Removing Non alpha numeric characters (C programming)

removenonalphanum.c

char* stringRemoveNonAlphaNum(char *str)
{
    int i = 0, j = 0;
    int arraysize=strlen(str)+1;
    char c;
    char *temp=malloc(arraysize* sizeof(char)); //allocatin memory for new string

    for(i=0;i<arraysize;i++){
        c=str[i];
        if(isalnum(c)!= 0){ //if it is not a alpha num put in a new string temp
            temp[j++]=c;
        }
       
    }
    temp[j]='\0'; //make last char null
//    return string;
    return temp;
}