user: 请从给定的汇编指令推导出C源代码：

```asm
<strtoul>:
  endbr64
  push   %r15
  mov    %rdi,%r15
  push   %r14
  push   %r13
  push   %r12
  push   %rbp
  mov    %edx,%ebp
  push   %rbx
  sub    $0x28,%rsp
  mov    %rdi,0x10(%rsp)
  mov    %rsi,0x8(%rsp)
  nopl   0x0(%rax)
L53:
  mov    %r15,%rbx
  movzbl (%r15),%r14d
  add    $0x1,%r15
  mov    %r14d,%edi
  call   <isspace@plt>
  test   %rax,%rax
  jne    L53
  cmp    $0x2d,%r14d
  je     L148
  movb   $0x0,0x1f(%rsp)
  cmp    $0x2b,%r14d
  je     L12
L221:
  test   $0xffffffef,%ebp
  jne    L24
  cmp    $0x30,%r14d
  je     L80
  test   %ebp,%ebp
  jne    L24
  movq   $0xa,(%rsp)
  mov    $0xa,%ebp
  jmp    L163
L9:
  test   %ebp,%ebp
  je     L206
  cs nopw 0x0(%rax,%rax,1)
L24:
  movslq %ebp,%rax
  mov    %rax,(%rsp)
L163:
  mov    D19(%rip),%rax
  xor    %edx,%edx
  xor    %ebx,%ebx
  xor    %r12d,%r12d
  mov    (%rax),%rax
  divq   (%rsp)
  mov    %edx,0x18(%rsp)
  mov    %rax,%r13
  jmp    L252
  nopl   (%rax)
L27:
  cmp    %ecx,%ebp
  jle    L108
L233:
  cmp    $0xffffffff,%ebx
  je     L200
  cmp    %r13,%r12
  ja     L28
  jne    L251
  cmp    0x18(%rsp),%ecx
  jle    L251
L28:
  mov    D19(%rip),%rax
  mov    $0xffffffff,%ebx
  mov    (%rax),%r12
  mov    D0(%rip),%rax
  mov    (%rax),%edx
  mov    D7(%rip),%rax
  mov    %edx,(%rax)
L200:
  movzbl (%r15),%r14d
  add    $0x1,%r15
L252:
  mov    %r14d,%edi
  call   <isdigit@plt>
  lea    -0x30(%r14),%ecx
  test   %rax,%rax
  jne    L27
  mov    %r14d,%edi
  call   <isalpha@plt>
  test   %rax,%rax
  je     L108
  mov    %r14d,%edi
  call   <isupper@plt>
  mov    %r14d,%ecx
  cmp    $0x1,%rax
  sbb    %eax,%eax
  and    $0x20,%eax
  add    $0x37,%eax
  sub    %eax,%ecx
  cmp    %ecx,%ebp
  jg     L233
L108:
  test   %ebx,%ebx
  jle    L218
  cmpb   $0x0,0x1f(%rsp)
  je     L218
  neg    %r12
  cmpq   $0x0,0x8(%rsp)
  je     L149
L154:
  lea    -0x1(%r15),%rax
  mov    %rax,0x10(%rsp)
  mov    0x8(%rsp),%rax
  mov    0x10(%rsp),%rsi
  mov    %rsi,(%rax)
L149:
  add    $0x28,%rsp
  mov    %r12,%rax
  pop    %rbx
  pop    %rbp
  pop    %r12
  pop    %r13
  pop    %r14
  pop    %r15
  ret
  nopl   0x0(%rax,%rax,1)
L251:
  imul   (%rsp),%r12
  movslq %ecx,%rcx
  mov    $0x1,%ebx
  add    %rcx,%r12
  jmp    L200
  nopl   (%rax)
L218:
  cmpq   $0x0,0x8(%rsp)
  je     L149
  test   %ebx,%ebx
  jne    L154
  mov    0x8(%rsp),%rax
  mov    0x10(%rsp),%rsi
  mov    %rsi,(%rax)
  jmp    L149
  nopl   0x0(%rax,%rax,1)
L12:
  movsbl (%r15),%r14d
  lea    0x2(%rbx),%r15
  jmp    L221
  nopl   (%rax)
L148:
  movsbl (%r15),%r14d
  movb   $0x1,0x1f(%rsp)
  lea    0x2(%rbx),%r15
  jmp    L221
  nopw   0x0(%rax,%rax,1)
L80:
  movzbl (%r15),%eax
  and    $0xffffffdf,%eax
  cmp    $0x58,%al
  jne    L9
  movsbl 0x1(%r15),%r14d
  mov    $0x10,%ebp
  add    $0x2,%r15
  movq   $0x10,(%rsp)
  jmp    L163
L206:
  movq   $0x8,(%rsp)
  mov    $0x8,%ebp
  jmp    L163
```

assistant: ```c
unsigned long strtoul(const char *nptr, char **endptr, int base) {
  const char *s;
  unsigned long acc, cutoff;
  int c;
  int neg, any, cutlim;
  s = nptr;
  do {
    c = (unsigned char)*s++;
  } while (isspace(c));
  if (c == '-') {
    neg = 1;
    c = *s++;
  } else {
    neg = 0;
    if (c == '+')
      c = *s++;
  }
  if ((base == 0 || base == 16) && c == '0' && (*s == 'x' || *s == 'X')) {
    c = s[1];
    s += 2;
    base = 16;
  }
  if (base == 0)
    base = c == '0' ? 8 : 10;
  cutoff = ULONG_MAX / (unsigned long)base;
  cutlim = ULONG_MAX % (unsigned long)base;
  for (acc = 0, any = 0;; c = (unsigned char)*s++) {
    if (isdigit(c))
      c -= '0';
    else if (isalpha(c))
      c -= isupper(c) ? 'A' - 10 : 'a' - 10;
    else
      break;
    if (c >= base)
      break;
    if (any < 0)
      continue;
    if (acc > cutoff || (acc == cutoff && c > cutlim)) {
      any = -1;
      acc = ULONG_MAX;
      errno = ERANGE;
    } else {
      any = 1;
      acc *= (unsigned long)base;
      acc += c;
    }
  }
  if (neg && any > 0)
    acc = -acc;
  if (endptr != 0)
    *endptr = (char *)(any ? s - 1 : nptr);
  return (acc);
}
```