Syscall's calc

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

typedef enum operation_type{
    plus, minus,
    times, divide
} operation_type;

typedef struct operation{
    operation_type type;
    double *lnum, *rnum;
    int priority;
    struct operation *next;
} operation;

double evop(operation_type type, double lnum, double rnum){
    switch(type){
        case plus:
            return lnum + rnum;
        case minus:
            return lnum - rnum;
        case times:
            return lnum * rnum;
        case divide:
            return lnum / rnum;
    }
}

void ev(operation **root){
    if((*root) -> next && (*root) -> next -> priority > (*root) -> priority)
        ev(&(*root) -> next);
    * (*root) -> lnum = evop((*root) -> type, * (*root) -> lnum, * (*root) -> rnum);
    free((*root) -> rnum);
    if((*root) -> next){
        (*root) -> next -> lnum = (*root) -> lnum;
        (*root) = (*root) -> next;
        ev(root);
        free(root);
    }
}

int main(){
    char *exprMem = malloc(sizeof(char)), *expr = exprMem;
    assert(scanf("%100[^\n]", &expr));
    char *end = strlen(expr) + expr;
    double result;
    operation *opptr, *root = malloc(sizeof(operation));
    opptr = root;
    root -> lnum = &result;
    int adpr = 0;   //Additional priority
    while(expr <= end && (isspace(*expr) || *expr == '(')){
        if(*expr == '(') adpr += 2;
    }
    sscanf(expr, "%lf", root -> lnum);
    while(expr <= end){
        expr += strspn(expr, ".0123456789");
        opptr -> next = malloc(sizeof(operation));
        opptr -> next -> lnum = opptr -> rnum;
        opptr = opptr -> next;
        opptr -> next = NULL;
        opptr -> rnum = malloc(sizeof(double));
        while(expr <= end && (isspace(*expr) || *expr == ')')){
            if(*expr == ')') adpr -= 2;
        }
        char op;
        expr += sscanf(expr, "%c", &op); //symbol is 1 byte long, so if pattern was read, there was only one byte
        switch(op){
            case '+':
                opptr -> type = plus;
                opptr -> priority = 0 + adpr;
                break;
            case '-':
                opptr -> type = minus;
                opptr -> priority = 0 + adpr;
                break;
            case '*':
                opptr -> type = times;
                opptr -> priority = 1 + adpr;
                break;
            case ':':
            case '/':
                opptr -> type = divide;
                opptr -> priority = 1 + adpr;
                break;
            default:
                fprintf(stderr, "Error, operation %c is not allowed!", op);
        }
        while(expr <= end && (isspace(*expr) || *expr == '(')){
            if(*expr == '(') adpr += 2;
        }
        expr += sscanf(expr, "%lf", opptr -> rnum);
    }
    ev(&root);
    free(root);
    free(exprMem);
    printf("=%lf\n", result);
}