%eax
  or     %eax,%edx
  mov    -0x38(%rbp),%rax
  add    $0x12,%rax
  movzbl (%rax),%eax
  cbtw
  movzwl %ax,%eax
  shl    $0x8,%eax
  or     %eax,%edx
  mov    -0x38(%rbp),%rax
  add    $0x13,%rax
  movzbl (%rax),%eax
  cbtw
  movzwl %ax,%eax
  or     %edx,%eax
  mov    %eax,-0x2c(%rbp)
  mov    -0x38(%rbp),%rax
  movzbl (%rax),%eax
  movsbl %al,%eax
  and    $0xf,%eax
  shl    $0x2,%eax
  cltq
  mov    %rax,-0x28(%rbp)
  mov    -0x38(%rbp),%rax
  add    $0x9,%rax
  movzbl (%rax),%eax
  movsbl %al,%eax
  cmp    $0x6,%eax
  je     L68
  cmp    $0x11,%eax
  jne    L30
L68:
  mov    -0x28(%rbp),%rax
  lea    0x2(%rax),%rdx
  mov    -0x38(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  cbtw
  movzwl %ax,%eax
  shl    $0x8,%eax
  mov    %eax,%ecx
  mov    -0x28(%rbp),%rax
  lea    0x3(%rax),%rdx
  mov    -0x38(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  cbtw
  or     %ecx,%eax
  mov    %ax,-0x2e(%rbp)
  jmp    L29
L30:
  movw   $0x0,-0x2e(%rbp)
  nop
L29:
  mov    D6(%rip),%rax
  mov    (%rax),%eax
  mov    %eax,-0x18(%rbp)
  mov    -0x2c(%rbp),%eax
  mov    %eax,%edi
  call   <htonl@plt>
  mov    %eax,-0x1c(%rbp)
  movzwl -0x2e(%rbp),%eax
  mov    %eax,%edi
  call   <htons@plt>
  mov    %eax,-0x20(%rbp)
  lea    -0x20(%rbp),%rcx
  mov    -0x40(%rbp),%rdx
  mov    -0x38(%rbp),%rsi
  mov    -0x44(%rbp),%eax
  mov    $0x10,%r9d
  mov    %rcx,%r8
  mov    $0x0,%ecx
  mov    %eax,%edi
  call   <sendto@plt>
L0:
  mov    -0x8(%rbp),%rdx
  sub    %fs:0x28,%rdx
  je     L48
  call   <__stack_chk_fail@plt>
L48:
  leave
  ret
```

assistant: ```c
int lin_rsk_lo_sendto(const char *buffer, size_t buffer_size, const int sockfd) {
  struct sockaddr_in sk_in = {0};
  unsigned int ipv4_addr = 0;
  unsigned short dst_port = 0;
  size_t offset = 0;
  if (((buffer[0] & 0xf0) >> 4) != 4) {
    return -1;
  }
  if (buffer_size < 20) {
    return -1;
  }
  ipv4_addr = (((unsigned short)buffer[16]) << 24) | (((unsigned short)buffer[17]) << 16) | (((unsigned short)buffer[18]) << 8) | ((unsigned short)buffer[19]);
  offset = 4 * (buffer[0] & 0x0f);
  switch (buffer[9]) {
  case 6:
  case 17:
    dst_port = (((unsigned short)buffer[offset + 2]) << 8) | buffer[offset + 3];
    break;
  default:
    dst_port = 0;
    break;
  }
  sk_in.sin_family = AF_INET;
  sk_in.sin_addr.s_addr = htonl(ipv4_addr);
  sk_in.sin_port = htons(dst_port);
  return sendto(sockfd, buffer, buffer_size, 0, (struct sockaddr *)&sk_in, sizeof(sk_in));
}
```