Passing Entire Array of Structs to Pthread_Create in C

Question

I am having a bit of trouble with this one issue. I have an array that holds 2 structs. I need to pass that array into a thread function and be able to access and manipulate both structs in that array. All the examples I've looked at and found typically pass each element of the array to pthread_create, but in the context of my program, I can only call one instance of pthread_create at a time, so I cannot pass each element in the array one by one. Here is the simplified code I am looking at right now (let me know if you need more relevant information):

typedef struct foo
{
    int a;
    char b[100];
}foo;


void *threadFunction(void *arr);
.
.
.
int main(int argc, char *argv[])
{
    foo temp1, temp2;
    //Somewhere between here temp1 and temp2 are initialized and filled
    foo arr[2];
    arr[0] = temp1;
    arr[1] = temp2;
    pthread_create(&thread, &attr, threadFunction, (void *)arr);
.
.
.
}
void *threadFunction(void *arr)
{       
    foo *temp = (foo *)malloc(2 * sizeof(foo));
    temp = (foo *)arr;
    .
    .
    .
    free(temp);
    pthread_exit(NULL);
}

I also tried just saying

foo *temp = (foo *)arr;

The obvious solution was to assign each element in temp[] to an element in arr[] but that doesn't work.

The result is I only have access to the first element in the temp array, even if I try to access temp[1], which makes sense since it doesn't appear that I am assigning the 2nd element in arr[] to anything.

I'm sure I'm missing something obvious, but I can't seem to find it, and I haven't been able to find any relevant information about this specific situation. Thanks for any help.

Blagovest Buyukliev · Accepted Answer · 2011-02-25 21:59:02Z

1

It involves casting the void pointer to the pointer type of your struct, and then dereferencing that pointer expression to get the element at a particular offset:

*((foo *) arr); /* first element */
*((foo *) arr + 1); /* second element */

Edit: another important issue - the arr array has to be defined globally, outside of main. Because the newly created thread will have its own stack space, it's not very safe to access another thread's stack.

edited Feb 25, 2011 at 21:59

answered Feb 25, 2011 at 21:30

Blagovest Buyukliev

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4 Comments

235711131719 Over a year ago

I can't seem to get the first statement to work, since I have to uncast the void *. I have foo **temp = (foo *)arr. It gives me invalid initializer warning and then the assignment statements give me errors for void * not ignored as it ought to be.

Blagovest Buyukliev Over a year ago

void pointers don't have to be casted - they are meant to be generic and can be assigned directly to any other pointer type

Blagovest Buyukliev Over a year ago

After examining your code more closely, I discovered that my answer is not quite right, check my edit.

235711131719 Over a year ago

Ok it's compiled now, thanks! I'll see if it actually does what I want now, but hopefully the issues behind this question are solved. Thanks a lot.

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