How Do I Remove This Linker Error In This Compailation Can Any One Tell Me

#include<stdio.h>
#include<conio.h>
struct NODE
    {
        char data;
 
        struct NODE *lchild;
 
        struct NODE *rchild;
    };
 
typedef struct NODE *node;
 
node exptree( char[] );
void push_cha( node );
void push_op( char );
node pop_cha();
char pop_op();
void inorder( node );
int prec( char );
node create()
{
    return ( ( node ) malloc( sizeof( node ) ) );
}
 
node newnode = NULL, root, temp, nstack[ 50 ];
int optop = -1, ntop = -1, p, i, opstack[ 50 ];
 
int main()
{
    char str[ 50 ];
    int z;
    printf( "\nENTER EXPRESSION" );
    scanf( "%s", str );
    root = exptree( str );
    printf( "\nThe inorder expression is:" );
    inorder( root );
    getch();
    return 0;
}
 
node exptree( char str[] )
{
    for ( i = 0;str[ i ] != '\0';i++ )
        {
            if ( isalnum( str[ i ] ) )
                {
                    newnode = create();
                    newnode->data = str[ i ];
                    newnode->lchild = NULL;
                    newnode->rchild = NULL;
                    push_cha( newnode );
                }
            else if ( str[ i ] == '(' || str[ i ] == '+' || str[ i ] == '-' || str[ i ] == '*' || str[ i ] == '/' )
                {
                    if ( str[ i ] == '(' || ( optop == -1 ) )
                        push_op( str[ i ] );
                    else if ( prec( str[ i ] ) > prec( opstack[ optop ] ) )
                        push_op( str[ i ] );
                    else
                        {
                            while ( ( prec( str[ i ] ) <= prec( opstack[ optop ] ) && ( opstack[ optop ] != '(' ) ) && ( optop >= 0 ) )
                                {
                                    newnode = create();
                                    newnode->data = pop_op();
                                    newnode->rchild = pop_cha();
                                    newnode->lchild = pop_cha();
                                    push_cha( newnode );
                                }
 
                            push_op( str[ i ] );
                        }
                }
            else if ( str[ i ] == ')' )
                {
                    while ( opstack[ optop ] != '(' )
                        {
                            newnode = create();
                            newnode->data = pop_op();
                            newnode->rchild = pop_cha();
                            newnode->lchild = pop_cha();
                            push_cha( newnode );
                        }
 
                    pop_op();
                }
            else
                printf( "\nInvalid character" );
        }
 
    while ( optop != -1 )
        {
            newnode = create();
            newnode->data = pop_op();
            newnode->rchild = pop_cha();
            newnode->lchild = pop_cha();
            push_cha( newnode );
        }
 
    return pop_cha();
}
 
void push_cha( node ptr )
{
    ntop = ntop + 1;
    nstack[ ntop ] = ptr;
}
 
void push_op( char str )
{
    opstack[ ++optop ] = str;
}
 
node pop_cha()
{
    node ptr;
    ptr = nstack[ ntop ];
    ntop = ntop-1;
    return ptr;
}
 
int prec( char k )
{
    int p;
 
    switch ( k )
        {
 
        case '^':
            p = 3;
            break;
 
        case '*':
            p = 2;
            break;
 
        case '/':
            p = 2;
            break;
 
        case '+':
            p = 1;
            break;
 
        case '-':
            p = 1;
            break;
 
        case '(':
            p = 0;
            break;
        }
 
    return p;
}
 
char pop_op()
{
    char d;
    d = opstack[ optop ];
    optop = optop-1;
    return ( d );
}
 
void inorder( node temp )
{
    if ( temp != NULL )
        {
            inorder( temp->lchild );
            printf( "%c ", temp->data );
            inorder( temp->rchild );
        }
}