前端学习C语言 - 开篇

2023/6/10 1:22:29

编程Tag： c语言

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前端学习C语言 - 开篇

前端学习C语言有很多理由：工作、兴趣或其他。

C 语言几个常见的使用场景：

操作系统开发：Linux 操作系统的内核就是主要由 C 语言编写的。其他操作系统也广泛使用 C 语言进行核心部分的开发。
系统级开发和嵌入式编程：C 语言具有强大的底层控制能力和高效的代码执行效率，非常适合进行系统级开发和嵌入式编程。
游戏开发和图形处理：许多游戏引擎和各种图形应用程序都是使用 C 语言进行开发的。这是因为 C 语言相对于其他高级语言具有更好的性能和更直接的硬件控制能力，可以实现复杂的计算和高性能的图形处理。
数据库系统开发：C 语言的高效性使得其在数据库系统的内部组件中使用非常广泛。例如，Berkeley DB、MySQL、PostgreSQL 等数据库管理系统都是使用 C 语言进行开发的。
底层驱动开发：由于 C 语言直接操作内存和硬件资源，可以用来编写驱动程序、设备控制器和微控制器的固件等低级别的代码。
单片机（跟硬件打交道）

C 语言还可以用于许多其他方面的开发，包括网络编程、信号处理、人工智能等。

第一个程序

在 linux 中创建一个 .c 文件：

pjl@pjl-pc:~/$ cat hello.c
#include <stdio.h> // 头文件。提供了 I/O 功能，例如下面用的 printf() 函数

// int 函数返回值的类型
// main 入口函数，通常有且只有一个。没有 main 在 linux 中也可以编译通过，比如 gcc -nostartfiles 
int main(void) {
    // 必须加分号，否则编译会报错
    printf("Hello World\n");
    return 0;
}

编译

这里使用 gcc 来编译 c。

Tip：gcc 代表GNU Compiler Collection，是一个开源的编译器集合，由GNU项目开发。它主要用于编译C和C++等编程语言的源代码，并生成可执行程序

然后安装 gcc:

// 更新源
pjl@pjl-pc:~/$ sudo apt update
// 安装 gcc
pjl@pjl-pc:~/$ sudo apt-get install gcc

通过 gcc -v 确认以安装：

pjl@pjl-pc:~/$ gcc -v
Using built-in specs.
COLLECT_GCC=gcc
COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-linux-gnu/9/lto-wrapper
OFFLOAD_TARGET_NAMES=nvptx-none:hsa
OFFLOAD_TARGET_DEFAULT=1
Target: x86_64-linux-gnu
....
gcc version 9.3.0 (Ubuntu 9.3.0-10kylin2)

编译 hello.c 生成可执行文件 hello，通常没有扩展名，直接运行可执行文件，输出 Hello World。就像这样:

pjl@pjl-pc:~/$ gcc hello.c -o hello

pjl@pjl-pc:~/$ ls
hello  hello.c  stdio.h

// 直接运行
pjl@pjl-pc:~/$ ./hello
Hello World

例如故意去掉 printf() 函数后的分号，编译就会报错。就像这样：

pjl@pjl-pc:~/$ gcc hello.c -o hello
hello.c: In function ‘main’:
hello.c:4:28: error: expected ‘;’ before ‘return’
    4 |     printf("Hello World\n")
      |                            ^
      |                            ;
    5 |     return 0;
      |     ~~~~~~

Tip: 笔者找了两款在线编写 c 的环境，直接点击运行即可查看结果。语法错误也无需等待编译后查看。：

lightly - 点击在线使用，使用微信扫一下即可登录，通过云能保存自己的项目。
dotcpp - 运行一次需要等20秒，可能为了缓解服务器压力。

头文件

<stdio.h> 是 C 标准库的头文件之一，包含了许多与标准输入输出流（stdin、stdout 和 stderr）相关的函数和常量定义。

常见的函数包括：

printf() 和 fprintf()：格式化输出函数，可以将内容输出到终端或文件中；
scanf() 和 fscanf()：格式化读取函数，可以从终端或文件中读取输入数据；
fgets() 和 fputs()：以行为单位进行读写，可以读写字符串；
fgetc() 和 fputc()：以字符为单位进行读写；
fseek() 和 ftell()：文件定位函数，用于在文件中移动位置以及得到文件当前位置。
此外，<stdio.h> 头文件还包含许多输入和输出流相关的常量和宏定义，例如常见的 stdin、stdout、stderr 流对象指针等等。

总之，<stdio.h> 是 C 语言程序中使用最频繁的头文件之一，提供了强大的 I/O 功能支持，方便程序员完成输入输出操作。

在 Linux 系统中，<stdio.h> 一般存放在 /usr/include 目录下

pjl@pjl-pc:~/$ sudo find / -name stdio.h
输入密码
find: ‘/run/user/1000/gvfs’: 权限不够
/opt/linux-stable/tools/include/nolibc/stdio.h
/opt/linux-stable/arch/powerpc/boot/stdio.h
// stdio.h 就是这个
/usr/include/stdio.h
/usr/include/x86_64-linux-gnu/bits/stdio.h
/usr/share/python3-pycparser/fake_libc_include/stdio.h

基本数据类型

要存储数据，就得申请内存，就得告诉操作系统要申请多少字节，于是 c 语言就是通过不同的类型申请内存。

基本数据类型有：

空类型 void，常用于函数返回值，或指针，不会用于变量
字符型
数值型
  实型 - 表示小数
    单精度 float   精确到小数点6位
    双精度 doubble 精确到小数点15位，表示的数也更大
  整型
    短整型
        有符号短整型
        无符号短整型
    整型
        有符号整型
        无符号整型
    长整型
        有符号长整型
        无符号长整型

重点说一下整型。其中无符号整数类型（unsigned int）只能表示非负整数，有符号整型则可以表示正数和负数。

# 无符号
unsigned int variable_name;

# 有符号
int variable_name;

类型所占字节

字节是内存中最基本单元。

每个字节由8位组成，每一位将只有两种可能的状态：1或0。因此，一个字节可以表示256个不同的值（2^8=256），包括从0到255的所有数字。

Tip：由于计算机芯片上只有两个状态，通常为高电平和低电平，因此计算机只能处理0和1。虽然二进制数系统看起来比较简单，但它可以用来表示任意数字、字符和其他类型的数据。例如，整数和浮点数可以用二进制数表示，字符串可以将字符转换为二进制代码并存储在内存中。因此，虽然计算机只能使用0和1，但它们可以处理和存储各种不同类型的数据。

在C语言中，通常使用16进制表示内存地址，比如 0x100、0x101。把每个字节比作一个房间，内存中有太多房间，16进制比10进制数字看起来要小点，比如同样是数字10，16进制就能表示16个房间。

十进制
0 1 2 3 4 5 6 7 8 9 在加1 变成10

二进制
0 1 在加1 变成10

八进制
0 1 2 3 4 5 6 7 在加1 变成10

十六进制
0 1 2 3 4 5 6 7 8 9 A B C D E F 在加1 10

比如 long 在 64位系统中占8个字节，在32位系统中占4个字节。可以通过 sizeof 计算每种类型在系统中所占字节数。就像这样：

// 代码
pjl@pjl-pc:~/$ cat demo-4.c

#include <stdio.h>

int main() {
    printf("char类型所占字节数为：%zu\n", sizeof(char));
    printf("short类型所占字节数为：%zu\n", sizeof(short));
    printf("int类型所占字节数为：%zu\n", sizeof(int));
    printf("long类型所占字节数为：%zu\n", sizeof(long));
    printf("float类型所占字节数为：%zu\n", sizeof(float));
    printf("double类型所占字节数为：%zu\n", sizeof(double));
    printf("long long类型所占字节数为：%zu\n", sizeof(long long));
    return 0;
}
// 编译
pjl@pjl-pc:~/$ gcc demo-4.c -o demo-4

// 运行可执行文件
pjl@pjl-pc:~/$ ./demo-4
char类型所占字节数为：1
short类型所占字节数为：2
int类型所占字节数为：4
long类型所占字节数为：8
float类型所占字节数为：4
double类型所占字节数为：8
long long类型所占字节数为：8

Tip: sizeof 是一个关键字，虽然用法和函数类似，如果sizeof()是函数，那么 sizeof() 中的 a++ 就会执行，将结果作为实参传入 sizeof，那么结果应该是 12，然而结果是 11。sizeof(a++) 等同于 sizeof(int)。

#include <stdio.h>

int main(void) {
    int a = 10;
    a++;
    printf("%lu\n", sizeof(a++));
    printf("%d\n", a);
    return 0;
}
/*
4
11
*/

类型的范围

我们首先详细分析char能表示的范围，其他类型计算的方法也类似。

char

char，占1个字节，也就是8位。

无符号，最小是8位都是0，也就是0，最大就是8位全是1，也就是255。计算过程如下：

// 8位都是1
1111 1111

// 加1得到 2^8 - 1，即255
+ 1

1 0000 0000 = 2^8 = Math.pow(2, 8) = 256 

256 - 1 = 255

Tip: 比如二进制 1000 就是 Math.pow(2, 3)，也就是8。

无符合 char 的范围是 0 ~ 255，共 256 种。

有符号，最高位（即最左侧）0 表示正数，1 表示负数。正数则是 0111 1111，也就是 127。

// 0 表示正数
0111 1111

+1 

1000 0000 = 2^7 = 128

128 - 1 = 127

负数则是 1000 0000，而不是 1111 1111（通常我们认为负的值越大，就越小）。假如是后者，有符号的范围就是 -127 ~ 127，共255中可能，而无符号有256中可能。

1111 1111

// 只看这7位
 111 1111

+1

1000 0000 = 2^7 = 128

128 - 1 = 127

少的一个在这里，一个正0，一个负0，数学中没有这个说法，所以在计算机中是这么规定的，最小数是 1000 0000，也就是 -128。

有符合 char 的范围是 -128 ~ 127，也是 256 种。

有符号整型

singed char                      1                     -2^7(-128) ~ 2^7-1(127)
short int 或 short               2                     -2^15 ~ 2^15 -1
int                              4                     -2^31 ~ 2^31 -1
long 或 long int                 4（32位） 8（64位）    -2^31 ~ 2^31 -1
long long 或 long long int       8                     -2^63 ~ 2^63 -1

Tip: short 和 short int 等价；long 和 long int 等价；long long 和 long long int 等价。

无符号整型

unsinged char              1                     0~2^8 -1 (255)
unsinged short             2                     0~2^16 -1(65535)
unsinged int               4                     0~2^32 -1(4294 9672 95)
unsinged long              4（32位） 8（64位）    0~2^32 -1
unsinged long long         8                     0~2^64 -1

浮点

float 是4个字节，32位，范围是 -3.40282347e+38 ~ 3.40282347e+38。最大有38个数。

unsinged int 同样也是 4个字节，范围却只有 10个数。

同样是 4 个字节，为何 float 能表示的数更大？

这是因为它的表示方法不一样：最左侧 1 位表示符号，中间 8 位8 位表示指数，最后 23 位为尾数。符号位用来表示数字的正负，指数位则表示小数点在有效数字中向左或向右移动多少位（即阶码），而尾数位则包含实际的有效数字。

单精度浮点（float）数采用了分数形式的科学计数法（即尾数和指数分别表示大数整数部分和小数部分的方式）来表示实数，但由于单精度浮点数的尾数只有 23 位，而指数只有 8 位，因此这种近似值可能会存在精度误差，导致与原来的值略有不同。

双精度浮点数（double）和单精度 float 一样由三部分组成。双精度浮点数使用更多的位数表示指数和尾数，因此可以表示更广阔范围的实数，并具有更高的精度。

长双精度浮点数（long double）也由三部分组成：符号位、指数和尾数。长双精度浮点数的规格并没有在 IEEE 754 中具体规定。因此这一点取决于不同实现及其所用硬件平台的架构。在 x86 架构下，一般将 long double 定义为 80 位或者 128 位，而在 ARM 架构下则分别将其定义为 96 位和 128 位。通常情况下它的存储长度要比双精度浮点数更长，同时在精度方面也有所提升。

练习

char的计算

#include <stdio.h>

int main(void) {
    char a = -128;
    char num = a -1;
    printf("%d \n", num);
}

答案是 127。过程如下：

num 是 char 类型，范围是 -128 ~ 127，所以不可能是 -129
计算机中，有许多处理负数的方式，其中补码是最常见和普遍的一种。
-128 源码是 1000 0000，取反码 1111 1111（标志位不动，其他取反。0变1,1变0），反码加1得补码 1000 0000

源码 1000 0000
反码 1111 1111
补码 1 1000 0000 —— 由于只有七位，删除最高位（1000 0000）得到 000 0000
     1000 0000

-1 的补码是 1111 1111:

源码 1000 0001
反码 1111 1110
补码 1111 1111

两个补码相加得 0111 1111，也就是 127

1000 0000
1111 1111
1 0111 1111 —— 只有8位，删除最高位
0111 1111

有符号转无符号

pjl@pjl-pc:~/$ cat demo-3.c
#include <stdio.h>

int main(void) {
   int i = -20;
   unsigned int j = 10;
   if(i + j > 0){
       printf("i + j > 0\n");
   }
   printf("%u\n", i + j);

}

结果是：4294967286。

pjl@pjl-pc:~/$ gcc demo-3.c -o demo-3
pjl@pjl-pc:~/$ ./demo-3
i + j > 0
4294967286

解答过程：i是有符号int，j是无符号int，需要将有符号转为无符号。i是负数，得转为补码，最高位就不再表示符号，需要参与计算，补码转为十进制是 4294967276，在加上 10 就是结果。

-20 

源码 10000000 00000000 00000000 00010100
反码 11111111 11111111 11111111 11101011
补码 11111111 11111111 11111111 11101100

现在最高位也得参与运算，打开 windows 的计算器，从`查看`进入`程序员`，选择`二进制`，将补码粘贴，在点击`十进制`，得到4294967276

Tip：类型不同，比如 char + int，把1个字节的 char 转为 4 个字节的 int 很好理解。就是范围小的转为范围大的。比如有符号char 的范围是 -128~127，无符号 char 的范围是 0~255，计算机认为无符号的255比有符号的127更大!？

int的计算

#include <stdio.h>

int main(void) {
   unsigned int a = 3;
   printf("%u\n", -1 * a);
}

结果是 4294967293。

结果分析：

-1 转为补码，补码对应的十进制：4294967295
4294967295*3 结果12884901885，12884901885的二进制是 1011 11111111 11111111 11111111 11111101
int 就4个字节，也就是32位，这里是36位，超出int。删除最高4位，得到 11111111 11111111 11111111 11111101，转为十进制就是 4294967293。

-1

原码：10000000 00000000 00000000 00000001
反码：11111111 11111111 11111111 11111110
补码：11111111 11111111 11111111 11111111

补码对应的十进制：4294967295

4294967295*3

1011 11111111 11111111 11111111 11111101

删除高4位，值保留int的32位

11111111 11111111 11111111 11111101

小数的二进制表示

小数1.5625的二进制表示？

答案：1.1001。步骤如下：

首先，将1.5625转化为二进制整数部分1。

然后，将小数部分0.5625乘以2，得到1.125。将其整数部分1添加到二进制小数部分的末尾，得到1.1。

继续将小数部分0.125乘以2，得到0.25。将其整数部分0添加到二进制小数部分的末尾，得到1.10。

继续将小数部分0.25乘以2，得到0.50。将其整数部分0添加到二进制小数部分的末尾，得到1.100。

最后，将小数部分0.50继续乘以2，得到1.00。将其整数部分1添加到二进制小数部分的末尾，得到1.1001。

因此，1.5625的二进制表示为1.1001

扩展

计算机如何把人类理解的信息转为0和1

将人类理解的信息转换成0和1，需要使用编码方式来表示不同类型的信息。这个过程通常称为数字化。

例如，对于文字信息，可以使用ASCII编码或Unicode编码将每个字符映射到一个二进制数字。比如字母“A”在ASCII编码中对应的二进制数字是01000001，字母“B”对应的是01000010等等。通过这种方式，计算机可以识别并处理文本信息。

对于图像和音频等复杂的信息，数字化的过程更为复杂，需要使用更加高级的编码方式和算法来实现。例如，JPEG、PNG等格式用于存储图片，MP3、AAC等格式用于存储音频。这些格式都会将图像和音频分割成小块并使用压缩算法来减小文件大小，以节省存储空间和传输带宽，并且保持足够的质量以满足人类对图像和声音的要求。

总之，将人类理解的信息转换成0和1，是通过使用不同的编码方式来表示数据，从而使计算机能够理解和处理这些数据。

stdio.h 头文件

将该文件导出：

pjl@pjl-pc:~/$ cat /usr/include/stdio.h > stdio.h

stdio.h 文件一共 875 行，完整内容如下，例如 printf 等函数就在里面：

/* Define ISO C stdio on top of C++ iostreams.
   Copyright (C) 1991-2020 Free Software Foundation, Inc.
   This file is part of the GNU C Library.

   The GNU C Library is free software; you can redistribute it and/or
   modify it under the terms of the GNU Lesser General Public
   License as published by the Free Software Foundation; either
   version 2.1 of the License, or (at your option) any later version.

   The GNU C Library is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
   Lesser General Public License for more details.

   You should have received a copy of the GNU Lesser General Public
   License along with the GNU C Library; if not, see
   <https://www.gnu.org/licenses/>.  */

/*
 *	ISO C99 Standard: 7.19 Input/output	<stdio.h>
 */

#ifndef _STDIO_H
#define _STDIO_H	1

#define __GLIBC_INTERNAL_STARTING_HEADER_IMPLEMENTATION
#include <bits/libc-header-start.h>

__BEGIN_DECLS

#define __need_size_t
#define __need_NULL
#include <stddef.h>

#define __need___va_list
#include <stdarg.h>

#include <bits/types.h>
#include <bits/types/__fpos_t.h>
#include <bits/types/__fpos64_t.h>
#include <bits/types/__FILE.h>
#include <bits/types/FILE.h>
#include <bits/types/struct_FILE.h>

#ifdef __USE_GNU
# include <bits/types/cookie_io_functions_t.h>
#endif

#if defined __USE_XOPEN || defined __USE_XOPEN2K8
# ifdef __GNUC__
#  ifndef _VA_LIST_DEFINED
typedef __gnuc_va_list va_list;
#   define _VA_LIST_DEFINED
#  endif
# else
#  include <stdarg.h>
# endif
#endif

#if defined __USE_UNIX98 || defined __USE_XOPEN2K
# ifndef __off_t_defined
# ifndef __USE_FILE_OFFSET64
typedef __off_t off_t;
# else
typedef __off64_t off_t;
# endif
# define __off_t_defined
# endif
# if defined __USE_LARGEFILE64 && !defined __off64_t_defined
typedef __off64_t off64_t;
# define __off64_t_defined
# endif
#endif

#ifdef __USE_XOPEN2K8
# ifndef __ssize_t_defined
typedef __ssize_t ssize_t;
# define __ssize_t_defined
# endif
#endif

/* The type of the second argument to `fgetpos' and `fsetpos'.  */
#ifndef __USE_FILE_OFFSET64
typedef __fpos_t fpos_t;
#else
typedef __fpos64_t fpos_t;
#endif
#ifdef __USE_LARGEFILE64
typedef __fpos64_t fpos64_t;
#endif

/* The possibilities for the third argument to `setvbuf'.  */
#define _IOFBF 0		/* Fully buffered.  */
#define _IOLBF 1		/* Line buffered.  */
#define _IONBF 2		/* No buffering.  */


/* Default buffer size.  */
#define BUFSIZ 8192


/* The value returned by fgetc and similar functions to indicate the
   end of the file.  */
#define EOF (-1)


/* The possibilities for the third argument to `fseek'.
   These values should not be changed.  */
#define SEEK_SET	0	/* Seek from beginning of file.  */
#define SEEK_CUR	1	/* Seek from current position.  */
#define SEEK_END	2	/* Seek from end of file.  */
#ifdef __USE_GNU
# define SEEK_DATA	3	/* Seek to next data.  */
# define SEEK_HOLE	4	/* Seek to next hole.  */
#endif


#if defined __USE_MISC || defined __USE_XOPEN
/* Default path prefix for `tempnam' and `tmpnam'.  */
# define P_tmpdir	"/tmp"
#endif


/* Get the values:
   L_tmpnam	How long an array of chars must be to be passed to `tmpnam'.
   TMP_MAX	The minimum number of unique filenames generated by tmpnam
		(and tempnam when it uses tmpnam's name space),
		or tempnam (the two are separate).
   L_ctermid	How long an array to pass to `ctermid'.
   L_cuserid	How long an array to pass to `cuserid'.
   FOPEN_MAX	Minimum number of files that can be open at once.
   FILENAME_MAX	Maximum length of a filename.  */
#include <bits/stdio_lim.h>


/* Standard streams.  */
extern FILE *stdin;		/* Standard input stream.  */
extern FILE *stdout;		/* Standard output stream.  */
extern FILE *stderr;		/* Standard error output stream.  */
/* C89/C99 say they're macros.  Make them happy.  */
#define stdin stdin
#define stdout stdout
#define stderr stderr

/* Remove file FILENAME.  */
extern int remove (const char *__filename) __THROW;
/* Rename file OLD to NEW.  */
extern int rename (const char *__old, const char *__new) __THROW;

#ifdef __USE_ATFILE
/* Rename file OLD relative to OLDFD to NEW relative to NEWFD.  */
extern int renameat (int __oldfd, const char *__old, int __newfd,
		     const char *__new) __THROW;
#endif

#ifdef __USE_GNU
/* Flags for renameat2.  */
# define RENAME_NOREPLACE (1 << 0)
# define RENAME_EXCHANGE (1 << 1)
# define RENAME_WHITEOUT (1 << 2)

/* Rename file OLD relative to OLDFD to NEW relative to NEWFD, with
   additional flags.  */
extern int renameat2 (int __oldfd, const char *__old, int __newfd,
		      const char *__new, unsigned int __flags) __THROW;
#endif

/* Create a temporary file and open it read/write.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
#ifndef __USE_FILE_OFFSET64
extern FILE *tmpfile (void) __wur;
#else
# ifdef __REDIRECT
extern FILE *__REDIRECT (tmpfile, (void), tmpfile64) __wur;
# else
#  define tmpfile tmpfile64
# endif
#endif

#ifdef __USE_LARGEFILE64
extern FILE *tmpfile64 (void) __wur;
#endif

/* Generate a temporary filename.  */
extern char *tmpnam (char *__s) __THROW __wur;

#ifdef __USE_MISC
/* This is the reentrant variant of `tmpnam'.  The only difference is
   that it does not allow S to be NULL.  */
extern char *tmpnam_r (char *__s) __THROW __wur;
#endif


#if defined __USE_MISC || defined __USE_XOPEN
/* Generate a unique temporary filename using up to five characters of PFX
   if it is not NULL.  The directory to put this file in is searched for
   as follows: First the environment variable "TMPDIR" is checked.
   If it contains the name of a writable directory, that directory is used.
   If not and if DIR is not NULL, that value is checked.  If that fails,
   P_tmpdir is tried and finally "/tmp".  The storage for the filename
   is allocated by `malloc'.  */
extern char *tempnam (const char *__dir, const char *__pfx)
     __THROW __attribute_malloc__ __wur;
#endif


/* Close STREAM.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern int fclose (FILE *__stream);
/* Flush STREAM, or all streams if STREAM is NULL.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern int fflush (FILE *__stream);

#ifdef __USE_MISC
/* Faster versions when locking is not required.

   This function is not part of POSIX and therefore no official
   cancellation point.  But due to similarity with an POSIX interface
   or due to the implementation it is a cancellation point and
   therefore not marked with __THROW.  */
extern int fflush_unlocked (FILE *__stream);
#endif

#ifdef __USE_GNU
/* Close all streams.

   This function is not part of POSIX and therefore no official
   cancellation point.  But due to similarity with an POSIX interface
   or due to the implementation it is a cancellation point and
   therefore not marked with __THROW.  */
extern int fcloseall (void);
#endif


#ifndef __USE_FILE_OFFSET64
/* Open a file and create a new stream for it.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern FILE *fopen (const char *__restrict __filename,
		    const char *__restrict __modes) __wur;
/* Open a file, replacing an existing stream with it.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern FILE *freopen (const char *__restrict __filename,
		      const char *__restrict __modes,
		      FILE *__restrict __stream) __wur;
#else
# ifdef __REDIRECT
extern FILE *__REDIRECT (fopen, (const char *__restrict __filename,
				 const char *__restrict __modes), fopen64)
  __wur;
extern FILE *__REDIRECT (freopen, (const char *__restrict __filename,
				   const char *__restrict __modes,
				   FILE *__restrict __stream), freopen64)
  __wur;
# else
#  define fopen fopen64
#  define freopen freopen64
# endif
#endif
#ifdef __USE_LARGEFILE64
extern FILE *fopen64 (const char *__restrict __filename,
		      const char *__restrict __modes) __wur;
extern FILE *freopen64 (const char *__restrict __filename,
			const char *__restrict __modes,
			FILE *__restrict __stream) __wur;
#endif

#ifdef	__USE_POSIX
/* Create a new stream that refers to an existing system file descriptor.  */
extern FILE *fdopen (int __fd, const char *__modes) __THROW __wur;
#endif

#ifdef	__USE_GNU
/* Create a new stream that refers to the given magic cookie,
   and uses the given functions for input and output.  */
extern FILE *fopencookie (void *__restrict __magic_cookie,
			  const char *__restrict __modes,
			  cookie_io_functions_t __io_funcs) __THROW __wur;
#endif

#if defined __USE_XOPEN2K8 || __GLIBC_USE (LIB_EXT2)
/* Create a new stream that refers to a memory buffer.  */
extern FILE *fmemopen (void *__s, size_t __len, const char *__modes)
  __THROW __wur;

/* Open a stream that writes into a malloc'd buffer that is expanded as
   necessary.  *BUFLOC and *SIZELOC are updated with the buffer's location
   and the number of characters written on fflush or fclose.  */
extern FILE *open_memstream (char **__bufloc, size_t *__sizeloc) __THROW __wur;
#endif


/* If BUF is NULL, make STREAM unbuffered.
   Else make it use buffer BUF, of size BUFSIZ.  */
extern void setbuf (FILE *__restrict __stream, char *__restrict __buf) __THROW;
/* Make STREAM use buffering mode MODE.
   If BUF is not NULL, use N bytes of it for buffering;
   else allocate an internal buffer N bytes long.  */
extern int setvbuf (FILE *__restrict __stream, char *__restrict __buf,
		    int __modes, size_t __n) __THROW;

#ifdef	__USE_MISC
/* If BUF is NULL, make STREAM unbuffered.
   Else make it use SIZE bytes of BUF for buffering.  */
extern void setbuffer (FILE *__restrict __stream, char *__restrict __buf,
		       size_t __size) __THROW;

/* Make STREAM line-buffered.  */
extern void setlinebuf (FILE *__stream) __THROW;
#endif


/* Write formatted output to STREAM.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern int fprintf (FILE *__restrict __stream,
		    const char *__restrict __format, ...);
/* Write formatted output to stdout.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern int printf (const char *__restrict __format, ...);
/* Write formatted output to S.  */
extern int sprintf (char *__restrict __s,
		    const char *__restrict __format, ...) __THROWNL;

/* Write formatted output to S from argument list ARG.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern int vfprintf (FILE *__restrict __s, const char *__restrict __format,
		     __gnuc_va_list __arg);
/* Write formatted output to stdout from argument list ARG.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern int vprintf (const char *__restrict __format, __gnuc_va_list __arg);
/* Write formatted output to S from argument list ARG.  */
extern int vsprintf (char *__restrict __s, const char *__restrict __format,
		     __gnuc_va_list __arg) __THROWNL;

#if defined __USE_ISOC99 || defined __USE_UNIX98
/* Maximum chars of output to write in MAXLEN.  */
extern int snprintf (char *__restrict __s, size_t __maxlen,
		     const char *__restrict __format, ...)
     __THROWNL __attribute__ ((__format__ (__printf__, 3, 4)));

extern int vsnprintf (char *__restrict __s, size_t __maxlen,
		      const char *__restrict __format, __gnuc_va_list __arg)
     __THROWNL __attribute__ ((__format__ (__printf__, 3, 0)));
#endif

#if __GLIBC_USE (LIB_EXT2)
/* Write formatted output to a string dynamically allocated with `malloc'.
   Store the address of the string in *PTR.  */
extern int vasprintf (char **__restrict __ptr, const char *__restrict __f,
		      __gnuc_va_list __arg)
     __THROWNL __attribute__ ((__format__ (__printf__, 2, 0))) __wur;
extern int __asprintf (char **__restrict __ptr,
		       const char *__restrict __fmt, ...)
     __THROWNL __attribute__ ((__format__ (__printf__, 2, 3))) __wur;
extern int asprintf (char **__restrict __ptr,
		     const char *__restrict __fmt, ...)
     __THROWNL __attribute__ ((__format__ (__printf__, 2, 3))) __wur;
#endif

#ifdef __USE_XOPEN2K8
/* Write formatted output to a file descriptor.  */
extern int vdprintf (int __fd, const char *__restrict __fmt,
		     __gnuc_va_list __arg)
     __attribute__ ((__format__ (__printf__, 2, 0)));
extern int dprintf (int __fd, const char *__restrict __fmt, ...)
     __attribute__ ((__format__ (__printf__, 2, 3)));
#endif


/* Read formatted input from STREAM.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern int fscanf (FILE *__restrict __stream,
		   const char *__restrict __format, ...) __wur;
/* Read formatted input from stdin.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern int scanf (const char *__restrict __format, ...) __wur;
/* Read formatted input from S.  */
extern int sscanf (const char *__restrict __s,
		   const char *__restrict __format, ...) __THROW;

/* For historical reasons, the C99-compliant versions of the scanf
   functions are at alternative names.  When __LDBL_COMPAT is in
   effect, this is handled in bits/stdio-ldbl.h.  */
#if !__GLIBC_USE (DEPRECATED_SCANF) && !defined __LDBL_COMPAT
# ifdef __REDIRECT
extern int __REDIRECT (fscanf, (FILE *__restrict __stream,
				const char *__restrict __format, ...),
		       __isoc99_fscanf) __wur;
extern int __REDIRECT (scanf, (const char *__restrict __format, ...),
		       __isoc99_scanf) __wur;
extern int __REDIRECT_NTH (sscanf, (const char *__restrict __s,
				    const char *__restrict __format, ...),
			   __isoc99_sscanf);
# else
extern int __isoc99_fscanf (FILE *__restrict __stream,
			    const char *__restrict __format, ...) __wur;
extern int __isoc99_scanf (const char *__restrict __format, ...) __wur;
extern int __isoc99_sscanf (const char *__restrict __s,
			    const char *__restrict __format, ...) __THROW;
#  define fscanf __isoc99_fscanf
#  define scanf __isoc99_scanf
#  define sscanf __isoc99_sscanf
# endif
#endif

#ifdef	__USE_ISOC99
/* Read formatted input from S into argument list ARG.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern int vfscanf (FILE *__restrict __s, const char *__restrict __format,
		    __gnuc_va_list __arg)
     __attribute__ ((__format__ (__scanf__, 2, 0))) __wur;

/* Read formatted input from stdin into argument list ARG.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern int vscanf (const char *__restrict __format, __gnuc_va_list __arg)
     __attribute__ ((__format__ (__scanf__, 1, 0))) __wur;

/* Read formatted input from S into argument list ARG.  */
extern int vsscanf (const char *__restrict __s,
		    const char *__restrict __format, __gnuc_va_list __arg)
     __THROW __attribute__ ((__format__ (__scanf__, 2, 0)));

/* Same redirection as above for the v*scanf family.  */
# if !__GLIBC_USE (DEPRECATED_SCANF)
#  if defined __REDIRECT && !defined __LDBL_COMPAT
extern int __REDIRECT (vfscanf,
		       (FILE *__restrict __s,
			const char *__restrict __format, __gnuc_va_list __arg),
		       __isoc99_vfscanf)
     __attribute__ ((__format__ (__scanf__, 2, 0))) __wur;
extern int __REDIRECT (vscanf, (const char *__restrict __format,
				__gnuc_va_list __arg), __isoc99_vscanf)
     __attribute__ ((__format__ (__scanf__, 1, 0))) __wur;
extern int __REDIRECT_NTH (vsscanf,
			   (const char *__restrict __s,
			    const char *__restrict __format,
			    __gnuc_va_list __arg), __isoc99_vsscanf)
     __attribute__ ((__format__ (__scanf__, 2, 0)));
#  elif !defined __REDIRECT
extern int __isoc99_vfscanf (FILE *__restrict __s,
			     const char *__restrict __format,
			     __gnuc_va_list __arg) __wur;
extern int __isoc99_vscanf (const char *__restrict __format,
			    __gnuc_va_list __arg) __wur;
extern int __isoc99_vsscanf (const char *__restrict __s,
			     const char *__restrict __format,
			     __gnuc_va_list __arg) __THROW;
#   define vfscanf __isoc99_vfscanf
#   define vscanf __isoc99_vscanf
#   define vsscanf __isoc99_vsscanf
#  endif
# endif
#endif /* Use ISO C9x.  */


/* Read a character from STREAM.

   These functions are possible cancellation points and therefore not
   marked with __THROW.  */
extern int fgetc (FILE *__stream);
extern int getc (FILE *__stream);

/* Read a character from stdin.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern int getchar (void);

#ifdef __USE_POSIX199506
/* These are defined in POSIX.1:1996.

   These functions are possible cancellation points and therefore not
   marked with __THROW.  */
extern int getc_unlocked (FILE *__stream);
extern int getchar_unlocked (void);
#endif /* Use POSIX.  */

#ifdef __USE_MISC
/* Faster version when locking is not necessary.

   This function is not part of POSIX and therefore no official
   cancellation point.  But due to similarity with an POSIX interface
   or due to the implementation it is a cancellation point and
   therefore not marked with __THROW.  */
extern int fgetc_unlocked (FILE *__stream);
#endif /* Use MISC.  */


/* Write a character to STREAM.

   These functions are possible cancellation points and therefore not
   marked with __THROW.

   These functions is a possible cancellation point and therefore not
   marked with __THROW.  */
extern int fputc (int __c, FILE *__stream);
extern int putc (int __c, FILE *__stream);

/* Write a character to stdout.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern int putchar (int __c);

#ifdef __USE_MISC
/* Faster version when locking is not necessary.

   This function is not part of POSIX and therefore no official
   cancellation point.  But due to similarity with an POSIX interface
   or due to the implementation it is a cancellation point and
   therefore not marked with __THROW.  */
extern int fputc_unlocked (int __c, FILE *__stream);
#endif /* Use MISC.  */

#ifdef __USE_POSIX199506
/* These are defined in POSIX.1:1996.

   These functions are possible cancellation points and therefore not
   marked with __THROW.  */
extern int putc_unlocked (int __c, FILE *__stream);
extern int putchar_unlocked (int __c);
#endif /* Use POSIX.  */


#if defined __USE_MISC \
    || (defined __USE_XOPEN && !defined __USE_XOPEN2K)
/* Get a word (int) from STREAM.  */
extern int getw (FILE *__stream);

/* Write a word (int) to STREAM.  */
extern int putw (int __w, FILE *__stream);
#endif


/* Get a newline-terminated string of finite length from STREAM.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern char *fgets (char *__restrict __s, int __n, FILE *__restrict __stream)
     __wur;

#if __GLIBC_USE (DEPRECATED_GETS)
/* Get a newline-terminated string from stdin, removing the newline.

   This function is impossible to use safely.  It has been officially
   removed from ISO C11 and ISO C++14, and we have also removed it
   from the _GNU_SOURCE feature list.  It remains available when
   explicitly using an old ISO C, Unix, or POSIX standard.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern char *gets (char *__s) __wur __attribute_deprecated__;
#endif

#ifdef __USE_GNU
/* This function does the same as `fgets' but does not lock the stream.

   This function is not part of POSIX and therefore no official
   cancellation point.  But due to similarity with an POSIX interface
   or due to the implementation it is a cancellation point and
   therefore not marked with __THROW.  */
extern char *fgets_unlocked (char *__restrict __s, int __n,
			     FILE *__restrict __stream) __wur;
#endif


#if defined __USE_XOPEN2K8 || __GLIBC_USE (LIB_EXT2)
/* Read up to (and including) a DELIMITER from STREAM into *LINEPTR
   (and null-terminate it). *LINEPTR is a pointer returned from malloc (or
   NULL), pointing to *N characters of space.  It is realloc'd as
   necessary.  Returns the number of characters read (not including the
   null terminator), or -1 on error or EOF.

   These functions are not part of POSIX and therefore no official
   cancellation point.  But due to similarity with an POSIX interface
   or due to the implementation they are cancellation points and
   therefore not marked with __THROW.  */
extern __ssize_t __getdelim (char **__restrict __lineptr,
                             size_t *__restrict __n, int __delimiter,
                             FILE *__restrict __stream) __wur;
extern __ssize_t getdelim (char **__restrict __lineptr,
                           size_t *__restrict __n, int __delimiter,
                           FILE *__restrict __stream) __wur;

/* Like `getdelim', but reads up to a newline.

   This function is not part of POSIX and therefore no official
   cancellation point.  But due to similarity with an POSIX interface
   or due to the implementation it is a cancellation point and
   therefore not marked with __THROW.  */
extern __ssize_t getline (char **__restrict __lineptr,
                          size_t *__restrict __n,
                          FILE *__restrict __stream) __wur;
#endif


/* Write a string to STREAM.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern int fputs (const char *__restrict __s, FILE *__restrict __stream);

/* Write a string, followed by a newline, to stdout.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern int puts (const char *__s);


/* Push a character back onto the input buffer of STREAM.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern int ungetc (int __c, FILE *__stream);


/* Read chunks of generic data from STREAM.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern size_t fread (void *__restrict __ptr, size_t __size,
		     size_t __n, FILE *__restrict __stream) __wur;
/* Write chunks of generic data to STREAM.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern size_t fwrite (const void *__restrict __ptr, size_t __size,
		      size_t __n, FILE *__restrict __s);

#ifdef __USE_GNU
/* This function does the same as `fputs' but does not lock the stream.

   This function is not part of POSIX and therefore no official
   cancellation point.  But due to similarity with an POSIX interface
   or due to the implementation it is a cancellation point and
   therefore not marked with __THROW.  */
extern int fputs_unlocked (const char *__restrict __s,
			   FILE *__restrict __stream);
#endif

#ifdef __USE_MISC
/* Faster versions when locking is not necessary.

   These functions are not part of POSIX and therefore no official
   cancellation point.  But due to similarity with an POSIX interface
   or due to the implementation they are cancellation points and
   therefore not marked with __THROW.  */
extern size_t fread_unlocked (void *__restrict __ptr, size_t __size,
			      size_t __n, FILE *__restrict __stream) __wur;
extern size_t fwrite_unlocked (const void *__restrict __ptr, size_t __size,
			       size_t __n, FILE *__restrict __stream);
#endif


/* Seek to a certain position on STREAM.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern int fseek (FILE *__stream, long int __off, int __whence);
/* Return the current position of STREAM.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern long int ftell (FILE *__stream) __wur;
/* Rewind to the beginning of STREAM.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern void rewind (FILE *__stream);

/* The Single Unix Specification, Version 2, specifies an alternative,
   more adequate interface for the two functions above which deal with
   file offset.  `long int' is not the right type.  These definitions
   are originally defined in the Large File Support API.  */

#if defined __USE_LARGEFILE || defined __USE_XOPEN2K
# ifndef __USE_FILE_OFFSET64
/* Seek to a certain position on STREAM.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern int fseeko (FILE *__stream, __off_t __off, int __whence);
/* Return the current position of STREAM.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern __off_t ftello (FILE *__stream) __wur;
# else
#  ifdef __REDIRECT
extern int __REDIRECT (fseeko,
		       (FILE *__stream, __off64_t __off, int __whence),
		       fseeko64);
extern __off64_t __REDIRECT (ftello, (FILE *__stream), ftello64);
#  else
#   define fseeko fseeko64
#   define ftello ftello64
#  endif
# endif
#endif

#ifndef __USE_FILE_OFFSET64
/* Get STREAM's position.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern int fgetpos (FILE *__restrict __stream, fpos_t *__restrict __pos);
/* Set STREAM's position.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern int fsetpos (FILE *__stream, const fpos_t *__pos);
#else
# ifdef __REDIRECT
extern int __REDIRECT (fgetpos, (FILE *__restrict __stream,
				 fpos_t *__restrict __pos), fgetpos64);
extern int __REDIRECT (fsetpos,
		       (FILE *__stream, const fpos_t *__pos), fsetpos64);
# else
#  define fgetpos fgetpos64
#  define fsetpos fsetpos64
# endif
#endif

#ifdef __USE_LARGEFILE64
extern int fseeko64 (FILE *__stream, __off64_t __off, int __whence);
extern __off64_t ftello64 (FILE *__stream) __wur;
extern int fgetpos64 (FILE *__restrict __stream, fpos64_t *__restrict __pos);
extern int fsetpos64 (FILE *__stream, const fpos64_t *__pos);
#endif

/* Clear the error and EOF indicators for STREAM.  */
extern void clearerr (FILE *__stream) __THROW;
/* Return the EOF indicator for STREAM.  */
extern int feof (FILE *__stream) __THROW __wur;
/* Return the error indicator for STREAM.  */
extern int ferror (FILE *__stream) __THROW __wur;

#ifdef __USE_MISC
/* Faster versions when locking is not required.  */
extern void clearerr_unlocked (FILE *__stream) __THROW;
extern int feof_unlocked (FILE *__stream) __THROW __wur;
extern int ferror_unlocked (FILE *__stream) __THROW __wur;
#endif


/* Print a message describing the meaning of the value of errno.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern void perror (const char *__s);

/* Provide the declarations for `sys_errlist' and `sys_nerr' if they
   are available on this system.  Even if available, these variables
   should not be used directly.  The `strerror' function provides
   all the necessary functionality.  */
#include <bits/sys_errlist.h>


#ifdef	__USE_POSIX
/* Return the system file descriptor for STREAM.  */
extern int fileno (FILE *__stream) __THROW __wur;
#endif /* Use POSIX.  */

#ifdef __USE_MISC
/* Faster version when locking is not required.  */
extern int fileno_unlocked (FILE *__stream) __THROW __wur;
#endif


#ifdef __USE_POSIX2
/* Create a new stream connected to a pipe running the given command.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern FILE *popen (const char *__command, const char *__modes) __wur;

/* Close a stream opened by popen and return the status of its child.

   This function is a possible cancellation point and therefore not
   marked with __THROW.  */
extern int pclose (FILE *__stream);
#endif


#ifdef	__USE_POSIX
/* Return the name of the controlling terminal.  */
extern char *ctermid (char *__s) __THROW;
#endif /* Use POSIX.  */


#if (defined __USE_XOPEN && !defined __USE_XOPEN2K) || defined __USE_GNU
/* Return the name of the current user.  */
extern char *cuserid (char *__s);
#endif /* Use X/Open, but not issue 6.  */


#ifdef	__USE_GNU
struct obstack;			/* See <obstack.h>.  */

/* Write formatted output to an obstack.  */
extern int obstack_printf (struct obstack *__restrict __obstack,
			   const char *__restrict __format, ...)
     __THROWNL __attribute__ ((__format__ (__printf__, 2, 3)));
extern int obstack_vprintf (struct obstack *__restrict __obstack,
			    const char *__restrict __format,
			    __gnuc_va_list __args)
     __THROWNL __attribute__ ((__format__ (__printf__, 2, 0)));
#endif /* Use GNU.  */


#ifdef __USE_POSIX199506
/* These are defined in POSIX.1:1996.  */

/* Acquire ownership of STREAM.  */
extern void flockfile (FILE *__stream) __THROW;

/* Try to acquire ownership of STREAM but do not block if it is not
   possible.  */
extern int ftrylockfile (FILE *__stream) __THROW __wur;

/* Relinquish the ownership granted for STREAM.  */
extern void funlockfile (FILE *__stream) __THROW;
#endif /* POSIX */

#if defined __USE_XOPEN && !defined __USE_XOPEN2K && !defined __USE_GNU
/*  X/Open Issues 1-5 required getopt to be declared in this
   header.  It was removed in Issue 6.  GNU follows Issue 6.  */
# include <bits/getopt_posix.h>
#endif

/* Slow-path routines used by the optimized inline functions in
   bits/stdio.h.  */
extern int __uflow (FILE *);
extern int __overflow (FILE *, int);

/* If we are compiling with optimizing read this file.  It contains
   several optimizing inline functions and macros.  */
#ifdef __USE_EXTERN_INLINES
# include <bits/stdio.h>
#endif
#if __USE_FORTIFY_LEVEL > 0 && defined __fortify_function
# include <bits/stdio2.h>
#endif
#ifdef __LDBL_COMPAT
# include <bits/stdio-ldbl.h>
#endif

__END_DECLS

#endif /* <stdio.h> included.  */

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前端学习C语言 - 开篇

前端学习C语言 - 开篇

第一个程序

编译

头文件

基本数据类型

类型所占字节

类型的范围

char

有符号整型

无符号整型

浮点

练习

char的计算

有符号转无符号

int的计算

小数的二进制表示

扩展

计算机如何把人类理解的信息转为0和1

stdio.h 头文件

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