xref: /openbmc/linux/Documentation/bpf/libbpf/libbpf_naming_convention.rst (revision 9330303446382a33aa62a8a88a7a48555f76df0b)

.. SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)

API naming convention
=====================

libbpf API provides access to a few logically separated groups of
functions and types. Every group has its own naming convention
described here. It's recommended to follow these conventions whenever a
new function or type is added to keep libbpf API clean and consistent.

All types and functions provided by libbpf API should have one of the
following prefixes: ``bpf_``, ``btf_``, ``libbpf_``, ``xsk_``,
``btf_dump_``, ``ring_buffer_``, ``perf_buffer_``.

System call wrappers
--------------------

System call wrappers are simple wrappers for commands supported by
sys_bpf system call. These wrappers should go to ``bpf.h`` header file
and map one to one to corresponding commands.

For example ``bpf_map_lookup_elem`` wraps ``BPF_MAP_LOOKUP_ELEM``
command of sys_bpf, ``bpf_prog_attach`` wraps ``BPF_PROG_ATTACH``, etc.

Objects
-------

Another class of types and functions provided by libbpf API is "objects"
and functions to work with them. Objects are high-level abstractions
such as BPF program or BPF map. They're represented by corresponding
structures such as ``struct bpf_object``, ``struct bpf_program``,
``struct bpf_map``, etc.

Structures are forward declared and access to their fields should be
provided via corresponding getters and setters rather than directly.

These objects are associated with corresponding parts of ELF object that
contains compiled BPF programs.

For example ``struct bpf_object`` represents ELF object itself created
from an ELF file or from a buffer, ``struct bpf_program`` represents a
program in ELF object and ``struct bpf_map`` is a map.

Functions that work with an object have names built from object name,
double underscore and part that describes function purpose.

For example ``bpf_object__open`` consists of the name of corresponding
object, ``bpf_object``, double underscore and ``open`` that defines the
purpose of the function to open ELF file and create ``bpf_object`` from
it.

All objects and corresponding functions other than BTF related should go
to ``libbpf.h``. BTF types and functions should go to ``btf.h``.

Auxiliary functions
-------------------

Auxiliary functions and types that don't fit well in any of categories
described above should have ``libbpf_`` prefix, e.g.
``libbpf_get_error`` or ``libbpf_prog_type_by_name``.

AF_XDP functions
-------------------

AF_XDP functions should have an ``xsk_`` prefix, e.g.
``xsk_umem__get_data`` or ``xsk_umem__create``. The interface consists
of both low-level ring access functions and high-level configuration
functions. These can be mixed and matched. Note that these functions
are not reentrant for performance reasons.

ABI
---

libbpf can be both linked statically or used as DSO. To avoid possible
conflicts with other libraries an application is linked with, all
non-static libbpf symbols should have one of the prefixes mentioned in
API documentation above. See API naming convention to choose the right
name for a new symbol.

Symbol visibility
-----------------

libbpf follow the model when all global symbols have visibility "hidden"
by default and to make a symbol visible it has to be explicitly
attributed with ``LIBBPF_API`` macro. For example:

.. code-block:: c

        LIBBPF_API int bpf_prog_get_fd_by_id(__u32 id);

This prevents from accidentally exporting a symbol, that is not supposed
to be a part of ABI what, in turn, improves both libbpf developer- and
user-experiences.

ABI versionning
---------------

To make future ABI extensions possible libbpf ABI is versioned.
Versioning is implemented by ``libbpf.map`` version script that is
passed to linker.

Version name is ``LIBBPF_`` prefix + three-component numeric version,
starting from ``0.0.1``.

Every time ABI is being changed, e.g. because a new symbol is added or
semantic of existing symbol is changed, ABI version should be bumped.
This bump in ABI version is at most once per kernel development cycle.

For example, if current state of ``libbpf.map`` is:

.. code-block:: none

        LIBBPF_0.0.1 {
        	global:
                        bpf_func_a;
                        bpf_func_b;
        	local:
        		\*;
        };

, and a new symbol ``bpf_func_c`` is being introduced, then
``libbpf.map`` should be changed like this:

.. code-block:: none

        LIBBPF_0.0.1 {
        	global:
                        bpf_func_a;
                        bpf_func_b;
        	local:
        		\*;
        };
        LIBBPF_0.0.2 {
                global:
                        bpf_func_c;
        } LIBBPF_0.0.1;

, where new version ``LIBBPF_0.0.2`` depends on the previous
``LIBBPF_0.0.1``.

Format of version script and ways to handle ABI changes, including
incompatible ones, described in details in [1].

Stand-alone build
-------------------

Under https://github.com/libbpf/libbpf there is a (semi-)automated
mirror of the mainline's version of libbpf for a stand-alone build.

However, all changes to libbpf's code base must be upstreamed through
the mainline kernel tree.


API documentation convention
============================

The libbpf API is documented via comments above definitions in
header files. These comments can be rendered by doxygen and sphinx
for well organized html output. This section describes the
convention in which these comments should be formated.

Here is an example from btf.h:

.. code-block:: c

        /**
         * @brief **btf__new()** creates a new instance of a BTF object from the raw
         * bytes of an ELF's BTF section
         * @param data raw bytes
         * @param size number of bytes passed in `data`
         * @return new BTF object instance which has to be eventually freed with
         * **btf__free()**
         *
         * On error, error-code-encoded-as-pointer is returned, not a NULL. To extract
         * error code from such a pointer `libbpf_get_error()` should be used. If
         * `libbpf_set_strict_mode(LIBBPF_STRICT_CLEAN_PTRS)` is enabled, NULL is
         * returned on error instead. In both cases thread-local `errno` variable is
         * always set to error code as well.
         */

The comment must start with a block comment of the form '/\*\*'.

The documentation always starts with a @brief directive. This line is a short
description about this API. It starts with the name of the API, denoted in bold
like so: **api_name**. Please include an open and close parenthesis if this is a
function. Follow with the short description of the API. A longer form description
can be added below the last directive, at the bottom of the comment.

Parameters are denoted with the @param directive, there should be one for each
parameter. If this is a function with a non-void return, use the @return directive
to document it.

License
-------------------

libbpf is dual-licensed under LGPL 2.1 and BSD 2-Clause.

Links
-------------------

[1] https://www.akkadia.org/drepper/dsohowto.pdf
    (Chapter 3. Maintaining APIs and ABIs).