user: What would the C source look like for the assembly code provided here?

```asm
<udp4_checksum>:
  endbr64
  push   %rbp
  mov    %rsp,%rbp
  push   %r13
  push   %r12
  push   %rbx
  sub    $0x68,%rsp
  mov    %rdx,%rax
  mov    %rcx,%rdx
  mov    %r8,-0x78(%rbp)
  mov    %r9d,-0x7c(%rbp)
  mov    %rdi,-0x60(%rbp)
  mov    %esi,-0x58(%rbp)
  mov    %rax,-0x70(%rbp)
  mov    -0x68(%rbp),%eax
  and    $0x0,%eax
  or     %edx,%eax
  mov    %eax,-0x68(%rbp)
  mov    %fs:0x28,%rax
  mov    %rax,-0x28(%rbp)
  xor    %eax,%eax
  mov    %rsp,%rax
  mov    %rax,%rbx
  mov    D1(%rip),%rax
  mov    (%rax),%eax
  movslq %eax,%rdx
  sub    $0x1,%rdx
  mov    %rdx,-0x38(%rbp)
  movslq %eax,%rdx
  mov    %rdx,%r12
  mov    $0x0,%r13d
  movslq %eax,%rdx
  mov    %rdx,%r10
  mov    $0x0,%r11d
  cltq
  mov    $0x10,%edx
  sub    $0x1,%rdx
  add    %rdx,%rax
  mov    $0x10,%esi
  mov    $0x0,%edx
  div    %rsi
  imul   $0x10,%rax,%rax
  mov    %rax,%rcx
  and    $0xfffffffffffff000,%rcx
  mov    %rsp,%rdx
  sub    %rcx,%rdx
L13:
  cmp    %rdx,%rsp
  je     L35
  sub    $0x1000,%rsp
  orq    $0x0,0xff8(%rsp)
  jmp    L13
L35:
  mov    %rax,%rdx
  and    $0xfff,%edx
  sub    %rdx,%rsp
  mov    %rax,%rdx
  and    $0xfff,%edx
  test   %rdx,%rdx
  je     L19
  and    $0xfff,%eax
  sub    $0x8,%rax
  add    %rsp,%rax
  orq    $0x0,(%rax)
L19:
  mov    %rsp,%rax
  add    $0x0,%rax
  mov    %rax,-0x30(%rbp)
  movl   $0x0,-0x48(%rbp)
  mov    -0x30(%rbp),%rax
  mov    %rax,-0x40(%rbp)
  lea    -0x60(%rbp),%rax
  lea    0x8(%rax),%rcx
  mov    -0x40(%rbp),%rax
  mov    $0x4,%edx
  mov    %rcx,%rsi
  mov    %rax,%rdi
  call   <memcpy@plt>
  addq   $0x4,-0x40(%rbp)
  mov    -0x48(%rbp),%eax
  add    $0x4,%eax
  mov    %eax,-0x48(%rbp)
  lea    -0x60(%rbp),%rax
  lea    0x4(%rax),%rcx
  mov    -0x40(%rbp),%rax
  mov    $0x4,%edx
  mov    %rcx,%rsi
  mov    %rax,%rdi
  call   <memcpy@plt>
  addq   $0x4,-0x40(%rbp)
  mov    -0x48(%rbp),%eax
  add    $0x4,%eax
  mov    %eax,-0x48(%rbp)
  mov    -0x40(%rbp),%rax
  movb   $0x0,(%rax)
  addq   $0x1,-0x40(%rbp)
  addl   $0x1,-0x48(%rbp)
  lea    -0x60(%rbp),%rcx
  mov    -0x40(%rbp),%rax
  mov    $0x4,%edx
  mov    %rcx,%rsi
  mov    %rax,%rdi
  call   <memcpy@plt>
  addq   $0x4,-0x40(%rbp)
  mov    -0x48(%rbp),%eax
  add    $0x4,%eax
  mov    %eax,-0x48(%rbp)
  lea    -0x70(%rbp),%rcx
  mov    -0x40(%rbp),%rax
  mov    $0x4,%edx
  mov    %rcx,%rsi
  mov    %rax,%rdi
  call   <memcpy@plt>
  addq   $0x4,-0x40(%rbp)
  mov    -0x48(%rbp),%eax
  add    $0x4,%eax
  mov    %eax,-0x48(%rbp)
  lea    -0x70(%rbp),%rax
  lea    0x8(%rax),%rcx
  mov    -0x40(%rbp),%rax
  mov    $0x4,%edx
  mov    %rcx,%rsi
  mov    %rax,%rdi
  call   <memcpy@plt>
  addq   $0x4,-0x40(%rbp)
  mov    -0x48(%rbp),%eax
  add    $0x4,%eax
  mov    %eax,-0x48(%rbp)
  lea    -0x70(%rbp),%rax
  lea    0x4(%rax),%rcx
  mov    -0x40(%rbp),%rax
  mov    $0x4,%edx
  mov    %rcx,%rsi
  mov    %rax,%rdi
  call   <memcpy@plt>
  addq   $0x4,-0x40(%rbp)
  mov    -0x48(%rbp),%eax
  add    $0x4,%eax
  mov    %eax,-0x48(%rbp)
  lea    -0x70(%rbp),%rcx
  mov    -0x40(%rbp),%rax
  mov    $0x4,%edx
  mov    %rcx,%rsi
  mov    %rax,%rdi
  call   <memcpy@plt>
  addq   $0x4,-0x40(%rbp)
  mov    -0x48(%rbp),%eax
  add    $0x4,%eax
  mov    %eax,-0x48(%rbp)
  mov    -0x40(%rbp),%rax
  movb   $0x0,(%rax)
  addq   $0x1,-0x40(%rbp)
  mov    -0x40(%rbp),%rax
  movb   $0x0,(%rax)
  addq   $0x1,-0x40(%rbp)
  addl   $0x2,-0x48(%rbp)
  mov    -0x7c(%rbp),%edx
  mov    -0x78(%rbp),%rcx
  mov    -0x40(%rbp),%rax
  mov    %rcx,%rsi
  mov    %rax,%rdi
  call   <memcpy@plt>
  mov    -0x7c(%rbp),%eax
  cltq
  add    %rax,-0x40(%rbp)
  mov    -0x7c(%rbp),%eax
  add    %eax,-0x48(%rbp)
  movl   $0x0,-0x44(%rbp)
  jmp    L26
L12:
  mov    -0x40(%rbp),%rax
  movb   $0x0,(%rax)
  addq   $0x1,-0x40(%rbp)
  addl   $0x1,-0x48(%rbp)
  addl   $0x1,-0x44(%rbp)
  addq   $0x1,-0x40(%rbp)
L26:
  mov    -0x7c(%rbp),%eax
  cltd
  shr    $0x1f,%edx
  add    %edx,%eax
  and    $0x1,%eax
  sub    %edx,%eax
  cmp    %eax,-0x44(%rbp)
  jl     L12
  mov    -0x48(%rbp),%edx
  mov    -0x30(%rbp),%rax
  mov    %edx,%esi
  mov    %rax,%rdi
  call   <checksum@plt>
  mov    %rbx,%rsp
  mov    -0x28(%rbp),%rdx
  sub    %fs:0x28,%rdx
  je     L7
  call   <__stack_chk_fail@plt>
L7:
  lea    -0x18(%rbp),%rsp
  pop    %rbx
  pop    %r12
  pop    %r13
  pop    %rbp
  ret
```

assistant: ```c
uint16_t udp4_checksum(struct ip iphdr, struct udphdr udphdr, uint8_t *payload, int payloadlen) {
  char buf[IP_MAXPACKET];
  char *ptr;
  int chksumlen = 0;
  int i;
  ptr = &buf[0];
  memcpy(ptr, &iphdr.ip_src.s_addr, sizeof(iphdr.ip_src.s_addr));
  ptr += sizeof(iphdr.ip_src.s_addr);
  chksumlen += sizeof(iphdr.ip_src.s_addr);
  memcpy(ptr, &iphdr.ip_dst.s_addr, sizeof(iphdr.ip_dst.s_addr));
  ptr += sizeof(iphdr.ip_dst.s_addr);
  chksumlen += sizeof(iphdr.ip_dst.s_addr);
  *ptr = 0;
  ptr++;
  chksumlen += 1;
  memcpy(ptr, &iphdr.ip_p, sizeof(iphdr.ip_p));
  ptr += sizeof(iphdr.ip_p);
  chksumlen += sizeof(iphdr.ip_p);
  memcpy(ptr, &udphdr.len, sizeof(udphdr.len));
  ptr += sizeof(udphdr.len);
  chksumlen += sizeof(udphdr.len);
  memcpy(ptr, &udphdr.source, sizeof(udphdr.source));
  ptr += sizeof(udphdr.source);
  chksumlen += sizeof(udphdr.source);
  memcpy(ptr, &udphdr.dest, sizeof(udphdr.dest));
  ptr += sizeof(udphdr.dest);
  chksumlen += sizeof(udphdr.dest);
  memcpy(ptr, &udphdr.len, sizeof(udphdr.len));
  ptr += sizeof(udphdr.len);
  chksumlen += sizeof(udphdr.len);
  *ptr = 0;
  ptr++;
  *ptr = 0;
  ptr++;
  chksumlen += 2;
  memcpy(ptr, payload, payloadlen);
  ptr += payloadlen;
  chksumlen += payloadlen;
  for (i = 0; i < payloadlen % 2; i++, ptr++) {
    *ptr = 0;
    ptr++;
    chksumlen++;
  }
  return checksum((uint16_t *)buf, chksumlen);
}
```