> For the complete documentation index, see [llms.txt](https://book.bsdcn.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://book.bsdcn.org/man/misc/offsetof.3.md).

# offsetof.3

`offsetof` — 结构成员的偏移量

## 名称

`offsetof`

## 概要

`#include <stddef.h>`

`Ft size_t Fn offsetof type member`

## 描述

`Fn offsetof` 宏展开为类型为 `Ft size_t` 的整型常量表达式，其值为字段 `member` 相对于结构 `type` 起始处的偏移量（以字节为单位）。

如果 `member` 未按字节边界对齐（即它是位域），将导致编译器报错。

## 标准

`Fn offsetof` 宏遵循 ANSI X3.159-1989 ("ANSI C89") 标准。


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter:

```
GET https://book.bsdcn.org/man/misc/offsetof.3.md?ask=<question>&goal=<endgoal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.