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结构数组中的内存分配

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airPdata **airport = malloc(sizeof(airport) * (50+1));
    printf("Passes airPdata **airport\n");
//  buffer = malloc(sizeof(char) * (50+1));

// puts the strings into char line
while(fgets(line, 1024, fp) != NULL)
{
    // has pointer value point to line
    value = line;
    printf("Before creating space for struct members\n");
    // creating space for the struct members
    airport[j]->LocID = malloc(sizeof(char)*(50+1));

    airport[j]->fieldName = malloc(sizeof(char)*(50+1));

    airport[j]->city = malloc(sizeof(char)*(50+1));

    printf("after\n");

我正在尝试创建一个结构数组，但我无法弄清楚如何为结构的成员分配内存..它保持segfaulting . LocID，fieldName和city都是char *

编辑***我发现了问题 . 使用双指针不需要分配机场，但仍需要分配机场成员 .

//为struct airPdata **机场分配内存;

// buffer = malloc（sizeof（char）*（50 1））;

// puts the strings into char line
while(fgets(line, 1024, fp) != NULL)
{
    // has pointer value point to line
    value = line;
printf("Yes\n");
    // creating space for the struct members
    airport[j]->LocID = malloc(sizeof(char)*(50+1));    
    airport[j]->fieldName = malloc(sizeof(char)*(50+1));
    airport[j]->city = malloc(sizeof(char)*(50+1));
    j++;
 }

但是，当程序第二次绕着while循环返回并遇到 airport[j]->LocID = malloc 时，程序会出错 .

c memory-management struct dynamic-memory-allocation

2 回答

  • 0

    OP的代码最大的失败是没有为每个 airport[i] 分配内存

    使用 airPdata **airport 和I want to use an array of pointers，代码需要分配2个级别并使用arrray .

    数组内存 airport[]
    分配并分配给 airport[i] （ OP missed this part. ）的每个元素的内存
    内存已分配并分配给各个成员，如 airport[i].LocID

    数组 airport 的内存很简单，如下所示 . airPdata **airport 是指针而不是数组 . 而是使用数组，因为这是规定的设计目标 .

    // define array element count. 
#define AIRPORT_N 100 
// Declare the array.
airPdata *airport[AIRPORT_N];
// Keep tack of how much of the array is used.
size_t n = 0;

    现在分配，读取并开始填充数组，根据需要进行分配 .

    #define AIRPORT_STRING_SIZE (50 + 1)
char line[1024];
while(n < AIRPORT_N && fgets(line, sizeof line, fp)) {
  // Allocate memory for one element of `airport`
  // Notice no cast nor type coded here.
  airport[n] = malloc(sizeof *(airport[n]));
  if (airport[n] == NULL) {
    // Something simple for now.
    fprintf(stderr, "OOM\n");
    break;
  }
  // Create space for each string,  
  // TODO: add check for Out-of-Memory
  airport[n]->LocID = malloc(AIRPORT_STRING_SIZE);
  airport[n]->fieldName = malloc(AIRPORT_STRING_SIZE);
  airport[n]->city = malloc(AIRPORT_STRING_SIZE);

  // Code to parse `line` into `airport[n]` members.

  // Usually the parsing happens first and if successful, the above allocations occur.

  // If the `LocID` string (and others) need not change then 
  //   use below to allocate a right-sized memory
  //   after parsing instead of allocating to some max size, like above.
  airport[n]->LocID = strdup(LocID_string);

  n++;
}

    后来释放了一切

    for (size_t i = 0; i < n; i++) {
  free(airport[i]->LocID);
  free(airport[i]->fieldName);
  free(airport[i]->city);
  free(airport[i]);
}

    细节：请注意以下细微错误 . 它分配的大小为 airport ，类型为 airPdata ** .

    相反，它应该分配 * airport 的大小，类型为 airPdata * .

    通常，所有类型的对象指针都具有相同的大小，但是在C中的所有类型中都没有指定相同的指针 .

    最好分配到类型的去引用指针的大小 . 它更可能编码正确，更容易审查和维护 .

    // airPdata **airport = malloc(sizeof(airport) * (50+1));
airPdata **airport = malloc(sizeof *airport * (50+1));
    回复于
  • 1

    你正在混合使用两种方法 .

    你有一系列连续的机场

    airports -----> ap1 | ap2 | ap3

    B你有一系列指向机场的指针（不一定在内存中彼此相邻）

    airports --> aptr1 | aptr2 | aptr3
               |        |        |
               v        v        v
              ap1       ap2     ap3

    如果你想要A，你的malloc会混合A和B.

    做

    airPdata *airport = malloc(sizeof(airport) * (50+1));

    如果你想B做

    airPdata **airport = malloc(sizeof(airport*) * (50+1));

    然后，您将不得不为每个指针槽分配一个机场对象 .

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