1.前言
重新回来审视单片机的开发，发现C51居然也支持库函数的开发。库函数中支持printf的串口输出函数、memcpy类的内存操作函数、malloc/free类的内存申请与释放函数、sin类的数学计算函数。
库函数的使用不仅可以加快开发效率，还能提供友好的程序跨平台移植特性。
2.库函数
C51库函数的帮助文档可以在Keil4的安装目录下找到，相对地址为：Keil4\C51\hlp\c51.chm，文档内的Library Reference目录下有以类别划分的库函数（Routines By Category）、有头文件介绍（Include Files）、有单个函数的介绍（Reference）。其中划分的库函数类别主要有：
缓存操作类（Buffer Manipulation Routines）
字符类（Character Routines）
数据转换类（Data Conversion Routines）
数学类（Math Routines）
内存分配类（Memory Allocation Routines）
IO流类（Stream I/O Routines）
字符串类（String Routines）
可变长度参数（Variable Length Argument Routines）
本征库函数（Intrinsic Routines）
其他（Miscellaneous Routines）
3.详细分类
缓存操作类（Buffer Manipulation Routines）
| Routine | Include | Description |
|---|---|---|
| memccpy | string.h | Copies data bytes from one buffer to another until a specified character or specified number of characters has been copied. |
| memchr | string.h | Returns a pointer to the first occurrence of a specified character in a buffer. |
| memcmp | string.h | Compares a given number of characters from two different buffers. |
| memcpy | string.h | Copies a specified number of data bytes from one buffer to another. |
| memmove | string.h | Copies a specified number of data bytes from one buffer to another. This routine is typically used when the two buffers may overlap. |
| memset | string.h | Initializes a specified number of data bytes in a buffer to a specified character value. |
字符类（Character Routines）
| Routine | Include | Description |
|---|---|---|
| isalnum | ctype.h | Tests for an alphanumeric character. |
| isalpha | ctype.h | Tests for an alphabetic character. |
| iscntrl | ctype.h | Tests for a Control character. |
| isdigit | ctype.h | Tests for a decimal digit. |
| isgraph | ctype.h | Tests for a printable character with the exception of space. |
| islower | ctype.h | |
| isprint | ctype.h | Tests for a printable character. |
| ispunct | ctype.h | Tests for a punctuation character. |
| isspace | ctype.h | Tests for a whitespace character. |
| isupper | ctype.h | Tests for an uppercase alphabetic character. |
| isxdigit | ctype.h | Tests for a hexadecimal digit. |
| toascii | ctype.h | Converts a character to an ASCII code. |
| toint | ctype.h | Converts a hexadecimal digit to a decimal value. |
| tolower | ctype.h | Tests a character and converts it to lowercase if it is uppercase. |
| _tolower | ctype.h | Unconditionally converts a character to lowercase. |
| toupper | ctype.h | Tests a character and converts it to uppercase if it is lowercase. |
| _toupper | ctype.h | Unconditionally converts a character to uppercase. |
数据转换类（Data Conversion Routines）
| Routine | Include | Description |
|---|---|---|
| abs | math.h | Generates the absolute value of an integer type. |
| atof | stdlib.h | Converts a string to a float. |
| atoi | stdlib.h | Converts a string to an int. |
| atol | stdlib.h | Converts a string to a long. |
| cabs | math.h | Generates the absolute value of a character type. |
| labs | math.h | Generates the absolute value of a long type. |
| strtod | stdlib.h | Converts a string to a float. |
| strtol | stdlib.h | Converts a string to a long. |
| strtoul | stdlib.h | Converts a string to an unsigned long. |
数学类（Math Routines）
| Routine | Include | Description |
|---|---|---|
| acos | math.h | Calculates the arc cosine of a specified number. |
| asin | math.h | Calculates the arc sine of a specified number. |
| atan | math.h | Calculates the arc tangent of a specified number. |
| atan2 | math.h | Calculates the arc tangent of a fraction. |
| ceil | math.h | Finds the integer ceiling of a specified number. |
| cos | math.h | Calculates the cosine of a specified number. |
| cosh | math.h | Calculates the hyperbolic cosine of a specified number. |
| exp | math.h | Calculates the exponential function of a specified number. |
| fabs | math.h | |
| floor | math.h | Finds the largest integer less than or equal to a specified number. |
| fmod | math.h | Calculates the floating-point remainder. |
| log | math.h | Calculates the natural logarithm of a specified number. |
| log10 | math.h | Calculates the common logarithm of a specified number. |
| modf | math.h | Generates integer and fractional components of a specified number. |
| pow | math.h | Calculates a value raised to a power. |
| rand | stdlib.h | Generates a pseudo random number. |
| sin | math.h | Calculates the sine of a specified number. |
| sinh | math.h | Calculates the hyperbolic sine of a specified number. |
| srand | stdlib.h | Initializes the pseudo random number generator. |
| sqrt | math.h | Calculates the square root of a specified number. |
| tan | math.h | Calculates the tangent of a specified number. |
| tanh | math.h | Calculates the hyperbolic tangent of a specified number. |
| _chkfloat_ | intrins.h | Checks the status of a floating-point number. |
| _crol_ | intrins.h | Rotates an unsigned char left a specified number of bits. |
| _cror_ | intrins.h | Rotates an unsigned char right a specified number of bits. |
| _irol_ | intrins.h | Rotates an unsigned int left a specified number of bits. |
| _iror_ | intrins.h | Rotates an unsigned int right a specified number of bits. |
| _lrol_ | intrins.h | Rotates an unsigned long left a specified number of bits. |
| _lror_ | intrins.h | Rotates an unsigned long right a specified number of bits. |
内存分配类（Memory Allocation Routines）
| Routine | Include | Description |
|---|---|---|
| calloc | stdlib.h | Allocates storage for an array from the memory pool. |
| free | stdlib.h | Frees a memory block that was allocated using calloc, malloc, or realloc. |
| init_mempool | stdlib.h | Initializes the memory location and size of the memory pool. |
| malloc | stdlib.h | Allocates a block from the memory pool. |
| realloc | stdlib.h | Reallocates a block from the memory pool. |
IO流类（Stream I/O Routines）
| Routine | Include | Description |
|---|---|---|
| getchar | stdio.h | Reads and echoes a character using the _getkey and putchar routines. |
| _getkey | stdio.h | Reads a character using the 8051 serial interface. |
| gets | stdio.h | Reads and echoes a character string using the getchar routine. |
| printf | stdio.h | Writes formatted data using the putchar routine. |
| putchar | stdio.h | Writes a character using the 8051 serial interface. |
| puts | stdio.h | Writes a character string and newline ('\n') character using the putchar routine. |
| scanf | stdio.h | Reads formatted data using the getchar routine. |
| sprintf | stdio.h | Writes formatted data to a string. |
| sscanf | stdio.h | Reads formatted data from a string. |
| ungetchar | stdio.h | Places a character back into the getchar input buffer. |
| vprintf | stdio.h | Writes formatted data using the putchar function. |
| vsprintf | stdio.h | Writes formatted data to a string. |
字符串类（String Routines）
| Routine | Include | Description |
|---|---|---|
| strcat | string.h | Concatenates two strings. |
| strchr | string.h | Returns a pointer to the first occurrence of a specified character in a string. |
| strcmp | string.h | Compares two strings. |
| strcpy | string.h | Copies one string to another. |
| strcspn | string.h | Returns the index of the first character in a string that matches any character in a second string. |
| strlen | string.h | Returns the length of a string. |
| strncat | string.h | Concatenates up to a specified number of characters from one string to another. |
| strncmp | string.h | Compares two strings up to a specified number of characters. |
| strncpy | string.h | Copies up to a specified number of characters from one string to another. |
| strpbrk | string.h | Returns a pointer to the first character in a string that matches any character in a second string. |
| strpos | string.h | Returns the index of the first occurrence of a specified character in a string. |
| strrchr | string.h | Returns a pointer to the last occurrence of a specified character in a string. |
| strrpbrk | string.h | Returns a pointer to the last character in a string that matches any character in a second string. |
| strrpos | string.h | Returns the index of the last occurrence of a specified character in a string. |
| strspn | string.h | Returns the index of the first character in a string that does not match any character in a second string. |
| strstr | string.h | Returns a pointer in a string that is identical to a second sub-string. |
可变长度参数（Variable Length Argument Routines）
| Routine | Include | Description |
|---|---|---|
| va_arg | stdarg.h | Retrieves an argument from an argument list. |
| va_end | stdarg.h | Resets an argument pointer. |
| va_start | stdarg.h | Sets a pointer to the beginning of an argument list. |
本征库函数（Intrinsic Routines）
| Routine | Include | Description |
|---|---|---|
| _chkfloat_ | intrins.h | Checks the status of a floating-point number. |
| _crol_ | intrins.h | Rotates an unsigned char left a specified number of bits. |
| _cror_ | intrins.h | Rotates an unsigned char right a specified number of bits. |
| _irol_ | intrins.h | Rotates an unsigned int left a specified number of bits. |
| _iror_ | intrins.h | Rotates an unsigned int right a specified number of bits. |
| _nop_ | intrins.h | Executes one NOP instruction. |
| _pop_ | intrins.h | Pops an SFR from the hardware stack using the POP instruction. |
| _push_ | intrins.h | Pushes an SFR onto the hardware stack using the PUSH instruction. |
| _testbit_ | intrins.h | Tests the value of a bit and clears it to 0. |
其他（Miscellaneous Routines）
| Routine | Include | Description |
|---|---|---|
| setjmp | setjump.h | Saves the current stack condition and program address. |
| longjmp | setjump.h | Restores the stack condition and program address. |
| _nop_ | intrins.h | Inserts an 8051 NOP instruction. |
| _testbit_ | intrins.h | Tests the value of a bit and clears it to 0. |
4.例程
Keil4为C51提供了较为丰富的例程，例程在Keil的安装目录下，相对地址为：Keil4\C51\Examples。