1The standard C library 2###################### 3 4Library organization 5==================== 6 7The C library is, unlike in other POSIX systems, called libroot.so. It contains functions typically 8found in libc, libm, libpthread, librt, as well as a few BeOS extensions. 9 10It does not contain functions related to sockets networking, which are instead available in the 11separate libnetwork.so. 12 13POSIX, BSD and GNU extensions 14============================= 15 16overview 17-------- 18 19The C library tries to follow the specifications set by the C standard as well as POSIX. These 20provide a complete list of functions that should be implemented in the standard C library. The 21library should not export any other functions, or if it does, they should be in a private namespace, 22for example by prefixing their names with an underscore. 23 24The idea is to avoid symbol name conflicts: applications are allowed to use any function names they 25want, and there is a risk that the standard library accidentally uses the same name. 26 27However, because the standards are a bit conservative, they often don't include functions that 28would be very useful. Historically, other operating systems have provided non-standard extensions 29in their C libraries. In particular, this is common in both glibc and BSD C libraries. 30 31In Haiku, such extensions will be protected by a preprocessor ifdef guard, declared in separate 32headers, and, whenever possible, made available in separate libraries. This allows the main C 33library (libroot) to be more strictly conforming to the C and POSIX standards, while still making 34popular additions available to applications. 35 36features.h 37---------- 38 39The file headers/compatibility/bsd/features.h is used to enable the declaration of these additions 40in system header files. 41 42It does so by defining the _DEFAULT_SOURCE variable (for "reasonable defaults"), which is enabled 43if either of the following conditions are satisfied: 44 45- The _GNU_SOURCE or _BSD_SOURCE preprocessor define is defined. 46- The compiler is NOT in strict mode (__STRICT_ANSI__ is not defined) and _POSIX_C_SOURCE is not defined 47 48Typically the following common cases will be used: 49 50- The compiler is run without specific compiler flags: the default in gcc is to enable GNU extensions, 51 so __STRICT_ANSI__ is not defined. As a result, the extended functions are available. 52- The compiler is run with -D_POSIX_C_SOURCE: compiler GNU extensions to C and C++ are enabled, 53 but extended functions are not available. 54- The compiler is set to use a strict standard (for example -std=c11). In this case, __STRICT_ANSI__ 55 is defined, and the extended functions are not available. 56- The compiler is set to use a strict standard, but either _BSD_SOURCE or _GNU_SOURCE is defined. 57 In this case, the C and C++ language extensions are disabled, but the extra functions are available. 58 59header files organization 60------------------------- 61 62In addition to the _DEFAULT_SOURCE guard, the nonstandard functions are declared in separate headers, 63found in headers/compatibility/bsd and headers/compatibility/gnu, depending on where the function 64was first introduced. 65 66These directories are inserted before the standard ones (such as headers/posix). Since the extensions 67are usually added in existing headers, these headers are overridden in these directories. 68 69The overriden header uses #include_next (a gcc extension) to include the original one. It then 70defines any extensions available. 71 72There is a problem with this: #include_next is itself a nonstandard feature. So, in order to use a 73fully standard compiler which would not recognize it, the compatibility headers directories should 74be removed from the include paths. 75 76libgnu and libbsd 77----------------- 78 79Moving the declarations out of the system headers is fine, but it is not enough. The function 80definitions should also be moved out of libroot. So they are implemented in separate libraries, 81libgnu and libbsd. Applications using these extended functions will need to link with these. 82 83weak symbols 84------------ 85 86In some cases, the code can't easily be moved out of libroot. There are various cases where this 87can happen, for example: 88 89- Other code in libroot depends on the nonstandard function 90- The functions was in libroot in BeOS and can't be removed without breaking BeOS apps 91 92In these cases, the function will be provided with an underscore at the start of its name. This 93moves it to a namespace where libroot is allowed to define anything it needs. Then, a non-prefixed 94version can be exported as a weak symbol. This allows applications to define their own version of 95the function, since the one in libroot is "weak", the application one will be used instead for the 96application code. Since code in libroot references the underscore-prefixed function only, it will 97not be affected by this, and will still call the libroot-provided function. 98 99This creates extra complexity, so, whenever possible, the functions should be moved to separate 100libraries instead. 101 102BeOS and Haiku specific functions 103================================= 104 105In addition to the standard C library, libroot also contains functions specific to BeOS and Haiku. 106Unfortunately, no precautions were taken with these, and they can conflict with application defined 107symbols. 108 109We can't change this for symbols existing from BeOS, as it would break binary compatibility. We 110may need to change this in R2. However, we should keep this in mind when adding new functions to 111libroot. There is no well-defined solution for this currently, but the strategies documented above 112can be used if needed. 113