9.3 Unsupported Features

 

The following features in many C libraries are deliberately unsupported by the minimal C library, for reasons described below, and will remain unsupported unless a compelling counterargument arises:

• Locales: Typical programs that use the minimal C library, particularly kernels, are generally not the kinds of programs that need extensive internationalization support from the C library functions they use. In practice, the string-related minimal C library functions are typically used for printing diagnostic messages and allowing the user to select boot time parameters such as the root partition; for these purposes, simplicity and compactness are generally more important than multilingual flexibility. If a particular (rare) kernel does want full internationalization support in the C library functions it uses, and is prepared to pay the price in size and complexity, then it can instead use the full internationalized implementations from standard application-oriented C libraries, rather than the simple ones provided by the minimal C library.
• Multibyte characters: These are not supported for basically the same reasons as for locales.
• I/O buffering: Although the OSKit minimal C library provides high-level I/O functions such as fprintf, fputc, fread, etc., these functions do no buffering, and instead simply translate directly into calls to low-level I/O routines (e.g., read and write). We chose this strategy because typical programs that use the minimal C library only want to use high-level I/O functions for the convenience they provide (particularly formatted I/O), not for the performance benefits of buffering. Full I/O buffering generally comes with a great deal of C library code size and complexity, and add many additional dependencies to the environment (e.g., memory allocation for buffers, detection of line disciplines). Furthermore, the mere act of buffering I/O implies a major assumption about the environment and the use of these functions: in particular, it assumes that the underlying low-level I/O operations have high per-invocation overhead and that the high-level I/O operations are called at fine enough granularity to make this overhead a problem in practice. This assumption is often invalid for clients of the minimal C library, which generally use I/O functions only sporadically if at all, rather than intensively as many user-level applications do; and in any case, one of the primary goals of the minimal C library is to avoid such assumptions in the first place. For these reasons, we felt that I/O buffering is neither necessary nor appropriate for the minimal C library to perform.
• Floating-point math: In general, most kernels and other programs likely to use the minimal C library do not perform much, if any, floating point arithmetic; in many cases they never even access the FPU other than to save and restore its state on context switches. For this reason, all of the floating-point math functions that are a standard part of most C libraries are omitted from the minimal C library.

  There is limited support for printing floating point numbers, however. If this feature is desired, doprnt.c can be compiled with -DDOPRNT_FLOAT to enable the use of the %f format specifier.