README

/*
  This is free and unencumbered software released into the public domain.

  Anyone is free to copy, modify, publish, use, compile, sell, or
  distribute this software, either in source code form or as a compiled
  binary, for any purpose, commercial or non-commercial, and by any
  means.

  In jurisdictions that recognize copyright laws, the author or authors
  of this software dedicate any and all copyright interest in the
  software to the public domain. We make this dedication for the benefit
  of the public at large and to the detriment of our heirs and
  successors. We intend this dedication to be an overt act of
  relinquishment in perpetuity of all present and future rights to this
  software under copyright law.

  Unless you really want to, do not even mention that the copied content
  originates from this skeleton library. Its sole purpose is to be copied
  into other projects.

  The above statements apply to all content in this skeleton library, even
  when the COPYING files, or the headers in the files state otherwise,
  they are just common examples.
*/

Questions, feedback, patches please email:
        systemd-devel@lists.freedesktop.org (subscriber-only)
or:
        Kay Sievers <kay@vrfy.org>
        Lennart Poettering <lennart@poettering.net>

Why bother?
  - To make things easy for library users, distribution packagers and
    developers of library bindings for other programming languages. If
    you want your stuff to be used and commonly available, try to play
    nice, and give them what they are used to. It makes their life a
    lot easier.

Use autotools
  - Every custom config/makefile/build system is worse for everybody
    than autotools is.
  - We are all used to autotools, it works, nobody cares.
  - It's only two simple files to edit and include in git, which are
    well understood by many many people, not just you.
  - Ignore all crap autotools create in the source tree. Never check
    the created files into git.
  - Never, ever, install config.h. That's internal to your sources
    and is nothing to install.
  - And really, nothing but autotools is really an option. Just get
    over it. Everything else is an experiment, and it will come back
    to you sooner or later. Why? think cross compilation, installation/
    uninstallation, build root integration, separate object trees,
    standard adherence, tarball handling, make distcheck, testing,
    portability between distros, ...

If you use the GPL, always use the GPL's "(or later)" clause
  - Developers are not lawyers, libraries should be able to be linked
    to any version of the GPL. Remember that GPL2-only is incompatible
    with LGPL3!

Use LGPL (for the shared libraries) if you don't care about politics
  - It protects the code, but does not restrict its use. Low-level
    library interfaces are mostly used like kernel syscall or proc/sysfs
    interfaces, which are usually without any restrictions.

Zero global state -- Make your library threads-aware, but *not* thread-safe!
  - An app can use liba and libb. libb internally can also use liba --
    without you knowing. Both you and libb can run liba code at the
    very same time in different threads and operate at the same global
    variables, without telling you about that. Loadable modules make
    this problem even more prominent, since the libraries they pull in
    are generally completely unknown by the main application. And
    *every* program has loadable modules, think NSS!
  - Avoid locking and mutexes, they are very unlikely to work correctly,
    and incredibly hard to get right.
  - Always use a library context object. Every thread should then
    operate on its own context. Do not imply context objects via TLS. It
    won't work. TLS inheritance to new threads will get in your way. TLS
    is a problem in itself, not a solution.
  - Do not use gcc constructors, or destructors, you can only lose if
    you do. Do not use _fini() or _ini(), don't even use your own
    explicit library initializer/destructor functions. It just won't
    work if your library is pulled in indirectly from another library
    or even a shared module (i.e. dlopen())
  - Always use O_CLOEXEC, SOCK_CLOEXEC and friends. It's not an
    option, it's a must.
  - Don't use global variables (it includes static variables defined
    inside functions). Ever. And under no circumstances export global
    variables. It's madness.

Use a common prefix for _all_ exported symbols
  - Avoids namespace clashes
  - Also, hacking is not a contest of finding the shortest possible
    function name. And nobody cares about your 80ch line limit!
  - If you use a drop-in library in your own library make sure to hide its
    symbols with symbol versioning. Don't forget to hide *all* symbols, and
    don't install the header file of the used drop-in library.

Do not expose any complex structures in your API
  - Use get() and set() instead.
  - All objects should be opaque.
  - Exporting structs in headers is OK in very few cases only: usually
    those where you define standard binary formats (think: file
    formats, datagram headers, ...) or where you define well-known
    primitive types (think struct timeval, struct iovec, uuid
    type).
  - Why bother? Because struct stat, struct dirent and friends are
    disasters. Particularly horrible are structs with fixed-size
    strings.

Use the de-facto standardized function names
  - It's abc_new(), abc_free(), abc_ref(), abc_unref(). Don't invent
    your own names, and don't use the confusing kernel-style ref
    counting. Function names: _get() is for accessing properties of
    objects, not for refcounting.

Stick to kernel coding style
  - Just because you are otherwise not bound by the kernel guidelines
    when your write userspace libraries doesn't mean you have to give
    up the good things it defines.

Avoid callbacks in your API
  - Language bindings want iterators.
  - Programmers want iterators too.

Never call exit(), abort(), be very careful with assert()
  - Always return error codes.
  - Libraries need to be safe for usage in critical processes that
    need to recover from errors instead of getting killed (think PID 1!).

Avoid thinking about main loops/event dispatchers.
  - Get your stuff right in the kernel: fds are awesome, expose them
    in userspace and in the library, because people can easily integrate
    them with their own poll() loops of whatever kind they have.
  - Don't hide file descriptors away in your headers.
  - Never add blocking kernel syscalls, and never add blocking library
    calls either (with very few exceptions). Userspace code is primarily
    asynchronous around event loops, and blocking calls are generally
    incompatible with that.
  - Corollary of that: always O_NONBLOCK!

Functions should return int and negative errors instead of NULL
  - Return NULL in malloc() is fine, return NULL in fopen() is not!
  - Pass allocated objects as parameter (yes, ctx_t** is OK!)
  - Returning kernel style negative <errno.h> error codes is cool in
    userspace too. Do it!

Provide pkgconfig files
  - Apps want to add a single line to their configure file,
    they do not want to fiddle with the parameters, dependencies
    to setup and link your library.
  - It's just how we do these things today on Linux, and everything
    else is just horribly messy.

Avoid *hidden* fork()/exec() in libraries
  - Apps generally do not expect signals and react allergic to them.
  - Mutexes, locks, threads of the app might get confused. Mixing
    mutexes and fork() equals failure. It just can't work, and
    pthread_atfork() is not a solution for that, because it's broken
    (even POSIX acknowledges that, just read the POSIX man
    pages!). fork() safety for mutex-ridden code is not an
    afterthought, it's a broken right from the beginning.

Make your code safe for unexpected termination at any point:
  - Do not leave files dirty or temporary files around.
  - This is a tricky, since you need to design your stuff like this
    from the beginning, it's not an afterthought, since you generally
    do not have a place to clean up your stuff on exit. gcc
    destructors are NOT the answer.

Use symbol versioning
  - Only with that, RPM can handle dependencies for added symbols
  - Hide all internal symbols! *This is important!*

Always provide logging/debugging, but do not clutter stderr
  - Allow the app to hook the libs logging into its logging facility.
  - Use conditional logging, do not filter too late.
  - Do not burn cycles with printf() to /dev/null.
  - By default: do not generate any output on stdout/stderr.

Always use 'make distcheck' to create tarballs
  - Never release anything that does not pass distcheck. It will
    likely be broken for others too

Use ./autogen.sh to bootstrap the git repo
  - Always test bootstrapping with 'git clean -x -f -d' before
    release (careful, it force-deletes all uncommitted files).

Avoid any spec files or debian/ subdirs in git trees
  - Distribution specific things do not belong in upstream trees,
    but into distro packages

Update NEWS to let developers know what has changed
  - It's the history of the project, stuff that packagers need to know
    when putting a new version in the distro. The interesting changes
    or added/removed functionality from version to version. This is
    not a commit changelog.
  - If you want to provide ChangeLog, use the one generated
    by git, do not maintain your own.

Use standard types
  - The kernel's u8, u16, ... correspond to uint8_t, uint16_t in
    userspace from <inttypes.h>. Don't define your own typedefs
    for that, don't include the kernel types in common headers.
  - Use enums, not #define for constants, wherever possible. In
    userspace you have debuggers, and they are much nicer to use if
    you have proper enum identifiers instead of macro definitions,
    because the debugger can translate binary values back to enum
    identifiers, but not macros. However, be careful with enums in
    function prototypes: they might change the int type they are
    resolved to as you add new enum values.

Always guard for multiple inclusions of headers
  - You must place '#ifndef libabc, #define libabc, #endif' in your
    header files. There is no way around that.

Be careful with variadic functions
  - It's great if you provide them, but you must accompany them with
    "v" variants (i.e. functions taking a va_arg object), and provide
    non-variadic variants as well. This is important to get language
    wrappers right.

Don't put "extern" in front of your function prototypes in headers
  - It has no effect, no effect at all.

Never use sysv IPC, always use POSIX IPC
  - Shmops and semops are horrors. Don't use them, ever. POSIX IPC is
    much much much nicer.

Avoid multiplexed functions ala ioctl()/prctl() style variadic functions
  - Type-safety is awesome!

Executing out-of-process tools and parsing their output is usually
not acceptable in libraries
  - Tools should be built on top of their own lib.
  - Always separate 'mechanism' from 'policy'. Make access to functionality
    simple, but do not try to hide things that need to be decided by the
    caller. Keep automagic at its minimum. Don't do hidden fork() do not
    implicitly maintain cache files, ...

Function calls with 15 arguments are a bad idea. If you have tons of
booleans in a function call, then replace them by a flag argument!
  - Think about the invocation! foo(0, 1, 0, 1, 0, 0, 0, 1) is unreadable!
    foo(FOO_QUUX|FOO_BAR|FOO_WALDO) much nicer.

Don't be afraid of C99. Use it.
  - It's 12 years old. And it's nice.

Never expose fixed size strings in your API
  - Pass malloc()ed strings out, or ask the caller to provide you with
    a buffer, and return ENOSPC if too short.

Glibc has byteswapping calls, don't invent your own:
  - le32toh(), htole32() all those are documented in endian(3)
  - bswap_32(), bswap_16(), bswap_64(). #include <byteswap.h>
  - Don't use the versions provided by the linux kernel headers cpu_to_*() , *_to_cpu.
    stick to the glibc API.

Don't typedef pointers to structs!

Don't write your own LISP interpreter and do not include it in your
library. :)