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How very strange. For me, the problem only seems to happen when STDIN and STDERR are left at their defaults. For example, these work:
perl t50 </dev/tty perl t50 2>/dev/null
The problem may stem from this attribute of dup(2):
dup and dup2 create a copy of the file descriptor oldfd.

After successful return of dup or dup2, the old and new descriptors may be used interchangeably. They share locks, file position pointers and flags; for example, if the file position is modified by using lseek on one of the descriptors, the position is also changed for the other.

I suspect this dup happens in your login program, such as ssh or /bin/login.

Note that this behavior isn't perl-specific; I see the exact same thing with this C program:

#include <unistd.h> #include <stdio.h> #include <fcntl.h> void main() { char buf[8192]; int rr; fcntl(STDERR_FILENO,F_SETFL,O_NONBLOCK); while ( (rr = read(STDIN_FILENO,buf,8192)) > 0) { write(STDOUT_FILENO,buf,rr); } if (rr < 0) { perror("read error:"); exit(1); } exit(0); }

Whether or not this is a bug depends on POSIX, I suppose.

At any rate, hopefully one of the above workarounds will help. Good luck!

Update: It looks like this is How Things Have Always Worked. If you look at init.c from V7 Unix, the dfork function takes the tty on standard input and dup's it twice to create standard output and standard error. session.c from OpenSSH does essentially the same thing in its do_exec_pty function.


In reply to Re: What is wrong in blocking? by sgifford
in thread What is wrong in blocking? by madtoperl

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