Thread/mutex difficulty

[erasm]

How would i implement mutex with this code

Would i pass buffer and mutex into a struct? If so, could someone show me how or reference a tutorial?

pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;


void *print(void* string)
{
        printf("%s\n", string);
}

int main(void)
{

        char buffer[100];
        pthread_t thread;

        pthread_create(&thread, NULL, print, (void*)buffer);

        fgets(buffer, sizeof(buffer), stdin);

        pthread_join(thread, NULL);

}

[Codeplug]

Reading material:
https://computing.llnl.gov/tutorials/pthreads/
http://www.cs.umu.se/kurser/TDBC64/V...ead-primer.pdf

Posix standard:
The Open Group Base Specifications Issue 6 (2004)
The Open Group Base Specifications Issue 7 (2008)

Use your local man pages for API information above everything else.

gg

[erasm]

I am at a dead end with threads. How could i pass the buffer and a mutex to the child. Is the buffer suppose to be locked to avoid conflicts of variable access here? Do i lock the variable in the child function? Would this be the structure i pass into the child?

typedef struct myStruct
{
        pthread_mutex_t mutex_a;
        char buffer[100];

}myStruct;

[MK27]

You haven't described your problem AT ALL, you've just vaguely stated that you cannot get something to work the way you want, but it's not clear what you want or in what it is that happens instead.

I've used pthreads a bit, but never bothered with mutex's because I don't have any possible race conditions set up.

pthread_create(&thread, NULL, print, (void*)buffer);
pthread_detach(thread);

As long as print() is of type void* that should work fine.

[C_ntua]

The mutex can be a global variable. So all functions can use it. Otherwise yeah, you have to pass it with a struct.

No, the buffer isn't locked in any way. You have to do that, if needed. There isn't always the need to do so.

Where is exactly your problem or confusion?

[erasm]

The problem is stdin is not being printed.

The question asks to use a mutex to make the child wait for the parent to read a line before displaying it.

Hint: you will need to pass both the buffer and the mutex tot he child.

Also watchmen is leeeeeeeeeeeeet

[C_ntua]

You have bit confused the functions.

pthread_create will create and run the thread. It will print the un-initialized buffer.
pthread_join suspends the main thread until the child-thread is executed.

So, you first need fgets() and then the pthread_create().
Or do this:

#include <stdlib.h>
#include <stdio.h>
#include <pthread.h>

pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;

void *print(void* string)
{
	pthread_mutex_lock(&mutex);
        printf("%s\n", (char*)string);
	pthread_mutex_unlock(&mutex);
}

int main(void)
{

        char buffer[100];
        pthread_t thread;
	pthread_mutex_lock(&mutex);
        pthread_create(&thread, NULL, print, (void*)buffer);
        fgets(buffer, sizeof(buffer), stdin);
	pthread_mutex_unlock(&mutex);
	pthread_join(thread, NULL);
}

[erasm]

For my reference, how would you implement this with a struct?

[C_ntua]

Using your struct. This will avoid global variables

#include <stdlib.h>
#include <stdio.h>
#include <pthread.h>

typedef struct myStruct
{
        pthread_mutex_t mutex_a;
        char buffer[100];

} myStruct;

void *print(void* data)
{
        myStruct *d = (myStruct *)data;
	pthread_mutex_lock(&(d.mutex_a ));
        printf("%s\n", d.buffer;
	pthread_mutex_unlock(&(d.mutex_a ));
}

int main(void)
{
        myStruct data;
        data.mutex_a = PTHREAD_MUTEX_INITIALIZER;
        pthread_t thread;
	pthread_mutex_lock(&(data.mutex_a ));
        pthread_create(&thread, NULL, print, (void*)(&data));
        fgets(data.buffer, sizeof(data.buffer), stdin);
	pthread_mutex_unlock(&(data.mutex_a ));
	pthread_join(thread, NULL);
}

[erasm]

That code gets these errors. How can i fix this?


sync.c: In function âprintâ:
sync.c:14: error: request for member âmutex_aâ in something not a structure or union
sync.c:15: error: request for member âbufferâ in something not a structure or union
sync.c:16: error: request for member âmutex_aâ in something not a structure or union

[erasm]

#include <stdlib.h>
#include <stdio.h>
#include <pthread.h>

typedef struct myStruct
{
        pthread_mutex_t mutex_a;
        char buffer[100];

} myStruct;

void *print(void* data)
{
        myStruct *d = (myStruct *)data;
        pthread_mutex_lock(&(d->mutex_a ));
        printf("%s\n", d->buffer);
        pthread_mutex_unlock(&(d->mutex_a ));
}

int main(void)
{
        myStruct data;
        data.mutex_a = PTHREAD_MUTEX_INITIALIZER;
        pthread_t thread;
        pthread_mutex_lock(&(data.mutex_a ));
        pthread_create(&thread, NULL, print, (void*)(&data));
        fgets(data.buffer, sizeof(data.buffer), stdin);
        pthread_mutex_unlock(&(data.mutex_a ));
        pthread_join(thread, NULL);
}

This code now gets the error:
error: expected expression before â{â token
bafflezored i am

[cas]

PTHREAD_MUTEX_INITIALIZER is an initializer, and as such should be used during initialization only, not for assignment. Either initialize your struct, or use pthread_mutex_init().

Incidentally, those casts are unnecessary in C. Object pointers can be converted to void* and back without any need for explicit conversion.

[Sebastiani]

Eh? Care to tell us what line? Compiles fine for me.

[erasm]

How would i initialize the mutex?

[quzah]

You should have just said: "Hi, I don't know anything about threads, and I don't want to actually learn anything either. Please do it for me."

It would have saved us all some time.


Quzah.