C 库函数

C 中的标准函数库是一个巨大的子库库，每个子库都包含多个函数的代码。为了利用这些库，通过使用头文件链接更广泛的库中的每个库。这些函数的定义存在于它们各自的头文件中。为了使用这些函数，我们必须在程序中包含头文件。下面是一些带有描述的头文件：

S No.	Header Files	Description
1		It checks the value of an expression that we expect to be true under normal circumstances. If the expression is a nonzero value, the assert macro does nothing.
2		A set of functions for manipulating complex numbers.
3		Defines macro constants specifying the implementation-specific properties of the floating-point library.
4		These limits specify that a variable cannot store any value beyond these limits, for example- An unsigned character can store up to a maximum value of 255.
5		The math.h header defines various mathematical functions and one macro. All the Functions  in this library take double as an argument and return double as the result.
6		The stdio.h header defines three variable types, several macros, and various function for performing input and output.
7		Defines date and time handling functions.
8		Strings are defined as an array of characters. The difference between a character array  and a string is that a string is terminated with a special character ‘\0’.

实现：让我们用一个 C 程序来讨论基本库的实现：

1. stdio.h ：该库用于使用printf()函数，程序中应包含头文件。下面是实现上述方法的 C 程序：

C

// C program to implement
// the above approach
#include 
 
// Driver code
int main()
{
    printf("GEEKS FOR GEEKS");
    return 0;
}

输出

GEEKS FOR GEEKS

注意：如果使用 printf()函数而不包含头文件 ，则会显示错误。

2. math.h –要执行任何与数学相关的操作，必须包含 math.h 头文件。

示例 1： sqrt()

句法-

double sqrt(double x)

下面是计算任意数平方根的 C 程序：

C

// C program to implement
// the above approach
#include 
#include 
 
// Driver code
int main()
{
    double number, squareRoot;
 
    number = 12.5;
 
    // Computing the square root
    squareRoot = sqrt(number);
 
    printf("Square root of %.2lf =  %.2lf",
           number, squareRoot);
    return 0;
}

输出

Square root of 12.50 =  3.54

示例 2- pow() ：

句法：

double pow(double x, double y)

下面是计算任意数幂的 C 程序：

C

// C program to implement
// the above approach
#include 
#include 
 
// Driver code
int main()
{
    double base, power, result;
    base = 10.0;
    power = 2.0;
 
    // Calculate the result
    result = pow(base, power);
    printf("%.1lf^%.1lf = %.2lf",
           base, power, result);
    return 0;
}

输出

10.0^2.0 = 100.00

示例 3- sin() ：

句法：

double sin(double x)

下面是计算参数正弦的 C 程序：

C

// C program to implement
// the above approach
#include 
#include 
 
// Driver code
int main()
{
    double x;
    double result;
    x = 2.3;
    result = sin(x);
    printf("sin(%.2lf) = %.2lf\n",
           x, result);
    x = -2.3;
    result = sin(x);
    printf("sin(%.2lf) = %.2lf\n",
           x, result);
    x = 0;
    result = sin(x);
    printf("sin(%.2lf) = %.2lf\n",
           x, result);
    return 0;
}

输出

sin(2.30) = 0.75
sin(-2.30) = -0.75
sin(0.00) = 0.00

示例 4- cos() ：

句法：

double cos(double x);

下面是计算参数余弦的 C 程序：

C

// C program to implement
// the above approach
#include 
#include 
#define PI 3.141592654
 
// Driver code
int main()
{
    double arg = 30, result;
 
    // Converting to radian
    arg = (arg * PI) / 180;
 
    result = cos(arg);
    printf("cos of %.2lf radian = %.2lf",
           arg, result);
    return 0;
}

输出

cos of 0.52 radian = 0.87

示例 5- tan() ：

句法：

double tan(double x);

下面是计算参数正切的 C 程序：

C

// C program to implement
// the above approach
#include 
#include 
 
// Driver code
int main()
{
    double x;
    double result;
    x = 2.3;
    result = tan(x);
    printf("tan(%.2lf) = %.2lf\n",
           x, result);
    x = -2.3;
    result = tan(x);
    printf("tan(%.2lf) = %.2lf\n",
           x, result);
    return 0;
}

输出

tan(2.30) = -1.12
tan(-2.30) = 1.12

示例 6- log() ：

句法-

double log( double arg );

下面是计算参数的自然对数的 C 程序 -

C

// C program to implement
// the above approach
#include 
#include 
 
// Driver code
int main()
{
    double num = 5.6, result;
    result = log(num);
    printf("log(%.1f) = %.2f",
           num, result);
    return 0;
}

输出

log(5.6) = 1.72

3. float.h ： C 标准库的 float.h 头文件包含一组与浮点值相关的各种平台相关常量。下面是实现上述方法的 C 程序 -

C

// C program to implement
// the above approach
#include 
#include 
 
// Driver code
int main()
{
    printf("Maximum value of float = %.10e\n",
           FLT_MAX);
    printf("Minimum value of float = %.10e\n",
           FLT_MIN);
}

输出

Maximum value of float = 3.4028234664e+38
Minimum value of float = 1.1754943508e-38

4. limits.h中：该头limits.h中确定的各种变量类型的各种属性。此标头中定义的宏限制了各种变量类型（如 char、int 和 long）的值。下面是实现上述方法的 C 程序 -

C

// C program to implement
// the above approach
#include 
#include 
 
// Driver code
int main()
{
    printf("Number of bits in a byte %d\n",
           CHAR_BIT);
    printf("Minimum value of SIGNED CHAR = %d\n",
           SCHAR_MIN);
    printf("Maximum value of SIGNED CHAR = %d\n",
           SCHAR_MAX);
    printf("Maximum value of UNSIGNED CHAR = %d\n",
           UCHAR_MAX);
    printf("Minimum value of SHORT INT = %d\n",
           SHRT_MIN);
    printf("Maximum value of SHORT INT = %d\n",
           SHRT_MAX);
    printf("Minimum value of INT = %d\n",
           INT_MIN);
    printf("Maximum value of INT = %d\n",
           INT_MAX);
    printf("Minimum value of CHAR = %d\n",
           CHAR_MIN);
    printf("Maximum value of CHAR = %d\n",
           CHAR_MAX);
    printf("Minimum value of LONG = %ld\n",
           LONG_MIN);
    printf("Maximum value of LONG = %ld\n",
           LONG_MAX);
    return (0);
}

输出

Number of bits in a byte 8
Minimum value of SIGNED CHAR = -128
Maximum value of SIGNED CHAR = 127
Maximum value of UNSIGNED CHAR = 255
Minimum value of SHORT INT = -32768
Maximum value of SHORT INT = 32767
Minimum value of INT = -2147483648
Maximum value of INT = 2147483647
Minimum value of CHAR = -128
Maximum value of CHAR = 127
Minimum value of LONG = -9223372036854775808
Maximum value of LONG = 9223372036854775807

5. time.h ：这个头文件定义了日期和时间函数。下面是实现 time() 和 localtime() 函数的 C 程序-

C

// C program to implement
// the above approach
#include 
#include 
#define SIZE 256
 
// Driver code
int main(void)
{
    char buffer[SIZE];
    time_t curtime;
    struct tm* loctime;
 
    // Get the current time.
    curtime = time(NULL);
 
    // Convert it to local time
    // representation.
    loctime = localtime(&curtime);
 
    // Print out the date and time
    // in the standard format.
    fputs(asctime(loctime), stdout);
 
    // Print it out
    strftime(buffer, SIZE,
             "Today is %A, %B %d.\n",
             loctime);
 
    fputs(buffer, stdout);
    strftime(buffer, SIZE,
             "The time is %I:%M %p.\n",
             loctime);
    fputs(buffer, stdout);
    return 0;
}

输出

Sun May 30 17:27:47 2021
Today is Sunday, May 30.
The time is 05:27 PM.

6. 字符串.h ：为了使用字符串函数，需要在程序中包含字符串.h 头文件。

示例 1： strcat() ：在 C 编程中， strcat() 函数用于连接（连接）两个字符串。此函数连接目标字符串和源字符串，并将结果存储在目标字符串。

句法-

char *strcat(char *destination, const char *source)

下面是实现 strcat() 的 C 程序：

C

// C program to implement
// the above approach
#include 
#include 
 
// Driver code
int main()
{
    char str1[100] = "Geeks ",
         str2[100] = " For Geeks";
 
    // Concatenates str1 and str2
    strcat(str1, str2);
 
    // Resultant string is stored
    // in str1
    puts(str1);
 
    return 0;
}

输出

Geeks  For Geeks

示例 2- strcmp() ：它比较两个字符串。如果返回值为 0，则字符串相等；如果返回值为非零，则字符串不相等。

句法：

int strcmp (const char* str1, const char* str2);

下面是实现 strcmp() 的 C 程序：

C

// C program to implement
// the above approach
#include 
#include 
 
// Driver code
int main()
{
    char str1[] = "Geeks",
         str2[] = "gEeks",
         str3[] = "Geeks";
    int result;
 
    // Comparing strings str1
    // and str2
    result = strcmp(str1, str2);
 
    printf("strcmp(str1, str2) = %d\n",
           result);
 
    // Comparing strings str1 and str3
    result = strcmp(str1, str3);
 
    printf("strcmp(str1, str3) = %d\n",
           result);
    return 0;
}

输出

strcmp(str1, str2) = -32
strcmp(str1, str3) = 0

示例 3 – strcpy() ： strcpy()函数将源指向的字符串复制到目标。

句法：

char* strcpy(char* destination, const char* source);

下面是实现 strcpy() 的 C 程序：

C

// C program to implement
// the above approach
#include 
#include 
 
// Driver code
int main()
{
    char str1[20] = "Geeks For Geeks";
    char str2[20];
 
    // Copying str1 to str2
    strcpy(str2, str1);
 
    puts(str2);
    return 0;
}

输出

Geeks For Geeks

示例 4 – strlen() ：此函数计算给定字符串的长度。

句法：

int strlen(char a[]);

下面是实现 strlen() 的 C 程序：

C

// C program to implement
// the above approach
#include 
#include 
 
// Driver code
int main()
{
    char a[20] = "Program";
    char b[20] = { "Geeks for Geeks" };
    printf("Length of string a = %zu \n",
           strlen(a));
    printf("Length of string b = %zu \n",
           strlen(b));
    return 0;
}

输出

Length of string a = 7 
Length of string b = 15 

7. complex.h ：该头文件中的函数用于对复数进行各种操作。复数是具有实部和虚部的复数。

下面是实现复数共轭的 C 程序 -

C

// C program to implement
// the above approach
#include 
#include 
 
// Driver code
int main(void)
{
    double real = 1.3,
           imag = 4.9;
    double complex z = CMPLX(real,
                             imag);
    double complex conj_f = conjf(z);
    printf("z = %.1f% + .1fi\n",
           creal(conj_f),
           cimag(conj_f));
}

输出：

z = 1.3 - 4.9i

8. assert.h ：断言是用于测试程序员所做假设的语句。例如，我们可以使用断言来检查 malloc() 返回的指针是否为 NULL。

句法-

void assert(int expression); 

C++

// C program to implement
// the above approach
#include 
#include 
 
// Driver code
int main()
{
    int x = 7;
 
    // Some big code in between
    // and let's say x is accidentally
    // changed to 9
    x = 9;
 
    // Programmer assumes x to be 7
    // in rest of the code
    assert(x == 7);
 
    // Rest of the code
 
    return 0;
}

输出

Assertion failed: x==7, file test.cpp, line 13 
This application has requested the Runtime to terminate it in an unusual 
way. Please contact the application's support team for more information.

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