这种有效的类型转换与结构数组一起使用吗？

Question

在过去，我一直在努力理解指针算法、类型转换和结构数组，有时即使程序可以运行，它在技术上仍然是不正确的，所以我只想运行你们编写的这段代码并确保我正确理解所有概念。

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

typedef struct
{
  int id, jobType;
  float salary;
} Employee;

Employee * readFileInfo(FILE *fp)
{
  int numEmployees = 0;
  fscanf(fp, "%d", &numEmployees); //find the int at the start of the file to indicate how many employees there are
  int *array = malloc( (sizeof(int)) + ( sizeof(Employee) * numEmployees ) ); //allocate an extra spot for an integer
  *array = numEmployees; //hide the number of employees one integer behind the start of the array, increment on next line
  array++;
  Employee *test = (Employee *) array;
  for(int i = 0; i < 3; i++)
    fscanf(fp, "%d %d %f", &test[i].id, &test[i].jobType, &test[i].salary); //loop through and assign each value to its proper variable
  return test;
}

Employee * getEmployeeByID(Employee *arr, int ID)
{
  for(int i = 0; i < *((int*)arr - 1); i++)
    if(arr[i].id == ID)
        return ((arr) + i); //returns address of specified employee
   return 0;
}

void setID(Employee *p, int ID)
{
  p->id = ID;
  printf("\nset successfuly\n");
}

int getSize(Employee *arr)
{
  return *((int*)arr - 1); //returns the int value hidden before start of array
}

 void getAvgSalary(Employee *arr) //calculates average salary, prints out all employees who make more than that
 {
   float total = 0, avg = 0;
   for(int i = 0; i < getSize((Employee*)arr); i++)
     total += arr[i].salary;

   avg = total / (float)getSize((Employee*)arr);

   for(int i = 0; i < getSize((Employee*)arr); i++)
    if(arr[i].salary >= avg)
        printf("ID: %d has above the avg salary with a salary of $%.2f\n", arr[i].id, arr[i].salary);
 }

 int main(void)
 {
   FILE *fp = fopen("test.txt", "r+");
   Employee *array = readFileInfo(fp);
   Employee *emp2 = getEmployeeByID(array, 2);
   setID(emp2, 5);
   getAvgSalary(array);
 }

---

test.txt
3 1 5 20000 2 1 100000 3 3 50000

基本上，该程序应该打开 test.txt，调用 readFileInfo，它在文件中找到第一个 integer（表示要跟随的员工数量），使用 3 个东西为结构数组分配 memory：大小乘以employee 结构的大小，添加一个额外的 integer 以在数组开始之前“隐藏”该大小变量。 之后，在数组上调用各种函数来试验类型转换/指针算法

任何帮助、更正、建议、观察或回应都会很棒！

Answer 1

代码不正确。 假设float是double ，那么整个结构将具有 alignment 对double的要求，这在 x86-64 上是 8 个字节。 但是您已将其强制为 alignment 4. 它具有未定义的行为，这意味着它可以工作，或者它可能会灾难性地失败。

幸运的是，这个 hack 的需求为零。 只需使用具有灵活数组成员的struct ：

typedef struct
{
  int id, jobType;
  float salary;
} Employee;

typedef struct {
  size_t numEmployees; // use size_t for array sizes and indices
  Employee employees[];
} EmployeeArray;

// allocate sizeof (EmployeeArray) space for the struct
// and sizeof (Employee) space for each array element
EmployeeArray *array = malloc(sizeof *array + sizeof (Employee) * numEmployees);
array->numEmployees = numEmployees;
array->employees[0] = ...