学步园

Advanced Programming in the Unix  Environment练习题3.2：
题目：Write your own dup2 function that performs the same service as the dup2 function described in Section 3.12, without calling the fcntl function. Be sure to handle errors correctly.
思路： dup每次返回最小可用的文件描述符，通过循环调用dup， 来增加返回描述符的值，直到等于dup2的第二个参数filedes2。

废话少说，直接看代码：

#include <unistd.h>
#include <sys/types.h>
#include <errno.h>
#include <stdio.h>
#include <string.h>
#include <linux/limits.h>

#define OPEN_MAX 1024

int mydup2(int filedes, int filedes2)
{
	int open_fds[OPEN_MAX];
	int open_count = 0;
	int new_fds;
	int i;
	errno = 0;
	// test wether filedes is open and a valid file descriptor
	if((open_fds[open_count++] = dup(filedes)) == -1)
		return -1;
	// test wether filedes2 is a valid file descriptor
	if(filedes2 < 0 || filedes2 > OPEN_MAX)
	{
		errno = EBADF;
		return -1;
	}
	// if equal, return directly
	if(filedes == filedes2)
	{
		return filedes;
	}
	// test wether filedes2 is open, and close it if it is open
	if((open_fds[open_count++] = dup(filedes2)) >= 0)
	{
		if(close(filedes2) == -1)
		{
			return -1;
		}
	}
	// dup until the return file descriptor equals to filedes2
	new_fds = dup(filedes);
	open_fds[open_count++] = new_fds;
	while(new_fds != -1 && new_fds != filedes2)
	{
		open_fds[open_count++] = new_fds;
		new_fds = dup(filedes);
	}
	// close the des open by dup unless the last one
	for(i = 0; i < open_count - 1; ++i)
	{
		close(open_fds[i]);
	}
	return new_fds;

}

void test_mydup2(int (*fun_dup2)(int,int))
{
	// test case
	int old_fds[] = {0, 0, 2,  0, -1,  10,    -1, 65536};
	int new_fds[] = {0, 2, 2, 30,  2,  20, 65536, 65536};
	int count =  sizeof(old_fds)/sizeof(int);
	int ret;
	int i;
	// begin test
	for(i = 0; i < count; i++)
	{
		printf("dup2(%d, %d) : ", old_fds[i], new_fds[i]);
		ret = fun_dup2(old_fds[i], new_fds[i]);
		if(-1 == ret)
		{
			printf("%s\n", strerror(errno));
		}
		else if(ret == new_fds[i])
		{
			printf("success.\n");
			if(ret != 0 && ret != 1 && ret != 2)
				close(ret);
		}
		else 
		{
			printf("failed, not equal.\n");
		}
	}
	// test: dup stdout to 20, and print a string by 20
	printf("dup(1, 20), write(20, \"A test\\n\", 7) : ");
	fflush(stdout); // i don't know why fsync(1) not work here
	fun_dup2(1, 20);
	write(20, "A test\n", 7);
	printf("\n");
	close(20);
}

int main()
{
	// test my dup2
	printf("********* my dup2 ***********\n");
	test_mydup2(mydup2);
	// test standard dup2 for a contrast
	printf("********standard dup2********\n");
	test_mydup2(dup2);

	return 0;
}

代码中有注释，如有疑问，欢迎留言。