计算机语言和计算机操作系统是两个层面上的东西。
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C（包括C++）是贝尔实验室开发的，其所属公司ATT公司早已倒闭，所以已经是没有爹娘的孩子，根本没有什么标准化组织。现在主要的两个C语言标准：C89和C99都是各个厂商实现各自的C，经过市场竞争以后形成的事实标准，后经ANSI（美国国家标准化委员会）认定所形成的美国标准，继而又被ISO组织采纳为事实标准。
有标准以后各个厂商按照标准修改自己的产品，有的还提供各种选项供用户选择使用何种标准(如VC++工程属性里的各种设置)，以实现“源程序级兼容”。
C中各种函数，如printf函数，涉及到操作系统界面，在不同平台上的实现肯定是不一样的。stdio.h,conio.h在硬盘上都有确实的文件的，可以打开看看函数的定义，都是一些#define之类的。具体实现功能的东西在.obj，.lib这些文件里面，有部分是用汇编写的，全部都是没有源代码的。 但是其他的通用函数可能是一样的，比如C++STL，都是HP和SGI的标准版。
linux开源，开得是linux的源，当然不包括库函数。
操作系统的API，全称application programming interface，即"应用程序接口"，是操作系统提供的功能，操作系统将硬件裸机包装成虚拟机，提供多种接口供用户调用。Linux、Windows的API是基于C、C++的，DOS的API是基于汇编语言的。
既然C是接近硬件的语言，只要不违反操作系统的安全性、稳定性等限制，大可以跳开API直接操作硬件。这样无异于自己编写操作系统，所以C语言如果真的有楼主所述“只用这些函数就能解决问题”这一天，那么那个时候的C语言就已经发展成为操作系统了。
开源是unix系统发展过程中产生的一个词，意思是开放源代码。
开源的C语言库，就是自己可以得到源代码的C语言库，可以不断地改进这个库中的源代码，大家共同进步，百家争鸣，有利于C语言的发展。
C语言是一个由ISO组织中的ANSI制定的标准，任何个人或者组织都可以根据这个标准将其实现。现今，世界上有许多不同的C语言实现，比较著名的有：GCC、Watcom、MS C等，其中前两者是开源的，后者是闭源的。下面粘贴几个老外的回答（原回答链接）。
The C language is not a piece of software but a defined standard, so one wouldn't say that it's open-source, but rather that it's an open standard.
There are a gazillion different compilers for C however, and many of those are indeed open-source. The most notable example is GCC's C compiler, which is all under the GNU General Public License (GPL), an open-source license.
There are more options. Watcom is open-source, for instance. There is no shortage of open-source C compilers, but without a doubt the most widespread one, at least in the non-Windows world, is GCC.
For Windows, your best bet is probably Watcom or GCC by using Cygwin or MinGW.
C is a standard which specifies how C compilers should generate programs.
C itself doesn't have any source code, just like a musical note doesn't have any plastic.
Some C compilers, such as GCC, are open source.
C is just a language, and a standardised one at that, too. It pretty much is the compiler that "does all the work". Different compilers did have different dialects; before the the C99 ANSI standard, you had things like Borland C and other competing compilers, that implemented the C language in their own fantastic ways.
stdlib is just an agreed-upon collection of standard libraries that are required to be present in any ANSI C implementation.
关于C++开源与否：
与C语言类似，C++也是由ISO/ANSI制定的一个标准，所谓的“官方”并未给出确切的实现，任何组织与个人都可以根据标准自己开发一个C++编译器出来。出名的C++编译器有：GCC/G++、libc/libc++、clang（++）、 Visual studio和MS´ runtime等。也把老外的几个回答贴出来（原回答链接）。
C++ itself is only a description what the language should be,
without a definite implementation.
Anyone can make his own implementations (compiler etc, runtime library, ...)
and call it C++ if it fits to the description.
And if a implementation is open source depends on the creator.
Examples of implementation (parts):
GCC/G++, libc/libc++, clang (++ too), Visual studio and MS´ runtime...
C++ is developed by an ISO standard committee. There's also a C++ foundation that runs a web site you might want to read.
C++ itself is a language, not a specific implementation, so there's no source code available for the standard/language itself.
Some C++ implementations are open source (e.g., Gnu and Clang).
1. C++ is a code standard defined by the International Organization of Standardization (ISO). There are many different implementations of the language, but they all tend to conform to C++11. Unlike Linux or Qt, C++ is just a standard, and to use any code written in the language you'll need a compiler. The major compilers (list from Wikipedia) are LLVM Clang, GCC, Microsoft Visual C++, and the Intel C++ Compiler.
2. C++ revisions are dealt with by ISO, and are influenced primarily by the maintainers of the above four implementations.
3. Clang and GCC are both open-source, I'm sure if you poke around you can find other conforming compilers but those are the two most used.
总之，跟Java、Python和PHP这样所谓的开源语言不同，C语言与C++没有官方提供的各自确切的实现代码（库），ISO/ANSI仅仅提供了C和C++的标准。
这些都是我从自个儿博客摘抄来的，也不见得有人看得到。
