无法在Linux中写入管道

Question

I have a third party utility that I have to call from my application to initialize some of their stuff. I've chosen to call it by using popen because I need to write a password to stdin.

I's a simple use case:

...
FILE *f = popen("utility-bin", "w");
fwrite(myPassword.c_str(), 1, myPassword.size(), f);
fflush(f);
pclose(f);
...

However, no matter how I try, the stream is never sent, and the utility remains blocked waiting for the password.

If I call the utility from a regular Linux shell, on the other hand, I can just type the password and everything works.

So my question is: Is it possible for an application to block data from coming from pipes, but still accepting from a normal user shell? And, if that's the case, is there anything I can do to make it accept my pipe input?

PS: When I call the utility from the shell, many signals are beeing handled. ctrl+c and ctrl+z , for example, do nothing.

Answer 1

Is it possible for an application to block data from coming from pipes, but still accepting from a normal user shell

It can call isatty which will tell it if input comes from a terminal or not.

if that's the case, is there anything I can do to make it accept my pipe input

There is a way, but you might not like it:

1. Open a pseudoterminal using posix_openpt grantpt and unlockpt . You now have the "master" fd
2. Fork a new process
3. Call setsid() in the new process to kill its terminal association
4. In the child call ptsname on the fd obtained in step 1
5. Open the name obtained from ptsname and call TIOCSTTY on it - it becomes the controlling terminal
6. Duplicate the descriptor obtained in step 5 into STDIN_FILENO
7. Exec your program

You can probably adapt the function ptyFork from TLPI for this or the function pty_fork from APUE .

At this point you can write into the master fd - as if it were a pipe - from your process and the child will think it comes from a terminal.