问题都在第二个printf语句中调用的dup2
dup2的第二个参数是1，1是什么 STDIN_FILENO宏的本质就是1.这里还是那句话“fd2 is already open, it is first closed”
三个标准流在文件进程开始的时候肯定是都被打开的，already open是一定的事情。
So。。。dup2非常“老实本分"的把STDOUT_FILENO在第二printf的时候关闭了
这个时候printf的输出就不会被打印在标准输出设备上面了，因为STDOUT_FILENO已经被关闭了!!
如果这个时候只要把第二个printf里面dup2的第二个参数改成大于2的数字就没事了。
code：
#include
#include"fcntl.h"
#include
int main()
{
int file_descriptor;
if((file_descriptor = open("./text.t",O_RDWR)) < 0)
{
printf("error\n");
}
else
{
printf("file descriptor is %d\n",file_descriptor);
}
printf("dup2 return value:%d\n dup return value\
%d\n",dup2(file_descriptor,0),dup(file_descriptor));
printf("dup2 return value:%d\ndup return value\
%d\n",dup2(file_descriptor,10),dup(file_descriptor));
printf("dup2 return value:%d\ndup return value\
%d\n",dup2(file_descriptor,11),dup(file_descriptor));
close(file_descriptor);
return 0;
}
运行结果：
file descriptor is 3
dup2 return value:0
dup return value 4
dup2 return value:10
dup return value 5
dup2 return value:11
dup return value 6
这个时候就很清楚了。
Similarly,the call
dup2(fd, fd2);
is equivalent to
close(fd2);
fcntl(fd, F_DUPFD, fd2);
当然也不完全相同，不同点如下：
1. dup2 is an atomic operation, whereas the alternate form involves two function
calls. It is possible in the latter case to have a signal catcher called between the
closeand the fcntlthat could modify the file descriptors. (W edescribe
signals in Chapter 10.) The same problem could occur if a different thread
changes the file descriptors. (Wedescribe threads in Chapter 11.)
2. Ther eare s ome errnodifferences between dup2 and fcntl.