重定向子进程的输出

Question

There are several ways of redirecting the output of a child process:

1. using freopen(3)
2. using dup(3)
3. using popen(3)
4. ...

What should one pick if all is wanted is to execute a child process and have it output saved in a given file, pretty much like the ls > files.txt works?

What is normally used by shells?

Answer 1

You can discover what your favorite shell uses by strace(1) ing your shell.

In one terminal:

echo $$

In another terminal:

strace -o /tmp/shell -f -p [PID from the first shell]

In the first terminal again:

ls > files.txt

In the second terminal, ^C your strace(1) command and then edit the /tmp/shell output file to see what system calls it made to do the redirection.

---

freopen(3) manipulates the C standard IO FILE* pointers. All this will be thrown away on the other side of the execve(2) call, because it is maintained in user memory . You could use this after the execve(2) call, but that would be awkward to use generically.

popen(3) opens a single unidirectional pipe(7) . This is useful, but extremely limited -- you get either the standard output descriptor or the standard input descriptor. This would fail for something like ls | grep foo | sort ls | grep foo | sort ls | grep foo | sort where both input and output must be redirected. So this is a poor choice.

dup2(2) will manage file descriptors -- a kernel-implemented resource -- so it will persist across execve(2) calls and you can set up as many file descriptors as you need, which is nice for ls > /tmp/output 2> /tmp/error or handling both input and output: ls | sort | uniq ls | sort | uniq ls | sort | uniq .

There is another mechanism: pty(7) handling. The forkpty(3) , openpty(3) , functions can manage a new pseudo-terminal device created specifically to handle another program. The Advanced Programming in the Unix Environment, 2nd edition book has a very nice pty example program in its source code, though if you're having trouble understanding why this would be useful, take a look at the script(1) program -- it creates a new pseudo-terminal and uses it to record all input and output to and from programs and stores the transcript to a file for later playback or documentation. You can also use it to script actions in interactive programs, similar to expect(1) .

Answer 2

I would expect to find dup2() used mainly.

Neither popen() nor freopen() is designed to handle redirections such as 3>&7 . Up to a point, dup() could be used, but the 3>&7 example shows where dup() starts to creak; you'd have to ensure that file descriptors 4, 5, and 6 are open (and 7 is not) before it would handle what dup2() would do without fuss.