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/* 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: firstname.lastname@example.org (subscriber-only) or: Kay Sievers <email@example.com> Lennart Poettering <firstname.lastname@example.org> 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. :)