How do I wait inside a child process until the parent process reached a certain point?

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PSEUDO-CODE:

Child:

...

wait until parent prints "blabla"

...



Parent:

...

print "blabla"

...

I can't use wait, because that would be the other way around and furthermore, I want to continue the parent process after the print "blabla" parallel to the child process.
I could imagine to share with mmap a pthread_mutex_t variable and solve the issue with mutual exclusion, but I am not sure about that and wanted to ask if there might exists a very easy solution.

edited Jan 3 at 5:01

asked Jan 3 at 4:55

Lavair

1446

2

As you suspect, you need to use some form of interprocess communication. Injecting a message into the child's standard input is often one of the simpler ones to set up and fairly portable. But your question is really both too broad and too system dependent.

– Chris Stratton
Jan 3 at 5:03

@ChrisStratton okay, I am going to overthink it, thank you. Gonna get some sleep now ^^'

– Lavair
Jan 3 at 5:27

Depends on how you created the child. If the child was created with pthread_create, then all you need is a pthread_mutex_t. There's no need for mmap.

– user3386109
Jan 3 at 5:42

2

If it was created with pthread_create() it's not a child process but rather a thread.

– Chris Stratton
Jan 3 at 5:47

add a comment |

PSEUDO-CODE:

Child:

...

wait until parent prints "blabla"

...



Parent:

...

print "blabla"

...

edited Jan 3 at 5:01

asked Jan 3 at 4:55

Lavair

1446

2

As you suspect, you need to use some form of interprocess communication. Injecting a message into the child's standard input is often one of the simpler ones to set up and fairly portable. But your question is really both too broad and too system dependent.

– Chris Stratton
Jan 3 at 5:03

@ChrisStratton okay, I am going to overthink it, thank you. Gonna get some sleep now ^^'

– Lavair
Jan 3 at 5:27

Depends on how you created the child. If the child was created with pthread_create, then all you need is a pthread_mutex_t. There's no need for mmap.

– user3386109
Jan 3 at 5:42

2

If it was created with pthread_create() it's not a child process but rather a thread.

– Chris Stratton
Jan 3 at 5:47

add a comment |

PSEUDO-CODE:

Child:

...

wait until parent prints "blabla"

...



Parent:

...

print "blabla"

...

edited Jan 3 at 5:01

asked Jan 3 at 4:55

Lavair

1446

PSEUDO-CODE:

Child:

...

wait until parent prints "blabla"

...



Parent:

...

print "blabla"

...

edited Jan 3 at 5:01

asked Jan 3 at 4:55

Lavair

1446

edited Jan 3 at 5:01

asked Jan 3 at 4:55

Lavair

1446

edited Jan 3 at 5:01

asked Jan 3 at 4:55

Lavair

1446

asked Jan 3 at 4:55

Lavair

1446

asked Jan 3 at 4:55

Lavair

1446

2

As you suspect, you need to use some form of interprocess communication. Injecting a message into the child's standard input is often one of the simpler ones to set up and fairly portable. But your question is really both too broad and too system dependent.

– Chris Stratton
Jan 3 at 5:03

@ChrisStratton okay, I am going to overthink it, thank you. Gonna get some sleep now ^^'

– Lavair
Jan 3 at 5:27

Depends on how you created the child. If the child was created with pthread_create, then all you need is a pthread_mutex_t. There's no need for mmap.

– user3386109
Jan 3 at 5:42

2

If it was created with pthread_create() it's not a child process but rather a thread.

– Chris Stratton
Jan 3 at 5:47

add a comment |

2

As you suspect, you need to use some form of interprocess communication. Injecting a message into the child's standard input is often one of the simpler ones to set up and fairly portable. But your question is really both too broad and too system dependent.

– Chris Stratton
Jan 3 at 5:03

@ChrisStratton okay, I am going to overthink it, thank you. Gonna get some sleep now ^^'

– Lavair
Jan 3 at 5:27

Depends on how you created the child. If the child was created with pthread_create, then all you need is a pthread_mutex_t. There's no need for mmap.

– user3386109
Jan 3 at 5:42

2

If it was created with pthread_create() it's not a child process but rather a thread.

– Chris Stratton
Jan 3 at 5:47

As you suspect, you need to use some form of interprocess communication. Injecting a message into the child's standard input is often one of the simpler ones to set up and fairly portable. But your question is really both too broad and too system dependent.

– Chris Stratton
Jan 3 at 5:03

@ChrisStratton okay, I am going to overthink it, thank you. Gonna get some sleep now ^^'

– Lavair
Jan 3 at 5:27

Depends on how you created the child. If the child was created with pthread_create, then all you need is a pthread_mutex_t. There's no need for mmap.

– user3386109
Jan 3 at 5:42

If it was created with pthread_create() it's not a child process but rather a thread.

– Chris Stratton
Jan 3 at 5:47

add a comment |

2 Answers
2

active

oldest

votes

You can use Signals for this,
After completing a certain task parent process should send a signal to child process by using Kill system call and by updating the signal handler of the signal child process can do further task.

Child:
signal_handler()
{
...
}
main()
{
signal(SIGINT, signal_handler);
while(1);//wait until parent prints "blabla"
}

Parent:

...

print "blabla"

 kill(pid_of_child,signal)

 ...

edited Jan 3 at 5:40

answered Jan 3 at 5:30

Ganesh_Shinde

483

That's possible but it's probably not the best way. Generally a signal would be used for something abnormal.

– Chris Stratton
Jan 3 at 5:46

@ChrisStratton while true that that could be not the best way, it is simple and reasonable. Signals are often used for things that are not at all abnormal, see for example SIGWINCH en.wikipedia.org/wiki/Signal_(IPC)#SIGWINCH

– linuxfan
Jan 3 at 6:11

1

This is going to be quite challenging to get right. If the parent sends the signal before the child is ready for it, the signal may get lost or cause mischief for the child. To do this reliably, the parent would have to wait for a signal from the child that indicates that the child is ready to accept the signal from the parent, otherwise race conditions and unpredictable behavior could result. For this reason, a pipe or socketpair is probably a better choice.

– David Schwartz
Jan 3 at 6:21

add a comment |

Have two global variables,one for mutex lock and another for a signal variable like this.

pthread_cond_t signal = PTHREAD_MUTEX_INITIALIZER;

pthread_mutex_t cond_mutex = PTHREAD_COND_INITIALIZER;

In your parent thread when you print blabla signal the child thread :

pthread_mutex_lock(&cond_mutex);

pthread_cond_signal(&signal);

pthread_mutex_unlock(&cond_mutex);

In your child thread wait for the signal :

struct timespec time_to_wait = {0, 0};

time_to_wait.tv_sec = time(NULL) + 10;//10 secs

pthread_mutex_lock(&cond_mutex);

status = pthread_cond_timedwait(&signal,&cond_mutex, &time_to_wait);

pthread_mutex_unlock(&cond_mutex);

You can extend this code to handle repeatedly wait for the condition to occur for every 't' seconds within a loop, or simply wait once for 't' seconds and quit.
Refer this doc for the return values.

answered Jan 3 at 5:52

vinoth h

371110

2

Threads are all peers and don't have parent/child relationships. In any event, this question wasn't about threads at all anyway -- it was about processes (which do have parent/child relationships).

– David Schwartz
Jan 3 at 6:15

add a comment |

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2 Answers
2

active

oldest

votes

2 Answers
2

active

oldest

votes

Child:
signal_handler()
{
...
}
main()
{
signal(SIGINT, signal_handler);
while(1);//wait until parent prints "blabla"
}

Parent:

...

print "blabla"

 kill(pid_of_child,signal)

 ...

edited Jan 3 at 5:40

answered Jan 3 at 5:30

Ganesh_Shinde

483

That's possible but it's probably not the best way. Generally a signal would be used for something abnormal.

– Chris Stratton
Jan 3 at 5:46

@ChrisStratton while true that that could be not the best way, it is simple and reasonable. Signals are often used for things that are not at all abnormal, see for example SIGWINCH en.wikipedia.org/wiki/Signal_(IPC)#SIGWINCH

– linuxfan
Jan 3 at 6:11

1

This is going to be quite challenging to get right. If the parent sends the signal before the child is ready for it, the signal may get lost or cause mischief for the child. To do this reliably, the parent would have to wait for a signal from the child that indicates that the child is ready to accept the signal from the parent, otherwise race conditions and unpredictable behavior could result. For this reason, a pipe or socketpair is probably a better choice.

– David Schwartz
Jan 3 at 6:21

add a comment |

Child:
signal_handler()
{
...
}
main()
{
signal(SIGINT, signal_handler);
while(1);//wait until parent prints "blabla"
}

Parent:

...

print "blabla"

 kill(pid_of_child,signal)

 ...

edited Jan 3 at 5:40

answered Jan 3 at 5:30

Ganesh_Shinde

483

That's possible but it's probably not the best way. Generally a signal would be used for something abnormal.

– Chris Stratton
Jan 3 at 5:46

@ChrisStratton while true that that could be not the best way, it is simple and reasonable. Signals are often used for things that are not at all abnormal, see for example SIGWINCH en.wikipedia.org/wiki/Signal_(IPC)#SIGWINCH

– linuxfan
Jan 3 at 6:11

1

This is going to be quite challenging to get right. If the parent sends the signal before the child is ready for it, the signal may get lost or cause mischief for the child. To do this reliably, the parent would have to wait for a signal from the child that indicates that the child is ready to accept the signal from the parent, otherwise race conditions and unpredictable behavior could result. For this reason, a pipe or socketpair is probably a better choice.

– David Schwartz
Jan 3 at 6:21

add a comment |

Child:
signal_handler()
{
...
}
main()
{
signal(SIGINT, signal_handler);
while(1);//wait until parent prints "blabla"
}

Parent:

...

print "blabla"

 kill(pid_of_child,signal)

 ...

edited Jan 3 at 5:40

answered Jan 3 at 5:30

Ganesh_Shinde

483

Child:
signal_handler()
{
...
}
main()
{
signal(SIGINT, signal_handler);
while(1);//wait until parent prints "blabla"
}

Parent:

...

print "blabla"

 kill(pid_of_child,signal)

 ...

edited Jan 3 at 5:40

answered Jan 3 at 5:30

Ganesh_Shinde

483

edited Jan 3 at 5:40

answered Jan 3 at 5:30

Ganesh_Shinde

483

answered Jan 3 at 5:30

Ganesh_Shinde

483

answered Jan 3 at 5:30

Ganesh_Shinde

483

That's possible but it's probably not the best way. Generally a signal would be used for something abnormal.

– Chris Stratton
Jan 3 at 5:46

@ChrisStratton while true that that could be not the best way, it is simple and reasonable. Signals are often used for things that are not at all abnormal, see for example SIGWINCH en.wikipedia.org/wiki/Signal_(IPC)#SIGWINCH

– linuxfan
Jan 3 at 6:11

1

This is going to be quite challenging to get right. If the parent sends the signal before the child is ready for it, the signal may get lost or cause mischief for the child. To do this reliably, the parent would have to wait for a signal from the child that indicates that the child is ready to accept the signal from the parent, otherwise race conditions and unpredictable behavior could result. For this reason, a pipe or socketpair is probably a better choice.

– David Schwartz
Jan 3 at 6:21

add a comment |

That's possible but it's probably not the best way. Generally a signal would be used for something abnormal.

– Chris Stratton
Jan 3 at 5:46

@ChrisStratton while true that that could be not the best way, it is simple and reasonable. Signals are often used for things that are not at all abnormal, see for example SIGWINCH en.wikipedia.org/wiki/Signal_(IPC)#SIGWINCH

– linuxfan
Jan 3 at 6:11

1

This is going to be quite challenging to get right. If the parent sends the signal before the child is ready for it, the signal may get lost or cause mischief for the child. To do this reliably, the parent would have to wait for a signal from the child that indicates that the child is ready to accept the signal from the parent, otherwise race conditions and unpredictable behavior could result. For this reason, a pipe or socketpair is probably a better choice.

– David Schwartz
Jan 3 at 6:21

That's possible but it's probably not the best way. Generally a signal would be used for something abnormal.

– Chris Stratton
Jan 3 at 5:46

@ChrisStratton while true that that could be not the best way, it is simple and reasonable. Signals are often used for things that are not at all abnormal, see for example SIGWINCH en.wikipedia.org/wiki/Signal_(IPC)#SIGWINCH

– linuxfan
Jan 3 at 6:11

This is going to be quite challenging to get right. If the parent sends the signal before the child is ready for it, the signal may get lost or cause mischief for the child. To do this reliably, the parent would have to wait for a signal from the child that indicates that the child is ready to accept the signal from the parent, otherwise race conditions and unpredictable behavior could result. For this reason, a pipe or socketpair is probably a better choice.

– David Schwartz
Jan 3 at 6:21

add a comment |

Have two global variables,one for mutex lock and another for a signal variable like this.

pthread_cond_t signal = PTHREAD_MUTEX_INITIALIZER;

pthread_mutex_t cond_mutex = PTHREAD_COND_INITIALIZER;

In your parent thread when you print blabla signal the child thread :

pthread_mutex_lock(&cond_mutex);

pthread_cond_signal(&signal);

pthread_mutex_unlock(&cond_mutex);

In your child thread wait for the signal :

struct timespec time_to_wait = {0, 0};

time_to_wait.tv_sec = time(NULL) + 10;//10 secs

pthread_mutex_lock(&cond_mutex);

status = pthread_cond_timedwait(&signal,&cond_mutex, &time_to_wait);

pthread_mutex_unlock(&cond_mutex);

You can extend this code to handle repeatedly wait for the condition to occur for every 't' seconds within a loop, or simply wait once for 't' seconds and quit.
Refer this doc for the return values.

answered Jan 3 at 5:52

vinoth h

371110

2

Threads are all peers and don't have parent/child relationships. In any event, this question wasn't about threads at all anyway -- it was about processes (which do have parent/child relationships).

– David Schwartz
Jan 3 at 6:15

add a comment |

Have two global variables,one for mutex lock and another for a signal variable like this.

pthread_cond_t signal = PTHREAD_MUTEX_INITIALIZER;

pthread_mutex_t cond_mutex = PTHREAD_COND_INITIALIZER;

In your parent thread when you print blabla signal the child thread :

pthread_mutex_lock(&cond_mutex);

pthread_cond_signal(&signal);

pthread_mutex_unlock(&cond_mutex);

In your child thread wait for the signal :

struct timespec time_to_wait = {0, 0};

time_to_wait.tv_sec = time(NULL) + 10;//10 secs

pthread_mutex_lock(&cond_mutex);

status = pthread_cond_timedwait(&signal,&cond_mutex, &time_to_wait);

pthread_mutex_unlock(&cond_mutex);

You can extend this code to handle repeatedly wait for the condition to occur for every 't' seconds within a loop, or simply wait once for 't' seconds and quit.
Refer this doc for the return values.

answered Jan 3 at 5:52

vinoth h

371110

2

Threads are all peers and don't have parent/child relationships. In any event, this question wasn't about threads at all anyway -- it was about processes (which do have parent/child relationships).

– David Schwartz
Jan 3 at 6:15

add a comment |

Have two global variables,one for mutex lock and another for a signal variable like this.

pthread_cond_t signal = PTHREAD_MUTEX_INITIALIZER;

pthread_mutex_t cond_mutex = PTHREAD_COND_INITIALIZER;

In your parent thread when you print blabla signal the child thread :

pthread_mutex_lock(&cond_mutex);

pthread_cond_signal(&signal);

pthread_mutex_unlock(&cond_mutex);

In your child thread wait for the signal :

struct timespec time_to_wait = {0, 0};

time_to_wait.tv_sec = time(NULL) + 10;//10 secs

pthread_mutex_lock(&cond_mutex);

status = pthread_cond_timedwait(&signal,&cond_mutex, &time_to_wait);

pthread_mutex_unlock(&cond_mutex);

You can extend this code to handle repeatedly wait for the condition to occur for every 't' seconds within a loop, or simply wait once for 't' seconds and quit.
Refer this doc for the return values.

answered Jan 3 at 5:52

vinoth h

371110

Have two global variables,one for mutex lock and another for a signal variable like this.

pthread_cond_t signal = PTHREAD_MUTEX_INITIALIZER;

pthread_mutex_t cond_mutex = PTHREAD_COND_INITIALIZER;

In your parent thread when you print blabla signal the child thread :

pthread_mutex_lock(&cond_mutex);

pthread_cond_signal(&signal);

pthread_mutex_unlock(&cond_mutex);

In your child thread wait for the signal :

struct timespec time_to_wait = {0, 0};

time_to_wait.tv_sec = time(NULL) + 10;//10 secs

pthread_mutex_lock(&cond_mutex);

status = pthread_cond_timedwait(&signal,&cond_mutex, &time_to_wait);

pthread_mutex_unlock(&cond_mutex);

You can extend this code to handle repeatedly wait for the condition to occur for every 't' seconds within a loop, or simply wait once for 't' seconds and quit.
Refer this doc for the return values.

answered Jan 3 at 5:52

vinoth h

371110

answered Jan 3 at 5:52

vinoth h

371110

answered Jan 3 at 5:52

vinoth h

371110

answered Jan 3 at 5:52

vinoth h

371110

2

Threads are all peers and don't have parent/child relationships. In any event, this question wasn't about threads at all anyway -- it was about processes (which do have parent/child relationships).

– David Schwartz
Jan 3 at 6:15

add a comment |

2

Threads are all peers and don't have parent/child relationships. In any event, this question wasn't about threads at all anyway -- it was about processes (which do have parent/child relationships).

– David Schwartz
Jan 3 at 6:15

Threads are all peers and don't have parent/child relationships. In any event, this question wasn't about threads at all anyway -- it was about processes (which do have parent/child relationships).

– David Schwartz
Jan 3 at 6:15

add a comment |

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