# 2020q3 Homework6 (kcalc)
contributed by < `Rsysz` >.
###### tags: `sysprog`

{%hackmd theme-dark %}

## kcalc原理
首先必須先了解 `Character Device` 程式的結構與原理，接著才看內部數值計算方式，了解完 `Dev` 的使用方式後，這邊先看到 `main.c` 內的 `calc`，這邊使用到了 `struct expr` 這個結構體與 `expr_create` 對其初始化

static void calc(void)
{
    struct expr_var_list vars = {0};
    struct expr *e = expr_create(message, strlen(message), &vars, user_funcs);
    if (!e) {
        pr_alert("Syntax error");
        return;
    }

    result = expr_eval(e);
    pr_info("Result: %" PRIu64 "\n", result);
    expr_destroy(e, &vars);
}

再來看到 `expression.h` 與 `expression.c` 內定義了對應的結構體

struct expr {
    int type;
    union {
        struct {
            uint64_t value;
        } num;
        struct {
            uint64_t *value;
        } var;
        struct {
            vec_expr_t args;
        } op;
        struct {
            struct expr_func *f;
            vec_expr_t args;
            void *context;
        } func;
    } param;
};

/*
 * Functions
 */
struct expr_func {
    const char *name;
    exprfn_t f;
    exprfn_cleanup_t cleanup;
    size_t ctxsz;
};

struct expr_func *expr_func(struct expr_func *funcs, const char *s, size_t len);

/*
 * Variables
 */
struct expr_var {
    uint64_t value;
    struct expr_var *next;
    char name[];
};

struct expr_var_list {
    struct expr_var *head;
};

struct expr_var *expr_var(struct expr_var_list *vars,
                          const char *s,
                          size_t len);

如下圖所示

digraph EXPR{
        
        rankdir=LR
        subgraph cluster{
        label = "expr"
        type [label="int type" shape=box]
            subgraph cluster{
            label ="param"
            f [label="func" shape=box]
            num [label="num" shape=box]
            var [label="var" shape=box]
            op [label="op" shape=box]
            {rank=same}
            }
        }
	}