Pass a struct to pthread_create's startup routine

Question

So I've got an assignment that I'm having trouble with. I'm trying use pthreads to sum the elements of a matrix with 3 different processors. I have a struct

typedef struct{
    int rows;
    int cols;
    pid;
    int localsum;
}ThreadData;

some global variabls

int processors=3;
int rows=4;
int cols=4;
int matrix[10][10];

and a sum function

void *matrixSum(void *p){

    //cast *a to struct ThreadData?
    int sum=0;
    int i=p->pid;
    int size=p->rows*p->cols;

    //to sequentially add a processor's 'owned' cells
    int row=p-pid/p-cols;
    int col=p-pid%p->cols;

    int max_partition_size = ((size/processors)+1);

    for(i;i<max_partition_size*processors;i+=processors){
        col=i%p->cols;
        row=i/p->cols;

        if(i<=size-1){
            sum+=matrix[row][col]+1;
        }
    }

    p->localsum=sum;
}

so my main method looks like this:

int main(){

int  totalsum=0;

ThreadData *a;
a=malloc(processors*(sizeof(ThreadData));
int i;
for(i=0;i<processors;i++){
    a[i].rows=rows;
    a[i].cols=cols;
    a[i].pid=i;
    a[i].localsum=0;
}

//just a function that iterates over the matrix to assign it some contents
fillmatrix(rows, cols);

pthread_t tid[processors];
for(i=0;i<processors;i++){
    pthread_create(tid,NULL,matrixSum,(void *)&a);
    totalsum+=a[i].localsum;
}
pthread_join();
}

My ultimate goal is to pass my matrixSum() with a ThreadData struct as the argument.

So I think I have to cast the void pointer given in matrixSum() to a struct, but I'm having trouble doing so.

I tried doing so like this

ThreadData *a=malloc(sizeof(ThreadData));
a=(struct ThreadData*)p;

But I get a warning: assignment from incompatible pointer type error. So what's the proper way to do this - that is to cast the void pointer taken from the parameters, and operate on it like the struct it is meant to be?

Answer 1

Try using a=(ThreadData*)p;.

In C language, struct ThreadData is differ to ThreadData.

In this case, you used typedef and defined no tag to the struct, so you mustn't use struct to use the struct.