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在 程序设计里，C 标准函数库（C Standard library）收集所有目前符合标准的（head file），以及常用的实现程序，例如 和控制。不像 、 和 等编程语言，在 C 语言的工作任务里不会包含嵌入的，所以几乎所有的 C 语言程序都是由标准函数库的函数来创建的。
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每一个函数的名称与特性会被写成一个电脑文件，这个文件就称为，但是实际的函数实现是被分存到里。头文件的命名和领域是很常见的，但是函数库的组织架构也会因为不同的编译器而有所不同。标准函数库通常会随附在上。因为 C 编译器常会提供一些额外的非 函数功能，所以某个随附在特定编译器上的标准函数库，对其他不同的编译器来说，是不兼容的。
大多 C 标准函数库在设计上做得相当不错。有些少部分的，会为了商业优势和利益，会把某些旧函数视同错误或提出警告。字符串输入函数 gets()（以及 scanf() 读取字符串输入的使用上）是很多的原因，而且大多的程序设计指南会建议避免使用它。另一个较为奇特的函数是 strtok()，它原本是作为早期的用途，但是它非常容易出错（fragile），而且很难使用。
1995年，Normative Addendum 1 (NA1)批准了三个头文件(iso646.h, wchar.h, and wctype.h)增加到C标准函数库中。标准增加了六个头文件(complex.h, fenv.h, inttypes.h, stdbool.h, stdint.h, and tgmath.h)。标准中又新增了5个头文件(stdalign.h, stdatomic.h, stdnoreturn.h, threads.h, and uchar.h)。至此，C标准函数库共29个头文件：
| 名字 | 源自 | 描述 |
|---|---|---|
| Contains the assert macro, used to assist with detecting logical errors and other types of bug in debugging versions of a program. | |
| C99 | A set of functions for manipulating . |
| Defines set of functions used to classify characters by their types or to convert between upper and lower case in a way that is independent of the used character set (typically or one of its extensions, although implementations utilizing are also known). | |
| For testing error codes reported by library functions. | |
| C99 | Defines a set of functions for controlling environment. |
| Defines macro constants specifying the implementation-specific properties of the library. | |
| C99 | Defines exact width integer types. |
| NA1 | Defines several macros that are equivalent to some of the operators in C. For programming in variant character sets. |
| Defines macro constants specifying the implementation-specific properties of the integer types. | |
| Defines C localization functions. | |
| Defines C mathematical functions. | |
| Declares the macros setjmp and longjmp, which are used for non-local exits. | |
| Defines C signal handling functions. | |
| C11 | For querying and specifying the data structure alignment of objects. |
| For accessing a varying number of arguments passed to functions. | |
| C11 | For atomic operations on data shared between threads. |
| C99 | Defines a boolean data type. |
| Defines several useful types and macros. | |
| C99 | Defines exact width integer types. |
| Defines core input and output functions | |
| Defines numeric conversion functions, pseudo-random numbers generation functions, dynamicmemory allocation, process control functions | |
| C11 | For specifying non-returning functions. |
| Defines C string handling functions. | |
| C99 | Defines type-generic mathematical functions. |
| C11 | Defines functions for managing multiple threads as well as mutexes and condition variables. |
| Defines date and time handling functions | |
| C11 | Types and functions for manipulating characters. |
| NA1 | Defines wide string handling functions. |
| NA1 | Defines set of functions used to classify wide characters by their types or to convert between upper and lower case |
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| C 标准函数库 · · · · · · | |
| 特性 | · · · · |
| 延伸的相关编程语言 | · · · |
| C 与其他的编程语言 | C and C++ ( · ) · · |
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