user: 提供的汇编代码用C语言表示会是什么样子:

```asm
<env_build>:
  endbr64
  push   %rbp
  mov    %rsp,%rbp
  push   %rbx
  sub    $0x38,%rsp
  mov    %rdi,-0x38(%rbp)
  mov    %rsi,-0x40(%rbp)
  movl   $0x0,-0x28(%rbp)
  movl   $0x0,-0x2c(%rbp)
  jmp    L13
L15:
  addl   $0x1,-0x2c(%rbp)
L13:
  mov    -0x2c(%rbp),%eax
  cltq
  lea    0x0(,%rax,8),%rdx
  mov    -0x38(%rbp),%rax
  add    %rdx,%rax
  mov    (%rax),%rax
  test   %rax,%rax
  jne    L15
  addl   $0x2,-0x2c(%rbp)
  mov    -0x2c(%rbp),%eax
  cltq
  shl    $0x3,%eax
  mov    %eax,%edi
  call   <malloc@plt>
  mov    %rax,-0x20(%rbp)
  cmpq   $0x0,-0x20(%rbp)
  jne    L104
  mov    $0x0,%eax
  jmp    L12
L104:
  cmpq   $0x0,-0x40(%rbp)
  je     L36
  mov    -0x40(%rbp),%rax
  mov    $0x3d,%esi
  mov    %rax,%rdi
  call   <strchr@plt>
  mov    %rax,-0x18(%rbp)
  cmpq   $0x0,-0x18(%rbp)
  je     L86
  mov    -0x18(%rbp),%rax
  sub    -0x40(%rbp),%rax
  mov    %eax,-0x28(%rbp)
  jmp    L36
L86:
  movq   $0x0,-0x40(%rbp)
L36:
  cmpq   $0x0,-0x40(%rbp)
  jne    L91
  movl   $0x0,-0x2c(%rbp)
  jmp    L19
L132:
  mov    -0x2c(%rbp),%eax
  cltq
  lea    0x0(,%rax,8),%rdx
  mov    -0x38(%rbp),%rax
  add    %rdx,%rax
  mov    (%rax),%rax
  mov    -0x2c(%rbp),%edx
  movslq %edx,%rdx
  lea    0x0(,%rdx,8),%rcx
  mov    -0x20(%rbp),%rdx
  lea    (%rcx,%rdx,1),%rbx
  mov    %rax,%rdi
  call   <strdup@plt>
  mov    %rax,(%rbx)
  addl   $0x1,-0x2c(%rbp)
L19:
  mov    -0x2c(%rbp),%eax
  cltq
  lea    0x0(,%rax,8),%rdx
  mov    -0x38(%rbp),%rax
  add    %rdx,%rax
  mov    (%rax),%rax
  test   %rax,%rax
  jne    L132
  mov    -0x2c(%rbp),%eax
  cltq
  lea    0x0(,%rax,8),%rdx
  mov    -0x20(%rbp),%rax
  add    %rdx,%rax
  movq   $0x0,(%rax)
  mov    -0x20(%rbp),%rax
  jmp    L12
L91:
  movl   $0x0,-0x24(%rbp)
  movl   $0x0,-0x2c(%rbp)
  jmp    L44
L49:
  mov    -0x24(%rbp),%eax
  cltq
  lea    0x0(,%rax,8),%rdx
  mov    -0x38(%rbp),%rax
  add    %rdx,%rax
  mov    (%rax),%rax
  mov    -0x28(%rbp),%edx
  mov    -0x40(%rbp),%rcx
  mov    %rcx,%rsi
  mov    %rax,%rdi
  call   <strncmp@plt>
  test   %rax,%rax
  je     L9
  mov    -0x24(%rbp),%eax
  cltq
  lea    0x0(,%rax,8),%rdx
  mov    -0x38(%rbp),%rax
  add    %rdx,%rax
  mov    (%rax),%rdx
  mov    -0x2c(%rbp),%eax
  lea    0x1(%rax),%ecx
  mov    %ecx,-0x2c(%rbp)
  cltq
  lea    0x0(,%rax,8),%rcx
  mov    -0x20(%rbp),%rax
  lea    (%rcx,%rax,1),%rbx
  mov    %rdx,%rdi
  call   <strdup@plt>
  mov    %rax,(%rbx)
L9:
  addl   $0x1,-0x24(%rbp)
L44:
  mov    -0x24(%rbp),%eax
  cltq
  lea    0x0(,%rax,8),%rdx
  mov    -0x38(%rbp),%rax
  add    %rdx,%rax
  mov    (%rax),%rax
  test   %rax,%rax
  jne    L49
  mov    -0x2c(%rbp),%eax
  lea    0x1(%rax),%edx
  mov    %edx,-0x2c(%rbp)
  cltq
  lea    0x0(,%rax,8),%rdx
  mov    -0x20(%rbp),%rax
  lea    (%rdx,%rax,1),%rbx
  mov    -0x40(%rbp),%rax
  mov    %rax,%rdi
  call   <strdup@plt>
  mov    %rax,(%rbx)
  mov    -0x2c(%rbp),%eax
  cltq
  lea    0x0(,%rax,8),%rdx
  mov    -0x20(%rbp),%rax
  add    %rdx,%rax
  movq   $0x0,(%rax)
  mov    -0x20(%rbp),%rax
L12:
  mov    -0x8(%rbp),%rbx
  leave
  ret
```

assistant: ```c
char **env_build(const char **envp, const char *envadd) {
  int n, len = 0, i;
  char **va, *p;
  for (n = 0; envp[n]; n++)
    ;
  n += 2;
  va = malloc(sizeof(char *) * n);
  if (!va)
    return NULL;
  if (envadd) {
    p = strchr(envadd, '=');
    if (p) {
      len = p - envadd;
    } else {
      envadd = NULL;
    }
  }
  if (!envadd) {
    for (n = 0; envp[n]; n++)
      va[n] = strdup(envp[n]);
    va[n] = NULL;
    return va;
  }
  for (i = 0, n = 0; envp[i]; i++) {
    if (strncmp(envp[i], envadd, len)) {
      va[n++] = strdup(envp[i]);
    }
  }
  va[n++] = strdup(envadd);
  va[n] = NULL;
  return va;
}
```