Question

过去，我一直在努力理解指针算术，类型转换和结构数组，有时即使该程序可以运行，但从技术上来说还是不正确的，因此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来查找文件中的第一个整数（表示要跟随的雇员数），使用3种东西为结构数组分配内存：那个大小号乘以员工结构的大小，然后在数组开始之前添加一个额外的整数以“隐藏”该大小变量。之后，在数组上调用各种函数以进行类型转换/指针算法实验

任何帮助，更正，建议，观察或回应都是很棒的！

Answer 1

代码不正确。假设float改为double，则整个结构将具有double的对齐要求，在x86-64上为8字节。但是您已将其强制对齐4。它具有未定义的行为，这意味着它可以工作，也可以fail catastrophically。

幸运的是，此黑客的需求为零。只需将struct与 flexible数组成员一起使用即可

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] = ...

这种有效的类型转换可用于结构体数组吗？

1 个答案: