C. average from string problem with strtok.

Hi everyone i'm practicing with C, this program takes numbers in input but in string and divided with commas ("1,2,3,4") and the output is the average of the numbers. I don't know where I go wrong but it gives me always segmentation fault, can anybody help me?

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

float* separa(char* string){

int dim=0;
int i=0;
float numero;

char* sep = "," ;
char* tokena;
char* token;
char* stringa;
strcpy (stringa, string);

for(token=strtok(stringa,sep); token!=NULL; token=strtok(NULL,sep))
{
dim++;
}

float *vet = (float*)malloc (dim*sizeof(float));

for(tokena=strtok(string,sep); tokena!=NULL; tokena=strtok(NULL,sep))
{
numero=atof(tokena);
vet[i]=numero;
i++;
}

return vet;
}

float media(char* stringa){

int i;
int dim=0;
char* token;
char* sep = ",";
float sum=0;
float risultato=0;
char* stringas;
strcpy/

for(token=strtok(stringas,sep); token!=NULL; token=strtok(NULL,sep))
{
dim++;
}

float* vettore = separa(stringa);
for(i=0;i<dim;i++){

sum = sum + vettore[i];
}

risultato = (sum/dim);

return risultato;

}

int main()
{
char* prova="3,2,1";
float a;
a=media(prova);
printf("%f\n", a);
return 0;
}

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C. average from string problem with strtok.