user: 提供的汇编代码用C语言表示会是什么样子:

```asm
<decode_base64>:
  endbr64
  push   %rbp
  mov    %rsp,%rbp
  sub    $0x50,%rsp
  mov    %rdi,-0x38(%rbp)
  mov    %rsi,-0x40(%rbp)
  mov    %rdx,-0x48(%rbp)
  mov    %fs:0x28,%rax
  mov    %rax,-0x8(%rbp)
  xor    %eax,%eax
  movl   $0x0,-0x24(%rbp)
  movl   $0x0,-0xc(%rbp)
  movl   $0x0,-0x20(%rbp)
  movl   $0x0,-0x1c(%rbp)
  movq   $0x0,-0x18(%rbp)
  jmp    L177
L242:
  mov    -0x40(%rbp),%rax
  lea    0x1(%rax),%rdx
  mov    %rdx,-0x40(%rbp)
  movzbl (%rax),%eax
  movsbl %al,%eax
  mov    %eax,-0x24(%rbp)
  cmpl   $0x40,-0x24(%rbp)
  jle    L47
  cmpl   $0x5a,-0x24(%rbp)
  jg     L47
  subl   $0x41,-0x24(%rbp)
  jmp    L258
L47:
  cmpl   $0x60,-0x24(%rbp)
  jle    L52
  cmpl   $0x7a,-0x24(%rbp)
  jg     L52
  subl   $0x47,-0x24(%rbp)
  jmp    L258
L52:
  cmpl   $0x2f,-0x24(%rbp)
  jle    L118
  cmpl   $0x39,-0x24(%rbp)
  jg     L118
  addl   $0x4,-0x24(%rbp)
  jmp    L258
L118:
  cmpl   $0x2f,-0x24(%rbp)
  jne    L161
  movl   $0x3f,-0x24(%rbp)
  jmp    L258
L161:
  cmpl   $0x2b,-0x24(%rbp)
  jne    L253
  movl   $0x3e,-0x24(%rbp)
  jmp    L258
L253:
  movl   $0xffffffff,-0x24(%rbp)
L258:
  cmpl   $0x3f,-0x24(%rbp)
  setle  %al
  movzbl %al,%eax
  mov    %eax,%edi
  call   <assert@plt>
  cmpl   $0xffffffff,-0x24(%rbp)
  jne    L159
  cmpl   $0x0,-0x1c(%rbp)
  jne    L167
  mov    $0x0,%eax
  jmp    L200
L59:
  addl   $0x1,-0x20(%rbp)
  mov    -0x20(%rbp),%eax
  cltd
  shr    $0x1e,%edx
  add    %edx,%eax
  and    $0x3,%eax
  sub    %edx,%eax
  cltq
  movb   $0x0,-0xc(%rbp,%rax,1)
L167:
  mov    -0x20(%rbp),%eax
  cltd
  shr    $0x1e,%edx
  add    %edx,%eax
  and    $0x3,%eax
  sub    %edx,%eax
  cmp    $0x3,%eax
  jne    L59
  jmp    L238
L159:
  mov    -0x20(%rbp),%eax
  cltd
  shr    $0x1e,%edx
  add    %edx,%eax
  and    $0x3,%eax
  sub    %edx,%eax
  mov    %eax,%ecx
  mov    -0x24(%rbp),%eax
  mov    %eax,%edx
  movslq %ecx,%rax
  mov    %dl,-0xc(%rbp,%rax,1)
  addq   $0x1,-0x18(%rbp)
  movl   $0x1,-0x1c(%rbp)
L238:
  mov    -0x20(%rbp),%eax
  cltd
  shr    $0x1e,%edx
  add    %edx,%eax
  and    $0x3,%eax
  sub    %edx,%eax
  cmp    $0x3,%eax
  jne    L107
  movzbl -0xc(%rbp),%eax
  movsbl %al,%eax
  shl    $0x2,%eax
  mov    %eax,%esi
  movzbl -0xb(%rbp),%eax
  movsbl %al,%eax
  sar    $0x4,%eax
  and    $0x3,%eax
  mov    %eax,%ecx
  mov    -0x38(%rbp),%rax
  lea    0x1(%rax),%rdx
  mov    %rdx,-0x38(%rbp)
  or     %ecx,%esi
  mov    %esi,%edx
  mov    %dl,(%rax)
  movzbl -0xb(%rbp),%eax
  movsbl %al,%eax
  shl    $0x4,%eax
  mov    %eax,%esi
  movzbl -0xa(%rbp),%eax
  movsbl %al,%eax
  sar    $0x2,%eax
  and    $0xf,%eax
  mov    %eax,%ecx
  mov    -0x38(%rbp),%rax
  lea    0x1(%rax),%rdx
  mov    %rdx,-0x38(%rbp)
  or     %ecx,%esi
  mov    %esi,%edx
  mov    %dl,(%rax)
  movzbl -0xa(%rbp),%eax
  movsbl %al,%eax
  shl    $0x6,%eax
  mov    %eax,%esi
  movzbl -0x9(%rbp),%eax
  and    $0x3f,%eax
  mov    %eax,%ecx
  mov    -0x38(%rbp),%rax
  lea    0x1(%rax),%rdx
  mov    %rdx,-0x38(%rbp)
  or     %ecx,%esi
  mov    %esi,%edx
  mov    %dl,(%rax)
  lea    -0xc(%rbp),%rax
  mov    $0x4,%edx
  mov    $0x0,%esi
  mov    %rax,%rdi
  call   <memset@plt>
L107:
  addl   $0x1,-0x20(%rbp)
L177:
  mov    -0x40(%rbp),%rax
  movzbl (%rax),%eax
  test   %al,%al
  je     L94
  mov    -0x48(%rbp),%rax
  mov    (%rax),%rax
  cmp    %eax,-0x20(%rbp)
  jge    L94
  cmpl   $0xffffffff,-0x24(%rbp)
  jne    L242
L94:
  mov    -0x18(%rbp),%rax
  shr    $0x2,%rax
  mov    %eax,%edx
  mov    %edx,%eax
  add    %eax,%eax
  add    %edx,%eax
  movslq %eax,%rdx
  mov    -0x48(%rbp),%rax
  mov    %rdx,(%rax)
  mov    -0x18(%rbp),%rax
  and    $0x3,%eax
  cmp    $0x2,%rax
  jne    L160
  mov    -0x48(%rbp),%rax
  mov    (%rax),%rax
  lea    0x1(%rax),%rdx
  mov    -0x48(%rbp),%rax
  mov    %rdx,(%rax)
L160:
  mov    -0x18(%rbp),%rax
  and    $0x3,%eax
  cmp    $0x3,%rax
  jne    L248
  mov    -0x48(%rbp),%rax
  mov    (%rax),%rax
  lea    0x2(%rax),%rdx
  mov    -0x48(%rbp),%rax
  mov    %rdx,(%rax)
L248:
  mov    -0x38(%rbp),%rax
L200:
  mov    -0x8(%rbp),%rdx
  sub    %fs:0x28,%rdx
  je     L89
  call   <__stack_chk_fail@plt>
L89:
  leave
  ret
```

assistant: ```c
char *decode_base64(char *dest, char *src, size_t *size) {
  int cc = 0;
  char buf[4] = {0, 0, 0, 0};
  int p = 0;
  int valid_data = 0;
  size_t size_out = 0;
  while (*src && p < (int)*size && (cc != -1)) {
    cc = *src++;
    if ((cc >= 'A') && (cc <= 'Z'))
      cc = cc - 'A';
    else if ((cc >= 'a') && (cc <= 'z'))
      cc = cc - 'a' + 26;
    else if ((cc >= '0') && (cc <= '9'))
      cc = cc - '0' + 52;
    else if (cc == '/')
      cc = 63;
    else if (cc == '+')
      cc = 62;
    else
      cc = -1;
    assert(cc < 64);
    if (cc == -1) {
      if (valid_data == 0) {
        return 0;
      }
      while (p % 4 != 3) {
        p++;
        buf[p % 4] = 0;
      }
    } else {
      buf[p % 4] = cc;
      size_out++;
      valid_data = 1;
    }
    if (p % 4 == 3) {
      *dest++ = (buf[0] << 2) | ((buf[1] & 0x30) >> 4);
      *dest++ = ((buf[1] & 0x0F) << 4) | ((buf[2] & 0x3C) >> 2);
      *dest++ = ((buf[2] & 0x03) << 6) | (buf[3] & 0x3F);
      memset(buf, 0, 4);
    }
    p++;
  }
  *size = ((int)(size_out / 4)) * 3;
  if (size_out % 4 == 2)
    *size += 1;
  if (size_out % 4 == 3)
    *size += 2;
  return (dest);
}
```