The C standard library provides string manipulation (string.h), basic I/O (stdio.h), memory allocation (stdlib.h), and other basic functionality to C programs. The interface is described in the C standard, with further additions described in POSIX as well as vendor extensions. On Unix platforms, the library is named libc and is linked automatically into every executable.
You need a C standard library implementation with the necessary features to run C programs on your operating system. C++ programs can usually use the C standard library as well and the C++ implementation is normally built on top of libc. It is possible to use the C standard interface in a kernel if the library implementation supports this.
Freestanding and Hosted
There are two flavors of the C compilation environment: Hosted, where the standard library is available; and freestanding, where only a few headers are usable that contains only defines and types. The hosted environment is meant for user-space programming while freestanding is meant for kernel programming. The hosted environment is default, but you can switch to the freestanding by passing -ffreestanding to your compiler.
The __STDC_HOSTED__ macro expands to 1 on hosted implementations, or 0 on freestanding ones. The freestanding headers are: <float.h>, <iso646.h>, <limits.h>, <stdalign.h>, <stdarg.h>, <stdbool.h>, <stddef.h>, <stdint.h>, and <stdnoreturn.h>. You should be familiar with these headers as they contain useful declarations you shouldn't do yourself. GCC also comes with additional freestanding headers for CPUID, SSE and such.
There are several open-source C library packages available, and using one may be a viable solution for you. All of them will require some degree of modification to suit your needs.
There is a comparison table of some of these.
- Main article: Creating a C Library
One of the more obvious solutions is to roll your own. You can get seamless integration with your kernel and the rest of the operating system, without portability layers. You also get the ability to progress with function implementations in step with the hardware functionality you add to your kernel. You can effectively write the standard and append custom interfaces to be available to all apps. It is also the one and only option if you happen to be one of the purists out there. It also serves as quite the learning experience.
On the flipside, the C library requires a significant amount of time, and using an existing version allows you to spend more effort on your own kernel. In addition, there are a great many caveats hidden in the C language specification, and it is exceedingly easy to write a non-compliant implementation that will later on cause unexpected issues when porting other third part software - or it so happens that your version is actually compliant and whatever you try to port has a hard dependency on glibc quirks.
Here are some existing C libraries for you to pursue:
- GPL license
- Should be absolutely complete (even has all the bloat)
- Supports almost every architecture
- Generates large programs
- Is not written with anything other than Linux in mind, making it a hard port.
- MIT license
- No kernel portability layer, uses the Linux system calls directly. You can add your own layer between musl and the kernel to translate Linux system calls into native system calls, which is the method used by midipix.
- A full set of math and printf functions
- Support for about 1200 functions
- Many system calls needs to be implemented as it assumes you are a full Linux
- The license is unrestricted (not GPL or LGPL), but each file likely has a different copyright notice.
- Requires threading, so is more appropriate for a runtime library
- About 400 functions supported
- Creative Commons Zero license (public domain)
- Under active development, and not at full working release 1.0 yet
- Good for linking into kernels
- Support for about 120 functions, currently
- 10 (plus one optional) required syscalls need to be implemented
- No ASM -- should be fully portable
- LGPL license
- GPL 2 license
- Optimized for small size
- Many features missing
- BSD license
- No support for locales
- No libthread_db or libm implementation
- Its own smallish implementation of pthreads based on Linux futexes
- Support for x86, ARM and ARM thumb CPU instruction sets and kernel interfaces
- ISC license.
- Implements large parts of the C and POSIX standards.
- Subset can be built as the kernel standard library libk.
- Supports over 70 pieces of third party software.
- The source code is well organized and fairly straightforward.
- Static linking only at this time.
- Part of Sortix and assumes the Sortix system call ABI, which makes it cleaner.
- Some modification is required to support other system call ABIs depending on how similar the ABI is.
- Public domain
- Written for the new C11 standards.
- Under development since November 2014. Currently still severely lacking in functionality.
Especially if you want to roll your own C lib, you may want to buy the ISO/IEC 9899 specification to work from. It is not free. Expect a PDF to cost somewhere around $250 (US) or 250 Swiss Francs, depending on currency conversions.
On the other hand, the INCITS republishes these standards for a lot less: INCITS/ISO/IEC 9899-2011 can be purchased for about $60 (US) from the ANSI web store or from TechStreet.
The older standards (C89/C90, C99) are not commercially available anymore. To find the current standard, go to one of the following sites and search for document "ISO/IEC 9899".
By the way, you can use latest draft of standard, it's publicly available for free and exactly the same as approved version. This is a path chosen by most open-source software, especially GCC and GLIBC relies on drafts published by ISO/IEC. Latest version of C2011 draft has the name ISO/IEC 9899 N1570 and can be downloaded here (PDF)
- INCITS/ISO/IEC 9899-2011 at ANSI's eStandards Store
- INCITS/ISO/IEC 9899-2011 at Thomson Reuters TechStreet