pthread_create changes values of an array

Question

I have a problem with pthread_create. In this test, I create an array of integers, and then try to use them as parameters for a function that must be executed into a thread.

This is where I create the indexes:

int *indexes = (int *) malloc(sizeof(int)*threadNumber);
int i;
for (i = 0; i < threadNumber; i++){
    indexes[i] = i;
}

And this is where I create threads:

int i;
for (i = 0; i < threadNumber; i++){
    printf("%i %i    ", i, indexes[i]);
}
for (i = 0; i < threadNumber; i++){
    printf("%i %i    ", i, indexes[i]);
    pthread_create(sons + sizeof(pthread_t)*i, NULL, sonSimulation, (void *) &indexes[i]);
}

The first printf prints the following:

0 0   1 1   2 2   3 3   4 4

The second one, which is supposed to print the same output, prints this:

0 0   1 1   2 2   3 3   4 23154684

The last number changes each time I executes the code. I'm not able to fix this. Any suggestion?

(sonSimulation just prints the parameter)

What is sons? How is it declared? Why don't you use plain array indexing for it? Also, you're threads doesn't overwrite the pointer they get? — Some programmer dude
– Some programmer dude, Commented Jun 22, 2015 at 8:23
Please post an MCVE. Try sons[i] or sons + i instead of sons + sizeof(pthread_t)*i. Also, please post the sonSimulation function. — Spikatrix
– Spikatrix, Commented Jun 22, 2015 at 8:25
Please see why not to cast the return value of malloc() and family in C. — Sourav Ghosh
– Sourav Ghosh, Commented Jun 22, 2015 at 8:25
This is where i declare sons pthread_t *sons = (pthread_t * ) malloc(sizeof(pthread_t)*threadNumber); — Luca
– Luca, Commented Jun 22, 2015 at 8:44
Please provide the definition of sonSimulation function or if possible the complete program. — Rndp13
– Rndp13, Commented Jun 22, 2015 at 8:47

Community · Accepted Answer · 2017-05-23 12:15:19Z

4

There is a problem here:

pthread_create(sons + sizeof(pthread_t)*i, NULL, sonSimulation, (void *) &indexes[i]);

You allocate sons using

pthread_t *sons = (pthread_t * ) malloc(sizeof(pthread_t)*threadNumber);

So, sons + sizeof(pthread_t)*i in the pthread_create is wrong. When you use sons+i, it automatically moves the sons pointer sizeof(pthread_t) bytes forward because of pointer arithmetic. Using sons + sizeof(pthread_t)*i moves the pointer into invalid memory locations invoking Undefined Behavior.

To fix the problem, use

pthread_create(sons + i, NULL, sonSimulation, (void *) &indexes[i]);

or

pthread_create(&sons[i], NULL, sonSimulation, (void *) &indexes[i]);

instead of

pthread_create(sons + sizeof(pthread_t)*i, NULL, sonSimulation, (void *) &indexes[i]);

Also, as pointed out in the comments, you need not cast the result of malloc(and family) in C.
The void* cast in the last argument of is not needed either as @JoachimPileborg comments.

edited May 23, 2017 at 12:15

CommunityBot

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answered Jun 22, 2015 at 8:49

Spikatrix

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3 Comments

Some programmer dude Over a year ago

That cast in pthread_create for the thread function argument is not needed either.

Spikatrix Over a year ago

I thought that void* can be converted to any other type without a cast, but the opposite needs a cast. So void* can be converted to and from without a cast?

alk Over a year ago

@CoolGuy: "So void* can be converted to and from without a cast?" to/from any other pointer type, yes.

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