Mixing
readline hangs on manual pipe()
up vote
3
down vote
favorite
I'm trying to dispel the magic fork variant of open:
# magic-fork.pl
if (open my $fh, '-|') { # fork self, make new fd for reading, attach child STDOUT to it
STDOUT->say('parent getpid: ', $$);
STDOUT->say('parent STDOUT->fileno: ', STDOUT->fileno);
STDOUT->say('parent $fh->fileno: ', $fh->fileno);
while (my $line = $fh->getline) {
STDOUT->print('parent readline from child: ', $line);
}
} else {
STDOUT->say('child getpid: ', $$);
STDOUT->say('child STDOUT->fileno: ', STDOUT->fileno);
}
It runs and finishes.
# plain-fork.pl
pipe my $r, my $w;
if (fork) {
STDOUT->say('parent getpid: ', $$);
STDOUT->say('parent STDOUT->fileno: ', STDOUT->fileno);
STDOUT->say('parent $r->fileno: ', $r->fileno);
STDOUT->say('parent $w->fileno: ', $w->fileno);
while (my $line = $r->getline) {
STDOUT->print('parent readline from child: ', $line);
}
} else {
$w->say('child getpid: ', $$);
$w->say('child $r->fileno: ', $r->fileno);
$w->say('child $w->fileno: ', $w->fileno);
$w->say('child STDOUT->fileno: ', STDOUT->fileno);
}
This program unexpectedly hangs in the loop.
I tried to no avail:
- calling $w->autoflush(1)
- explicit close on handles
- explicit exit
- POSIX::dup2 $w onto STDOUT
- checking strace -ff to see whether I missed a crucial syscall
What's the problem?
perl posix
asked yesterday
daxim
35.8k449112
add a comment |
up vote
3
down vote
favorite
I'm trying to dispel the magic fork variant of open:
# magic-fork.pl
if (open my $fh, '-|') { # fork self, make new fd for reading, attach child STDOUT to it
STDOUT->say('parent getpid: ', $$);
STDOUT->say('parent STDOUT->fileno: ', STDOUT->fileno);
STDOUT->say('parent $fh->fileno: ', $fh->fileno);
while (my $line = $fh->getline) {
STDOUT->print('parent readline from child: ', $line);
}
} else {
STDOUT->say('child getpid: ', $$);
STDOUT->say('child STDOUT->fileno: ', STDOUT->fileno);
}
It runs and finishes.
# plain-fork.pl
pipe my $r, my $w;
if (fork) {
STDOUT->say('parent getpid: ', $$);
STDOUT->say('parent STDOUT->fileno: ', STDOUT->fileno);
STDOUT->say('parent $r->fileno: ', $r->fileno);
STDOUT->say('parent $w->fileno: ', $w->fileno);
while (my $line = $r->getline) {
STDOUT->print('parent readline from child: ', $line);
}
} else {
$w->say('child getpid: ', $$);
$w->say('child $r->fileno: ', $r->fileno);
$w->say('child $w->fileno: ', $w->fileno);
$w->say('child STDOUT->fileno: ', STDOUT->fileno);
}
This program unexpectedly hangs in the loop.
I tried to no avail:
- calling $w->autoflush(1)
- explicit close on handles
- explicit exit
- POSIX::dup2 $w onto STDOUT
- checking strace -ff to see whether I missed a crucial syscall
What's the problem?
perl posix
asked yesterday
daxim
35.8k449112
I believe Shawn's answer is correct; reading continues to block because the pipe is still open for writing (by the parent). But you do say you tried explicitly closing handles; did you try closing the write handle in the parent (before the getline)?
– ysth
yesterday
add a comment |
up vote
3
down vote
favorite
up vote
3
down vote
favorite
I'm trying to dispel the magic fork variant of open:
# magic-fork.pl
if (open my $fh, '-|') { # fork self, make new fd for reading, attach child STDOUT to it
STDOUT->say('parent getpid: ', $$);
STDOUT->say('parent STDOUT->fileno: ', STDOUT->fileno);
STDOUT->say('parent $fh->fileno: ', $fh->fileno);
while (my $line = $fh->getline) {
STDOUT->print('parent readline from child: ', $line);
}
} else {
STDOUT->say('child getpid: ', $$);
STDOUT->say('child STDOUT->fileno: ', STDOUT->fileno);
}
It runs and finishes.
# plain-fork.pl
pipe my $r, my $w;
if (fork) {
STDOUT->say('parent getpid: ', $$);
STDOUT->say('parent STDOUT->fileno: ', STDOUT->fileno);
STDOUT->say('parent $r->fileno: ', $r->fileno);
STDOUT->say('parent $w->fileno: ', $w->fileno);
while (my $line = $r->getline) {
STDOUT->print('parent readline from child: ', $line);
}
} else {
$w->say('child getpid: ', $$);
$w->say('child $r->fileno: ', $r->fileno);
$w->say('child $w->fileno: ', $w->fileno);
$w->say('child STDOUT->fileno: ', STDOUT->fileno);
}
This program unexpectedly hangs in the loop.
I tried to no avail:
- calling $w->autoflush(1)
- explicit close on handles
- explicit exit
- POSIX::dup2 $w onto STDOUT
- checking strace -ff to see whether I missed a crucial syscall
What's the problem?
perl posix
asked yesterday
daxim
35.8k449112
I'm trying to dispel the magic fork variant of open:
# magic-fork.pl
if (open my $fh, '-|') { # fork self, make new fd for reading, attach child STDOUT to it
STDOUT->say('parent getpid: ', $$);
STDOUT->say('parent STDOUT->fileno: ', STDOUT->fileno);
STDOUT->say('parent $fh->fileno: ', $fh->fileno);
while (my $line = $fh->getline) {
STDOUT->print('parent readline from child: ', $line);
}
} else {
STDOUT->say('child getpid: ', $$);
STDOUT->say('child STDOUT->fileno: ', STDOUT->fileno);
}
It runs and finishes.
# plain-fork.pl
pipe my $r, my $w;
if (fork) {
STDOUT->say('parent getpid: ', $$);
STDOUT->say('parent STDOUT->fileno: ', STDOUT->fileno);
STDOUT->say('parent $r->fileno: ', $r->fileno);
STDOUT->say('parent $w->fileno: ', $w->fileno);
while (my $line = $r->getline) {
STDOUT->print('parent readline from child: ', $line);
}
} else {
$w->say('child getpid: ', $$);
$w->say('child $r->fileno: ', $r->fileno);
$w->say('child $w->fileno: ', $w->fileno);
$w->say('child STDOUT->fileno: ', STDOUT->fileno);
}
This program unexpectedly hangs in the loop.
I tried to no avail:
- calling $w->autoflush(1)
- explicit close on handles
- explicit exit
- POSIX::dup2 $w onto STDOUT
- checking strace -ff to see whether I missed a crucial syscall
What's the problem?
perl posix
perl posix
asked yesterday
daxim
35.8k449112
asked yesterday
daxim
35.8k449112
asked yesterday
daxim
35.8k449112
asked yesterday
daxim
35.8k449112
asked yesterday
daxim
35.8k449112
35.8k449112
I believe Shawn's answer is correct; reading continues to block because the pipe is still open for writing (by the parent). But you do say you tried explicitly closing handles; did you try closing the write handle in the parent (before the getline)?
– ysth
yesterday
add a comment |
I believe Shawn's answer is correct; reading continues to block because the pipe is still open for writing (by the parent). But you do say you tried explicitly closing handles; did you try closing the write handle in the parent (before the getline)?
– ysth
yesterday
I believe Shawn's answer is correct; reading continues to block because the pipe is still open for writing (by the parent). But you do say you tried explicitly closing handles; did you try closing the write handle in the parent (before the getline)?
– ysth
yesterday
I believe Shawn's answer is correct; reading continues to block because the pipe is still open for writing (by the parent). But you do say you tried explicitly closing handles; did you try closing the write handle in the parent (before the getline)?
– ysth
yesterday
add a comment |
1 Answer
1
active
oldest
votes
up vote
3
down vote
accepted
You pipe before forking (Like usual for this sort of IPC), so both processes have open copies of both the read and write file descriptors, so the read loop in the parent will just block waiting for more input which never comes from the still-open write end.
The child process needs to close $r;, the parent process needs to close $w; at the start of their respective blocks (Or after you print out the file descriptors for those handles).
answered yesterday
Shawn
3,0441513
Key piece of missing info: The pipe will only return EOF once there are no open handles to the write end of the pipe.
– ikegami
5 hours ago
add a comment |
1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
3
down vote
accepted
You pipe before forking (Like usual for this sort of IPC), so both processes have open copies of both the read and write file descriptors, so the read loop in the parent will just block waiting for more input which never comes from the still-open write end.
The child process needs to close $r;, the parent process needs to close $w; at the start of their respective blocks (Or after you print out the file descriptors for those handles).
answered yesterday
Shawn
3,0441513
Key piece of missing info: The pipe will only return EOF once there are no open handles to the write end of the pipe.
– ikegami
5 hours ago
add a comment |
up vote
3
down vote
accepted
You pipe before forking (Like usual for this sort of IPC), so both processes have open copies of both the read and write file descriptors, so the read loop in the parent will just block waiting for more input which never comes from the still-open write end.
The child process needs to close $r;, the parent process needs to close $w; at the start of their respective blocks (Or after you print out the file descriptors for those handles).
answered yesterday
Shawn
3,0441513
Key piece of missing info: The pipe will only return EOF once there are no open handles to the write end of the pipe.
– ikegami
5 hours ago
add a comment |
up vote
3
down vote
accepted
up vote
3
down vote
accepted
You pipe before forking (Like usual for this sort of IPC), so both processes have open copies of both the read and write file descriptors, so the read loop in the parent will just block waiting for more input which never comes from the still-open write end.
The child process needs to close $r;, the parent process needs to close $w; at the start of their respective blocks (Or after you print out the file descriptors for those handles).
answered yesterday
Shawn
3,0441513
You pipe before forking (Like usual for this sort of IPC), so both processes have open copies of both the read and write file descriptors, so the read loop in the parent will just block waiting for more input which never comes from the still-open write end.
The child process needs to close $r;, the parent process needs to close $w; at the start of their respective blocks (Or after you print out the file descriptors for those handles).
answered yesterday
Shawn
3,0441513
answered yesterday
Shawn
3,0441513
answered yesterday
Shawn
3,0441513
answered yesterday
Shawn
3,0441513
3,0441513
Key piece of missing info: The pipe will only return EOF once there are no open handles to the write end of the pipe.
– ikegami
5 hours ago
add a comment |
Key piece of missing info: The pipe will only return EOF once there are no open handles to the write end of the pipe.
– ikegami
5 hours ago
Key piece of missing info: The pipe will only return EOF once there are no open handles to the write end of the pipe.
– ikegami
5 hours ago
Key piece of missing info: The pipe will only return EOF once there are no open handles to the write end of the pipe.
– ikegami
5 hours ago
add a comment |
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I believe Shawn's answer is correct; reading continues to block because the pipe is still open for writing (by the parent). But you do say you tried explicitly closing handles; did you try closing the write handle in the parent (before the getline)?
– ysth
yesterday