The example in README doesn't compile about expr HOT 2 CLOSED

#include "expr.h"

// Custom function that returns the sum of its two arguments
static float add(struct expr_func *f, vec_expr_t *args, void *c) {
	(void)f, (void)c;
  float a = expr_eval(&vec_nth(args, 0));
  float b = expr_eval(&vec_nth(args, 1));
  return a + b;
}

static struct expr_func user_funcs[] = {
	{"add", add, 0},
	{NULL, NULL, 0},
};

int main() {
	char* a = (char*)malloc(40);
	const char *s = "add(1,2) + 3";
  struct expr_var_list vars = {0};
  struct expr *e = expr_create(s, strlen(s), &vars, user_funcs);
  if (e == NULL) {
    printf("Syntax error");
		return 1;
	}

	float result = expr_eval(e);
	printf("result: %f\n", result);

	expr_destroy(e, &vars);
	return 0;
}