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dup函数的使用

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

int main(int argc, const char *argv[])
{
	int fd = open("temp", O_RDWR | O_CREAT | O_TRUNC, 0664);
	if(fd == -1)
	{
		perror("open");
		exit(1);
	}

	// copy fd
	int fd1 = dup(fd);
	// write file
	char *p = "this is dup demo";
    
    printf("sizeof--%lu---\n", sizeof("this is dup demo"));
    printf("strlen--%lu---\n",  strlen("this is dup demo"));

    write(fd1, p, strlen(p)+1);
	close(fd1);

	// read file
	char buf[64]= { 0 };
	lseek(fd, 0, SEEK_SET);
	read(fd, buf, sizeof(buf));
	printf("buf = %s\n", buf);
    
	close(fd);
}

测试结果

dup2函数使用

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

int main(int argc, const char *argv[])
{
	int fd = open("temp", O_RDWR | O_CREAT | O_APPEND, 0664);
	if(fd == -1)
	{
		perror("open");
		exit(1);
	}

	int fd2 = open("temp1", O_RDWR | O_CREAT | O_APPEND, 0664);
	if(fd2 == -1)
	{
		perror("open-2");
		exit(1);
	}
	// copy fd
	dup2(fd, fd2);

	// write file
	char *p = "this is dup2 demo....";
	write(fd2, p, strlen(p)+1);
	close(fd2);

	// read file
	char buf[64]= { 0 };
	lseek(fd, 0, SEEK_SET);
	read(fd, buf, sizeof(buf));
	printf("buf = %s\n", buf);
	close(fd);
}

测试结果

/* 实现"+"为输入重定向，"-"为输出重定向 */

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

#define BUFF    (1024)

int my_copy(int fd_in, int fd_out)
{
	int fd_r = fd_in;
	int fd_w = fd_out;
	char buf[BUFF] = { 0 };
	int ret = 0;
	memset(buf, 0, sizeof(buf));
	while((ret = read(fd_r, buf, BUFF)) > 0)
	{
		write(fd_w, buf, ret);
	}
	close(fd_r);
	close(fd_w);

	return 0;
}


int main(int argc, const char *argv[])
{
	int ret     = -1;
	int fd      = -1;
	int i       = 0;
	int flag    = 0;

	if(argc < 2)
	{

		fprintf(stderr, "Usage %s [+/-] <src_file>!\n", argv[0]);
		return -1;
	}

	for(i=1; i<argc; i++)
	{
		if(0 == strcmp(argv[i], "+"))
		{
			fd = open(argv[++i], O_RDONLY);

			if(fd < 0)
			{
				perror("open fd_in");
				return -2;
			}
			if(STDIN_FILENO != dup2(fd, STDIN_FILENO))
			{
				return -3;
			} 
			close(fd);
		}
		else if(0 == strcmp(argv[i], "-"))
		{
			fd = open(argv[++i], O_WRONLY | O_CREAT | O_TRUNC, 0777);
			printf("fd =%d\n", fd);

			if(fd < 0)
			{
				perror("open fd_in");
				return -2;
			}
			if(STDOUT_FILENO != dup2(fd, STDOUT_FILENO))
			{
				printf("dup2 error!\n");
				return -3;
			} 

			close(fd);
		}
		else
		{
			flag = 1;
			fd = open(argv[i], O_RDONLY);
			if(fd < 0)
			{
				perror("open fd_in");
				return -5;
			}
			if(STDIN_FILENO != dup2(fd, STDIN_FILENO))
			{
				printf("dup2 error3!\n");
				return -6;
			} 
			my_copy(STDIN_FILENO, STDOUT_FILENO);
			close(fd);
		}
	}

	if(!flag)
		my_copy(STDIN_FILENO, STDOUT_FILENO);

	return 0;
}

测试结果