How can I access the tree in my printpostroder function? - c

Question

This is my tree struct :
typedef struct quad
{
struct quad *child[4];
char *names;
} quad;
and I need to build it then print it in postorder
but I can't access the memory for the tree in my printpostorder function :
void printpostorder(quad * tree)
{
if (tree->names[0] == 'G') {
printpostorder(tree->child[0]);
printpostorder(tree->child[1]);
printpostorder(tree->child[2]);
printpostorder(tree->child[3]);
printf("%s", tree->names);
}
else {
printf("%s", tree->names);
}
}
I can access it before calling this function in the main function.
int main(void){
int n = 0;
int size;
quad * t;
char * str1 = (char *)malloc(MAX * sizeof(char));
printf("Enter name: ");
scanf("%s", str1);
size = strlen(str1);
t = build_preorder_tree(str1,t,&n,size);
printpostorder(t);
}
here is the build tree function, basically we have to build from preorder, then print out the post order.
quad* build_preorder_tree(char *s_r, quad * tree, int * index_ptr,int size){
char c;
int s = 0;
int index = *index_ptr;
c = s_r[index];
char d = ']';
char * ptr = (char *)malloc(MAX * sizeof(char));
char * ptr1;
if(index == size){
return;
}
tree = malloc(sizeof(quad*));
tree -> names = (char *)malloc(MAX * sizeof(char));
if(c == 'G') {
tree->names = "G";
(*index_ptr)++;
tree->child[SW] = build_preorder_tree(s_r, tree->child[SW],index_ptr,size);
tree->child[SE] = build_preorder_tree(s_r, tree->child[SE],index_ptr,size);
tree->child[NW] = build_preorder_tree(s_r, tree->child[NW],index_ptr,size);
tree->child[NE] = build_preorder_tree(s_r, tree->child[NE],index_ptr,size);
}
if(c == 'W') {
tree->names = "W";
(*index_ptr)++;
}
if(c == 'B') {
strcpy(ptr,s_r);
ptr1 = strtok(ptr+index,"]");
strncat(ptr1,&d,1);
s = strlen(ptr1);
(*index_ptr)= (*index_ptr) + s;
tree->names = ptr1;
}
return tree;
}
And when I call the printpostorder(t), a seg fault occurred at first call of
if (tree->names[0] == 'G')
I tried to access the tree before the printpostorder is call, and I was able to access every element of the tree