Pass a struct to pthread_create's startup routine - c

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?


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.

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I'm trying to dynamically create an array of ints for grades that are inside of a structure but I have a small syntax error. Here is my code:
typedef struct
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int Stud_ID;
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The error I get says: assignment makes integer from pointer without cast.

enroll->enrollment_list[enroll->Enrollment_Count].Grade_Cap = malloc(sizeof(int)*2);
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Deferencing Void Pointer in c

i'm having troubles trying to deference a void pointer in C.
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typedef struct nodo
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char label; // S si es un switch, E si es un enemigo
struct nodo *siguiente;
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typedef struct
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char(*siguiente_movimiento)(void *,char **,int,int);
char tipo;
int maxmov;
int pasos;
char direccion;
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void *general;
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And here's my problem, i can't give the "info" that is located general, i read something about cast the void pointer, but i failed miserably.
char siguiente_movimiento(void *general, char **map, int x, int y)
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tEnemigo *enemigo;
enemigo = *(tEnemigo *) general ;
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I don't know how to do this, maybe i'm wrong with all my "idea" of the code...hope you guys can give me a hand.
EDIT: THIS IS THE ERROR CODE.
error: incompatible types when assigning to type ‘tEnemigo’ from type ‘struct tEnemigo *’
*(tEnemigo *) general = enemigo;

In your siguiente_movimiento function, enemigo is a pointer, but you are trying to assign a value to it that is not a pointer (because you dereference it with *). Your function should look like this:
char siguiente_movimiento(void *general, char **map, int x, int y)
{
tEnemigo enemigo;
enemigo = *(tEnemigo *) general ;
}
Or:
char siguiente_movimiento(void *general, char **map, int x, int y)
{
tEnemigo *enemigo;
enemigo = (tEnemigo *) general ;
}
You currently have a mix of the two.

The error message gives the game away — you haven't declared a type struct tEnemigo.
You have:
typedef struct
{
...
}tEnemigo;
You either need to drop the struct when using the type, or you need:
typedef struct tEnemigo
{
...
} tEnemigo;
Both techniques work. I'd probably add the tag anyway, but I'd definitely write the code in terms of the typedef name (just tEnemigo), not struct tEnemigo.
And, as a side note, you should aim to avoid void * arguments. One advantage of the struct tEnemigo tag is that you can define opaque types by simply saying:
struct tEnemigo; // There is a type struct tEnemigo...
char siguiente_movimiento(struct tEnemigo *enemigo, char **map, int x, int y);
Your code that uses struct tEnemigo pointers does not need to know about the internals of the structure; only code that needs to look inside the structure needs to know the full type definition.
One reason for avoiding void * arguments is that any pointer type can be passed to a function that expects a void *. For example, if you have a FILE *fp = fopen("something", "r);, then the compiler cannot diagnose an error in:
char c = siguiente_movimiento(fp, map, 1, 3);
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