结构与未知大小的结构数组

Question

我一整天都试图绕过这个......

基本上,我有一个名为State的结构,它有一个名称,另一个名为StateMachine,带有名称,状态数组和添加的状态总数:

#include <stdio.h>
#include <stdlib.h>

typedef struct State {
  const char * name;

} State;

typedef struct StateMachine {
  const char * name;

  int total_states;
  State ** states;

} StateMachine;

StateMachine * create_state_machine(const char* name) {
  StateMachine * temp;

  temp = malloc(sizeof(struct StateMachine));

  if (temp == NULL) {
    exit(127);
  }

  temp->name = name;
  temp->total_states = 0;

  temp->states = malloc(sizeof(struct State));
  return temp;
}

void destroy_state_machine(StateMachine* state_machine) {
  free(state_machine);
}

State * add_state(StateMachine* state_machine, const char* name) {
  State * temp;

  temp = malloc(sizeof(struct State));

  if (temp == NULL) {
    exit(127);
  }

  temp->name = name;

  state_machine->states[state_machine->total_states]= temp;
  state_machine->total_states++;

  return temp;
}

int main(int argc, char **argv) {

  StateMachine * state_machine;

  State * init;
  State * foo;
  State * bar;

  state_machine = create_state_machine("My State Machine");

  init = add_state(state_machine, "Init");
  foo  = add_state(state_machine, "Foo");
  bar  = add_state(state_machine, "Bar");

  int i = 0;

  for(i; i< state_machine->total_states; i++) {
    printf("--> [%d] state: %s\n", i, state_machine->states[i]->name);
  }

}

出于某种原因(阅读低C-fu/ruby​​/python/php的年份)我无法表达状态是状态数组的事实.上面的代码打印:

--> [0] state: ~
--> [1] state: Foo
--> [2] state: Bar

第一个州增加了什么？

如果我在添加的第一个状态上malloc状态数组(例如state_machine = malloc(sizeof(temp));那么我得到第一个值而不是第二个值.

有什么建议吗？

这是一个C问题.我正在使用gcc 4.2.1编译样本.

Answer 1

看起来你没有为机器中的状态分配空间而不是第一个.

StateMachine * create_state_machine(const char* name) {
  StateMachine * temp;

  temp = malloc(sizeof(struct StateMachine));

  if (temp == NULL) {
    exit(127);
  }

  temp->name = name;
  temp->total_states = 0;

  temp->states = malloc(sizeof(struct State)); // This bit here only allocates space for 1.
  return temp;
}

你可能最好在状态机结构中放置一个固定大小的状态数组.如果这不合适,你将不得不重新分配并移动整个集合或分配块并跟踪当前长度,或制作链表.

顺便说一句,init,foo和bar永远不会被使用.

编辑:我建议的内容如下:

#define MAX_STATES 128 // Pick something sensible.
typedef struct StateMachine {
  const char * name;
  int total_states;
  State *states[MAX_STATES];
} StateMachine;

Answer 2

看起来你想在每个状态机中拥有可变数量的状态,但是你错误地分配了内存.在create_state_machine,这一行:

temp->states = malloc(sizeof(struct State));

分配单个State对象,而不是指针数组(这是您使用它的方式).

有两种方法可以改变这种情况.

1. 声明states为State states[<some-fixed-size>];但是你不能拥有超过固定数量的状态.
2. 添加另一个成员以指示已分配了多少存储空间states,因此您可以跟踪它以及使用的数量(这是total_states用于的内容).

后者看起来像这样:

#include <stdlib.h>
#include <string.h>

typedef struct 
{
    const char *name;
} State;

typedef struct 
{
    const char *name;
    int total_states;
    int states_capacity;
    State *states;
} StateMachine;

StateMachine *create_state_machine(const char *name)
{
    StateMachine *temp = malloc(sizeof(StateMachine));
    memset(temp, 0, sizeof(*temp));

    temp->name = name;
    temp->states_capacity = 10;
    temp->states = malloc(sizeof(State) * temp->states_capacity);

    return temp;
}

State *add_state(StateMachine *machine, const char *name)
{
    if (machine->total_states == machine->states_capacity)
    {
        // could grow in any fashion.  here i double the size, could leave
        // half the memory wasted though.
        machine->states_capacity *= 2;

        machine->states = realloc(
            machine->states, 
            sizeof(State) * machine->states_capacity);
    }

    State *state = (machine->states + machine->total_states);
    state->name = name;

    machine->total_states++;

    return state;
}