【Linux】基础I/O＞文件系统&&软硬链接&&动静态库详解

#include <stdio.h>
#include <string.h>
int main()
{
	FILE* fp = fopen("myfile", "w");
	if (!fp) {
		printf("fopen error!\n");
	}
	const char* msg = "hello bit!\n";
	int count = 5;
	while (count--) {
		fwrite(msg, strlen(msg), 1, fp);
	}
	fclose(fp);
	return 0;
}

#include <stdio.h>
#include <string.h>
int main()
{
	FILE* fp = fopen("myfile", "r");
	if (!fp) {
		printf("fopen error!\n");
	}
	char buf[1024];
	const char* msg = "hello bit!\n";
	while (1) {
		//注意返回值和参数，此处有坑，仔细查看man手册关于该函数的说明
		size_t s = fread(buf, 1, strlen(msg), fp);
		if (s > 0) {
			buf[s] = 0;
			printf("%s", buf);
		}
		if (feof(fp)) {
			break;
		}
	}
	fclose(fp);
	return 0;
}

#include <stdio.h>
#include <string.h>
int main()
{
	const char* msg = "hello fwrite\n";
	fwrite(msg, strlen(msg), 1, stdout);
	printf("hello printf\n");
	fprintf(stdout, "hello fprintf\n");
	return 0;
}

r	Open text file for reading.
	The stream is positioned at the beginning of the file.

r +	Open for readingand writing.
	The stream is positioned at the beginning of the file.

w	Truncate(缩短) file to zero length or create text file for writing.
	The stream is positioned at the beginning of the file.

w + Open for readingand writing.
	The file is created if it does not exist, otherwise it is truncated.
	The stream is positioned at the beginning of the file.
a   Open for appending(writing at end of file).
	The file is created if it does not exist.
	The stream is positioned at the end of the file.

a + Open for readingand appending(writing at end of file).
	The file is created if it does not exist.The initial file position
	for reading is at the beginning of the file,
	but output is always appended to the end of the file

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
int main()
{
	umask(0);
	int fd = open("myfile", O_WRONLY | O_CREAT, 0644);
	if (fd < 0) {
		perror("open");
		return 1;
	}
	int count = 5;
	const char* msg = "hello bit!\n";
	int len = strlen(msg);
	while (count--) {
		write(fd, msg, len);// msg：缓冲区首地址， len: 本次读取，期望写入多少个字节的数据。 返回值：实际写了多少字节数据
	}
	close(fd);
	return 0;
}

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
#include <string.h>
int main()
{
	int fd = open("myfile", O_RDONLY);
	if (fd < 0) {
		perror("open");
		return 1;
	}
	const char* msg = "hello bit!\n";
	char buf[1024];
	while (1) {
		size_t s = read(fd, buf, strlen(msg));//类比write
		if (s > 0) {
			printf("%s", buf);
		}
		else {
			break;
		}
	}
	close(fd);
	return 0;
}

#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
int open(const char* pathname, int flags);
int open(const char* pathname, int flags, mode_t mode);

pathname: 要打开或创建的目标文件
flags : 打开文件时，可以传入多个参数选项，用下面的一个或者多个常量进行“或”运算，构成flags。

参数 :
O_RDONLY: 只读打开
O_WRONLY : 只写打开
O_RDWR : 读，写打开

这三个常量，必须指定一个且只能指定一个

O_CREAT : 若文件不存在，则创建它。需要使用mode选项，来指明新文件的访问权限
O_APPEND : 追加写

返回值：
成功：新打开的文件描述符
失败： - 1

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <string.h>
int main()
{
	char buf[1024];
	size_t s = read(0, buf, sizeof(buf));
	if (s > 0) {
		buf[s] = 0;
		write(1, buf, strlen(buf));
		write(2, buf, strlen(buf));
	}
	return 0;
}

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
int main()
{
	int fd = open("myfile", O_RDONLY);
	if (fd < 0) {
		perror("open");
		return 1;
	}
	printf("fd: %d\n", fd);
	close(fd);
	return 0;
}

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <unistd.h>
int main()
{
	close(0);
	//close(2);
	int fd = open("myfile", O_RDONLY);
	if (fd < 0) {
		perror("open");
		return 1;
	}
	printf("fd: %d\n", fd);
	close(fd);
	return 0;
}

#include <stdio.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdlib.h>
#include <unistd.h>
int main()
{
	close(1);
	int fd = open("myfile", O_WRONLY | O_CREAT, 00644);
	if (fd < 0) {
		perror("open");
		return 1;
	}
	printf("fd: %d\n", fd);
	fflush(stdout);

	close(fd);
	exit(0);
}

#include <unistd.h>
int dup2(int oldfd, int newfd);

#include <unistd.h>
#include <sys/types.h>
#include <sys/stat.h>
#include <fcntl.h>
#include <stdio.h>
#include <string.h>

int main() {
    int fd = open("log.txt", O_CREAT | O_RDWR, 0644);
    if (fd < 0)
    {
        perror("open");
    }
    dup2(fd, 1);                      // 文件重定向

    printf("newfd: hello, world\n");  // 向1号文件描述符写入内容

    const char* buf = "oldfd: hello, world\n";
    write(fd, buf, strlen(buf));      // 向原本的log.txt对应的文件描述符写入内容

    return 0;
}

#include <stdio.h>
#include <string.h>
#include <unistd.h>
int main()
{
	const char* msg0 = "hello printf\n";
	const char* msg1 = "hello fwrite\n";
	const char* msg2 = "hello write\n";
	printf("%s", msg0);
	fwrite(msg1, strlen(msg0), 1, stdout);
	write(1, msg2, strlen(msg2));
	fork();
	return 0;
}

[root@localhost linux]# touch abc
[root@localhost linux]# ls -i abc
263466 abc

[root@localhost linux]# touch abc 
[root@localhost linux]# ln abc def 
[root@localhost linux]# ls -1i
    abc def 263466 abc 263466 def

263563 -rw-r--r--. 2 root root 0 9月 15 17:45 abc
261678 lrwxrwxrwx. 1 root root 3 9月 15 17:53 abc.s -> abc
263563 -rw-r--r--. 2 root root 0 9月 15 17:45 def

测试程序
/add.h/
#ifndef __ADD_H__
#define __ADD_H__ 
int add(int a, int b);
#endif // __ADD_H__
/add.c/
#include "add.h"
int add(int a, int b)
{
	return a + b;
}
/sub.h/
#ifndef __SUB_H__
#define __SUB_H__ 
int sub(int a, int b);
#endif // __SUB_H__
/add.c/
#include "add.h"
int sub(int a, int b)
{
	return a - b;
}
///main.c
#include <stdio.h>
#include "add.h"
#include "sub.h"

int main(void)
{
	int a = 10;
	int b = 20;
	printf("add(10, 20)=%d\n", a, b, add(a, b));
	a = 100;
	b = 20;
	printf("sub(%d,%d)=%d\n", a, b, sub(a, b));
}

[root@localhost linux]# ls
add.c add.h main.c sub.c sub.h
[root@localhost linux]# gcc -c add.c -o add.o
[root@localhost linux]# gcc -c sub.c -o sub.o

生成静态库
[root@localhost linux]# ar -rc libmymath.a add.o sub.o 
ar是gnu归档工具，rc表示(replace and create)

查看静态库中的目录列表
[root@localhost linux]# ar -tv libmymath.a 
rw-r--r-- 0/0 1240 Sep 15 16:53 2017 add.o
rw-r--r-- 0/0 1240 Sep 15 16:53 2017 sub.o
t:列出静态库中的文件
v:verbose 详细信息

[root@localhost linux]# gcc main.c -L. -lmymath
-L 指定库路径
-l 指定库名
测试目标文件生成后，静态库删掉，程序照样可以运行

示例： 
[root@localhost linux]# gcc -fPIC -c sub.c add.c 
[root@localhost linux]# gcc -shared -o libmymath.so *.o 
[root@localhost linux]# ls add.c add.h add.o libmymath.so main.c sub.c sub.h sub.o

示例： 
gcc main.o -o main –L. -lhello

 [root@localhost linux]# export LD_LIBRARY_PATH=.
 [root@localhost linux]# gcc main.c -lmymath
 [root@localhost linux]# ./a.out
 add(10, 20)=30
 sub(100, 20)=80

 [root@localhost linux]# cat /etc/ld.so.conf.d/bit.conf 
 /root/tools/linux
 [root@localhost linux]# ldconfig

#include <math.h>
#include <stdio.h>
int main(void)
{
     double x = pow(2.0, 3.0);
     printf("The cubed is %f\n", x);
     return 0;
}

gcc -Wall calc.c -o calc -lm