Unix环境高级编程习题3.2

题目

3.2 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.（实现自己的dup2函数，不能使用fcntl函数，注意处理错误值。）

函数原型

int dup2(int filedes, int filedes2);

函数功能

makes filedes2 be the copy of filedes, closing newfd first if necessary.（复制filedes到filedes2，若filedes2打开，则先关闭）

思路

要判断filedes2是否存在，可以使用dup函数，判断返回值。若ok或者，返回-1时的错误值不为EBADF，则表明filedes2是打开的。

我的解

参见如下代码。测试环境（cygwin）

#include <apue.h>

// excise 3.2 by fred yu
// implementation your own dup2 function without using fcntl.
int my_dup2(int fd, int fd2)
{
int tmpfd = -1, i, lastfd;
int * fds;

if (fd2 < 0
#ifdef OPEN_MAX
|| fd2 > OPEN_MAX
#endif
) {
errno = EBADF;
printf("my_dup2, %d] fd2 is not a valid number.\n", __LINE__);
return -1;
}

tmpfd = dup(fd2);
if (tmpfd == -1) {
switch (errno) {
case EBADF:
// fd2 is not opened.
break;
case EMFILE:
if (close(fd2) == -1) {
printf("my_dup2, %d] close fd2 failed.\n", __LINE__);
return -1;
}
break;
default:
printf("my_dup2, %d] dup fd2 failed.\n", __LINE__);
return -1;
}
} else { //fd2是打开了的
if(fd == fd2){ // 如果fd == fd2，则返回fd2，而不关闭
return;
}
if (close(tmpfd) == -1) {
printf("my_dup2, %d] close tmpfd failed.\n", __LINE__);
return -1;
}
if (close(fd2) == -1) {
printf("my_dup2, %d] close fd2 failed.\n", __LINE__);
return -1;
}
}

fds = (int *)malloc(fd2 * sizeof(int));
if (!fds) {
printf("my_dup2, %d] malloc fds failed.\n", __LINE__);
errno = ENOMEM;
return -1;
}

for (i = 0; i < fd2; ++i) {
fds[i] = -1;
}

lastfd = -1;
while (tmpfd = dup(fd)) { //在循环中复制fd，
if (tmpfd == -1) {
printf("my_dup2, %d] dup fd failed, fd=%d, lastfd=%d, err=%d.\n", __LINE__, fd, lastfd, errno);
goto FAIL_EXIT;
} else if (tmpfd == fd2) { //直到复制的tmpfd == fd2才退出
break;
} else {
fds[tmpfd] = tmpfd; //保存中间循环过程中复制了的描述符
lastfd = tmpfd; //保存最后一个可复制的描述符，用于出错的时候输出
}
}

for (i = 0; i < fd2; ++i) {
if (fds[i] != -1) close(fds[i]);
}

free(fds);
return fd2;

FAIL_EXIT:
for (i = 0; i < fd2; ++i) {
if (fds[i] != -1) close(fds[i]);
}
free(fds);
return -1;
}

int test_my_dup2(int argc, char * argv[])
{
int fd;

if (argc < 3) {
printf("usage: excise oldfd newfd\n");
return 0;
}

fd = my_dup2(atoi(argv[1]), atoi(argv[2]));
if (fd == -1) {
printf("dup2 fd failed\n");
return -1;
}
printf("return fd by my_dup2 is %d", fd);

return 0;
}

int main(int argc, char * argv[])
{
test_my_dup2(argc, argv);
exit(0);
}

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