c - 使用指针将结构体添加到数组

Question

我对 C 语言和一般的内存分配/使用指针相当陌生。不管怎样，我正在尝试读取文件，将这些值放入结构中，等等。我确切地知道我想要做什么，当然程序会运行，但输出不正确，并且出现某种困惑的数字和字母。

有一个文本文件，其中每行都有新信息。每一行代表一个对象。

文件中的一行可能如下所示:

meat sirloin 6.55 8 8.50 4

Overall I want to be able to store all of my PRODUCT objects in an array (so I have an array of structs). So I attempted to allocate memory with a pointer, use a line count, and then send the pointer to a function called read. In read I add each struct to the array via a pointer. The program doesn't crash, the output is just not correct and I have no clue why not. It's prob something with pointers. If anyone could help me I would really appreciate it. Any help at all would be great.

      //prototype
void read(pointerToArr);


typedef struct
{
    char supType[15];
    char prodName[15];
    double wholePrice;
    int quantWhole;
    double retPrice;
    int retProdQuantity;
}PRODUCT;

FILE *fr;
int lineCount = 0;
int main()
{
    PRODUCT *ptr;

    int i;
    char check[50];


     fr = fopen("ttt.txt", "r");

      while(fgets(check, sizeof(check), fr)!= NULL)
      {
          if(check[0] != '\n')
          {
              lineCount++;
          }
      }
    // allocate memory for array based on line count.
     PRODUCT prodContainter[lineCount];
     ptr = (PRODUCT*)malloc(sizeof(PRODUCT)*lineCount);
     ptr = prodContainter;

     read(ptr);

      //print after adding to array via pointer from other
      //function. this was a test.

      for(i = 0; i < lineCount; i++)
      {
          printf("%s ", prodContainter[i].supType);
          printf("%s ", prodContainter[i].prodName);
          printf("%f ", prodContainter[i].wholePrice);
          printf("%d ", prodContainter[i].quantWhole);
          printf("%f ", prodContainter[i].retPrice);
          printf("%d\n\n", prodContainter[i].retProdQuantity);

      }

    return 0;
}



void read(PRODUCT *pointerToArr)
{

    // objective in this method is to read in data from the file, create an object for every line and
    // then use the pointer array to add those objects to prodConstainer up above.

       char supplyName[15];
       char productName[15];
       double wholeP = 0;
       int  quantityWhole = 0;
       double retailPrice = 0;
       int retailProductQuant = 0;

    while(fscanf(fr, "%s %s %lf %d %lf %d", supplyName, productName, &wholeP, &quantityWhole, &retailPrice, &retailProductQuant) == 6)
    {
        PRODUCT record;
        int i;

        strcpy(record.supType, supplyName);
        strcpy(record.prodName, productName);
        record.wholePrice = wholeP;
        record.quantWhole = quantityWhole;
        record.retPrice = retailPrice;
        record.retProdQuantity = retailProductQuant;

        for(i = 0; i < lineCount; i++)
        {
            pointerToArr[i] = record;
        }
    }

    fclose(fr);
}

Answer 1

您永远不会倒带文件，因此计算行数后的所有读取都会失败。

您打印的内容正是内存中的内容。

当然，有很多方法可以解决这个问题。

1. 使用rewind()倒带文件
2. 关闭文件并让您的 read() 函数(顺便说一句，其名称与 POSIX 标准函数冲突)重新打开该文件。这还涉及删除可怕的全局变量fr。
3. 重新构建结构，这样您就无需计算行数，只需读取 ptr 数组并让其根据需要增长(请参阅 realloc() )。

此外，you should really avoid casting the return value of malloc() in C 。这:

ptr = (PRODUCT*)malloc(sizeof(PRODUCT)*lineCount);

最好写成:

ptr = malloc(lineCount * sizeof *ptr);

这消除了强制转换，并且还对指向的类型的值使用 sizeof 来自动计算要分配的正确字节数。