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

```asm
<rfile_fn_rel2abs>:
  endbr64
  push   %rbp
  mov    %rsp,%rbp
  push   %r12
  push   %rbx
  sub    $0x40,%rsp
  mov    %rdi,-0x48(%rbp)
  mov    %rsi,-0x50(%rbp)
  mov    %fs:0x28,%rax
  mov    %rax,-0x18(%rbp)
  xor    %eax,%eax
  mov    %rsp,%rax
  mov    %rax,%r12
  mov    D16(%rip),%rax
  mov    (%rax),%eax
  lea    (%rax,%rax,1),%ebx
  movslq %ebx,%rax
  sub    $0x1,%rax
  mov    %rax,-0x38(%rbp)
  movslq %ebx,%rax
  mov    %rax,%r10
  mov    $0x0,%r11d
  movslq %ebx,%rax
  mov    %rax,%r8
  mov    $0x0,%r9d
  movslq %ebx,%rax
  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
L78:
  cmp    %rdx,%rsp
  je     L26
  sub    $0x1000,%rsp
  orq    $0x0,0xff8(%rsp)
  jmp    L78
L26:
  mov    %rax,%rdx
  and    $0xfff,%edx
  sub    %rdx,%rsp
  mov    %rax,%rdx
  and    $0xfff,%edx
  test   %rdx,%rdx
  je     L106
  and    $0xfff,%eax
  sub    $0x8,%rax
  add    %rsp,%rax
  orq    $0x0,(%rax)
L106:
  mov    %rsp,%rax
  add    $0x0,%rax
  mov    %rax,-0x30(%rbp)
  mov    -0x48(%rbp),%rax
  mov    %rax,%rdi
  call   <rfile_fn_is_abs@plt>
  test   %rax,%rax
  je     L66
  movq   $0x0,-0x40(%rbp)
  jmp    L54
L66:
  cmpq   $0x0,-0x50(%rbp)
  je     L128
  mov    -0x50(%rbp),%rax
  mov    $0x0,%esi
  mov    %rax,%rdi
  call   <rfile_fn_rel2abs>
  mov    %rax,-0x28(%rbp)
  mov    -0x28(%rbp),%rax
  mov    %rax,%rdi
  call   <strlen@plt>
  cltq
  mov    %rax,-0x40(%rbp)
  mov    -0x40(%rbp),%rax
  lea    0x1(%rax),%rdx
  movslq %ebx,%rax
  cmp    %rax,%rdx
  jbe    L47
  mov    D34(%rip),%rax
  mov    (%rax),%edx
  mov    D32(%rip),%rax
  mov    %edx,(%rax)
  mov    $0x0,%eax
  jmp    L142
L47:
  mov    -0x40(%rbp),%rdx
  mov    -0x28(%rbp),%rcx
  mov    -0x30(%rbp),%rax
  mov    %rcx,%rsi
  mov    %rax,%rdi
  call   <memcpy@plt>
  mov    -0x28(%rbp),%rax
  mov    %rax,%rdi
  call   <free@plt>
  jmp    L32
L128:
  mov    %ebx,%eax
  sub    $0x1,%eax
  mov    %eax,%edx
  mov    -0x30(%rbp),%rax
  mov    %edx,%esi
  mov    %rax,%rdi
  call   <getcwd@plt>
  test   %eax,%eax
  jne    L85
  mov    $0x0,%eax
  jmp    L142
L85:
  mov    -0x30(%rbp),%rax
  mov    %rax,%rdi
  call   <strlen@plt>
  cltq
  mov    %rax,-0x40(%rbp)
L32:
  mov    -0x40(%rbp),%rax
  lea    -0x1(%rax),%rdx
  mov    -0x30(%rbp),%rax
  movzbl (%rax,%rdx,1),%eax
  cmp    $0x2f,%al
  je     L54
  mov    -0x40(%rbp),%rax
  lea    0x1(%rax),%rdx
  mov    %rdx,-0x40(%rbp)
  mov    -0x30(%rbp),%rdx
  movb   $0x2f,(%rdx,%rax,1)
L54:
  mov    -0x30(%rbp),%rdx
  mov    -0x40(%rbp),%rax
  add    %rdx,%rax
  movb   $0x0,(%rax)
  mov    -0x48(%rbp),%rax
  mov    %rax,%rdi
  call   <strlen@plt>
  add    $0x1,%eax
  cltq
  mov    %rax,-0x20(%rbp)
  mov    -0x40(%rbp),%rdx
  mov    -0x20(%rbp),%rax
  add    %rax,%rdx
  movslq %ebx,%rax
  cmp    %rax,%rdx
  jbe    L123
  mov    D34(%rip),%rax
  mov    (%rax),%edx
  mov    D32(%rip),%rax
  mov    %edx,(%rax)
  mov    $0x0,%eax
  jmp    L142
L123:
  mov    -0x30(%rbp),%rdx
  mov    -0x40(%rbp),%rax
  lea    (%rdx,%rax,1),%rcx
  mov    -0x20(%rbp),%rdx
  mov    -0x48(%rbp),%rax
  mov    %rax,%rsi
  mov    %rcx,%rdi
  call   <memcpy@plt>
  mov    -0x30(%rbp),%rax
  mov    %rax,%rdi
  call   <rfile_fn_tidy@plt>
L142:
  mov    %r12,%rsp
  mov    -0x18(%rbp),%rdx
  sub    %fs:0x28,%rdx
  je     L28
  call   <__stack_chk_fail@plt>
L28:
  lea    -0x10(%rbp),%rsp
  pop    %rbx
  pop    %r12
  pop    %rbp
  ret
```

assistant: ```c
char *rfile_fn_rel2abs(const char *rel, const char *base) {
  char bd[MAXPATHLEN * 2];
  size_t blen, rlen;
  if (rfile_fn_is_abs(rel)) {
    blen = 0;
  } else {
    if (base) {
      char *ab = rfile_fn_rel2abs(base, NULL);
      blen = strlen(ab);
      if (blen + 1 > sizeof(bd)) {
        errno = EINVAL;
        return NULL;
      }
      memcpy(bd, ab, blen);
      free(ab);
    } else {
      if (!getcwd(bd, sizeof(bd) - 1))
        return NULL;
      blen = strlen(bd);
    }
    if (bd[blen - 1] != '/')
      bd[blen++] = '/';
  }
  bd[blen] = '\0';
  rlen = strlen(rel) + 1;
  if (blen + rlen > sizeof(bd)) {
    errno = EINVAL;
    return NULL;
  }
  memcpy(bd + blen, rel, rlen);
  return rfile_fn_tidy(bd);
}
```