How to wait for parent process to exit in a child?

I want to fork a process and wait for the parent to exit before doing something in the child.

asked Feb 9 '18 at 10:01

Florian Margaine

43.6k1074107

What have you tried? Please refer to the help section on how to properly ask a question. 👌

– ndugger
Feb 9 '18 at 10:11

1

@ndugger this is a self-answered question, so please refer to the answers for "what I have tried".

– Florian Margaine
Feb 9 '18 at 10:16

@Arne it was a joke 👺

– ndugger
Feb 9 '18 at 10:19

add a comment |

I want to fork a process and wait for the parent to exit before doing something in the child.

asked Feb 9 '18 at 10:01

Florian Margaine

43.6k1074107

What have you tried? Please refer to the help section on how to properly ask a question. 👌

– ndugger
Feb 9 '18 at 10:11

1

@ndugger this is a self-answered question, so please refer to the answers for "what I have tried".

– Florian Margaine
Feb 9 '18 at 10:16

@Arne it was a joke 👺

– ndugger
Feb 9 '18 at 10:19

add a comment |

I want to fork a process and wait for the parent to exit before doing something in the child.

asked Feb 9 '18 at 10:01

Florian Margaine

43.6k1074107

I want to fork a process and wait for the parent to exit before doing something in the child.

python linux fork

asked Feb 9 '18 at 10:01

Florian Margaine

43.6k1074107

asked Feb 9 '18 at 10:01

Florian Margaine

43.6k1074107

asked Feb 9 '18 at 10:01

Florian Margaine

43.6k1074107

asked Feb 9 '18 at 10:01

Florian Margaine

43.6k1074107

asked Feb 9 '18 at 10:01

Florian Margaine

43.6k1074107

What have you tried? Please refer to the help section on how to properly ask a question. 👌

– ndugger
Feb 9 '18 at 10:11

1

@ndugger this is a self-answered question, so please refer to the answers for "what I have tried".

– Florian Margaine
Feb 9 '18 at 10:16

@Arne it was a joke 👺

– ndugger
Feb 9 '18 at 10:19

add a comment |

What have you tried? Please refer to the help section on how to properly ask a question. 👌

– ndugger
Feb 9 '18 at 10:11

1

@ndugger this is a self-answered question, so please refer to the answers for "what I have tried".

– Florian Margaine
Feb 9 '18 at 10:16

@Arne it was a joke 👺

– ndugger
Feb 9 '18 at 10:19

What have you tried? Please refer to the help section on how to properly ask a question. 👌

– ndugger
Feb 9 '18 at 10:11

@ndugger this is a self-answered question, so please refer to the answers for "what I have tried".

– Florian Margaine
Feb 9 '18 at 10:16

@Arne it was a joke 👺

– ndugger
Feb 9 '18 at 10:19

add a comment |

1 Answer
1

active

oldest

votes

The naive way would be a busy loop like this:

# BAD WAY!

pid = os.fork()

if pid == 0:

    while True:

        if os.path.isdir("/proc/%s" % pid):

            break

        time.sleep(0.1)

    # Parent is dead there.

But this is vulnerable to PID reuse issues. If another process is created right after the parent exits and gets its PID, the child will never exit.

Another way would be to use a flock() on a specific file. But it's not going to work, because the child shares the same locks as the parent.

A foolproof way to do this is using a special trick: create a pipe in the parent, and in the child, you just wait until you get an EOF.

# Good way

read_fd, write_fd = os.pipe()



pid = os.fork()

if pid > 0:

    # Close the read pipe, so that only the child is the reader.

    os.close(read_fd)



    # It is important to voluntarily leak write_fd there,

    # so that the kernel will close it for the parent process

    # when it will exit, triggering our trick.



elif pid == 0:

    # Daemon ourselves first.

    os.setsid()

    for fd in {0, 1, 2}:

        os.close(fd)



    # Close the write pipe so that the parent is the only writer.

    # This will make sure we then get an EOF when the only writer exits.

    os.close(write_fd)



    # Now, we're waiting on the read pipe for an EOF.

    # PS: the assert is not necessary in production...

    assert os.read(read_fd, 1) == ""

    os.close(read_fd)



    # At this point, the parent is dead.

edited Nov 26 '18 at 13:12

answered Feb 9 '18 at 10:01

Florian Margaine

43.6k1074107

add a comment |

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1 Answer
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active

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votes

1 Answer
1

active

oldest

votes

The naive way would be a busy loop like this:

# BAD WAY!

pid = os.fork()

if pid == 0:

    while True:

        if os.path.isdir("/proc/%s" % pid):

            break

        time.sleep(0.1)

    # Parent is dead there.

But this is vulnerable to PID reuse issues. If another process is created right after the parent exits and gets its PID, the child will never exit.

Another way would be to use a flock() on a specific file. But it's not going to work, because the child shares the same locks as the parent.

A foolproof way to do this is using a special trick: create a pipe in the parent, and in the child, you just wait until you get an EOF.

# Good way

read_fd, write_fd = os.pipe()



pid = os.fork()

if pid > 0:

    # Close the read pipe, so that only the child is the reader.

    os.close(read_fd)



    # It is important to voluntarily leak write_fd there,

    # so that the kernel will close it for the parent process

    # when it will exit, triggering our trick.



elif pid == 0:

    # Daemon ourselves first.

    os.setsid()

    for fd in {0, 1, 2}:

        os.close(fd)



    # Close the write pipe so that the parent is the only writer.

    # This will make sure we then get an EOF when the only writer exits.

    os.close(write_fd)



    # Now, we're waiting on the read pipe for an EOF.

    # PS: the assert is not necessary in production...

    assert os.read(read_fd, 1) == ""

    os.close(read_fd)



    # At this point, the parent is dead.

edited Nov 26 '18 at 13:12

answered Feb 9 '18 at 10:01

Florian Margaine

43.6k1074107

add a comment |

The naive way would be a busy loop like this:

# BAD WAY!

pid = os.fork()

if pid == 0:

    while True:

        if os.path.isdir("/proc/%s" % pid):

            break

        time.sleep(0.1)

    # Parent is dead there.

But this is vulnerable to PID reuse issues. If another process is created right after the parent exits and gets its PID, the child will never exit.

Another way would be to use a flock() on a specific file. But it's not going to work, because the child shares the same locks as the parent.

A foolproof way to do this is using a special trick: create a pipe in the parent, and in the child, you just wait until you get an EOF.

# Good way

read_fd, write_fd = os.pipe()



pid = os.fork()

if pid > 0:

    # Close the read pipe, so that only the child is the reader.

    os.close(read_fd)



    # It is important to voluntarily leak write_fd there,

    # so that the kernel will close it for the parent process

    # when it will exit, triggering our trick.



elif pid == 0:

    # Daemon ourselves first.

    os.setsid()

    for fd in {0, 1, 2}:

        os.close(fd)



    # Close the write pipe so that the parent is the only writer.

    # This will make sure we then get an EOF when the only writer exits.

    os.close(write_fd)



    # Now, we're waiting on the read pipe for an EOF.

    # PS: the assert is not necessary in production...

    assert os.read(read_fd, 1) == ""

    os.close(read_fd)



    # At this point, the parent is dead.

edited Nov 26 '18 at 13:12

answered Feb 9 '18 at 10:01

Florian Margaine

43.6k1074107

add a comment |

The naive way would be a busy loop like this:

# BAD WAY!

pid = os.fork()

if pid == 0:

    while True:

        if os.path.isdir("/proc/%s" % pid):

            break

        time.sleep(0.1)

    # Parent is dead there.

But this is vulnerable to PID reuse issues. If another process is created right after the parent exits and gets its PID, the child will never exit.

Another way would be to use a flock() on a specific file. But it's not going to work, because the child shares the same locks as the parent.

A foolproof way to do this is using a special trick: create a pipe in the parent, and in the child, you just wait until you get an EOF.

# Good way

read_fd, write_fd = os.pipe()



pid = os.fork()

if pid > 0:

    # Close the read pipe, so that only the child is the reader.

    os.close(read_fd)



    # It is important to voluntarily leak write_fd there,

    # so that the kernel will close it for the parent process

    # when it will exit, triggering our trick.



elif pid == 0:

    # Daemon ourselves first.

    os.setsid()

    for fd in {0, 1, 2}:

        os.close(fd)



    # Close the write pipe so that the parent is the only writer.

    # This will make sure we then get an EOF when the only writer exits.

    os.close(write_fd)



    # Now, we're waiting on the read pipe for an EOF.

    # PS: the assert is not necessary in production...

    assert os.read(read_fd, 1) == ""

    os.close(read_fd)



    # At this point, the parent is dead.

edited Nov 26 '18 at 13:12

answered Feb 9 '18 at 10:01

Florian Margaine

43.6k1074107

The naive way would be a busy loop like this:

# BAD WAY!

pid = os.fork()

if pid == 0:

    while True:

        if os.path.isdir("/proc/%s" % pid):

            break

        time.sleep(0.1)

    # Parent is dead there.

But this is vulnerable to PID reuse issues. If another process is created right after the parent exits and gets its PID, the child will never exit.

Another way would be to use a flock() on a specific file. But it's not going to work, because the child shares the same locks as the parent.

A foolproof way to do this is using a special trick: create a pipe in the parent, and in the child, you just wait until you get an EOF.

# Good way

read_fd, write_fd = os.pipe()



pid = os.fork()

if pid > 0:

    # Close the read pipe, so that only the child is the reader.

    os.close(read_fd)



    # It is important to voluntarily leak write_fd there,

    # so that the kernel will close it for the parent process

    # when it will exit, triggering our trick.



elif pid == 0:

    # Daemon ourselves first.

    os.setsid()

    for fd in {0, 1, 2}:

        os.close(fd)



    # Close the write pipe so that the parent is the only writer.

    # This will make sure we then get an EOF when the only writer exits.

    os.close(write_fd)



    # Now, we're waiting on the read pipe for an EOF.

    # PS: the assert is not necessary in production...

    assert os.read(read_fd, 1) == ""

    os.close(read_fd)



    # At this point, the parent is dead.

edited Nov 26 '18 at 13:12

answered Feb 9 '18 at 10:01

Florian Margaine

43.6k1074107

edited Nov 26 '18 at 13:12

answered Feb 9 '18 at 10:01

Florian Margaine

43.6k1074107

answered Feb 9 '18 at 10:01

Florian Margaine

43.6k1074107

answered Feb 9 '18 at 10:01

Florian Margaine

43.6k1074107

add a comment |

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