是否每个进程都将其stdin stdout stderr定义为Keyboard，Terminal等？

Question

Does every process have stdin , stdout and stderr associated to it to the Keyboard and Terminal?

I have a small program. I want to replace the keyboard input to a file called new.txt . How do I go about it?

FILE *file1
fopen("new.txt", "r")
close(0);  // close the stdio
dup2(file1, 0);

Would this work? Now my stdio is redirected to the FILE ?

Answer 1

No, not every process. But on operating systems that give you a command-line window to type in, a program started from that command line will have stdin connected to the keyboard, and stdout and stderr both going to the terminal.

If one program starts another, then often the second program's standard streams are connected to the first program in some way; for example, the first program may have an open descriptor through which it can send text and pretend that it's the "keyboard" for the second process. The details vary by operating system, of course.

Answer 2

In response to your question:

Would this work ?

No. dup2() takes two file descriptors ( int s) while you're passing it a FILE * and an int . You can't mix file handles ( FILE * s) and file descriptors ( int s) like that.

You could use open instead of fopen to open your file as a file descriptor instead of a file handle, or you could use fileno to get the file descriptor from a file handle. Or you could use freopen to reopen the stdin file handle to a new file.

Note that file descriptors ( int s) are part of POSIX operating systems and are only portable to other POSIX systems, while file handles ( FILE * s) are part of the C standard and are portable everywhere. If you use file descriptors, you'll have to rewrite your code to make it work on Windows.