c - 带有位域和 char * 的 fwrite/fread 结构

Question

我正在测试一个自定义结构，其中有位字段和一个 unsigned char *(稍后将分配)。

结构如下:

struct test {
  unsigned int field1 : 1;
  unsigned int field2 : 15;
  unsigned int field3 : 32;
  unsigned int dataLength : 16;
  unsigned char * data;
} 

问题是当我尝试将此结构保存在十六进制文件中时。

例如:

int writeStruct(struct test *ptr, FILE *f) {

   // for data, suppose I know the length by dataLength  : 
   // this throw me : cannot take adress of bit field
   int count;
   count = fwrite( &(ptr->field2), sizeof(unsigned int), 1, f);

   // this throw me : makes pointer to integer without a cast
   count = fwrite( ptr->field2, sizeof(unsigned int), 1, f);

   // same for length
   count = fwrite( htons(ptr->data) , ptr->dataLength, 1,f);       

   // so , how to do that ?
}

fread 也有同样的问题:

int readAnStructFromFile(struct test *ptr, FILE *f) {
   // probably wrong
   fread(ptr->field1, sizeof(unsigned int), 1, f);
}

那么，我怎样才能写/读这样的结构呢？

感谢您的帮助

PS for fread，如果没有这些位字段，这可以工作:How to fread() structs?

Answer 1

无法获取位域的地址。处理它的通常方法是在读/写端使用临时变量或将整个结构保存为单个实体。对于临时变量，它看起来如下所示:

int count;
int field2 = ptr->field2;
count = fwrite( &field2, sizeof(unsigned int), 1, f);
...
int field1;
fread(&field1, sizeof(unsigned int), 1, f);
ptr->field1 = field1;

或者对于整个结构:

count = fwrite( ptr, sizeof(struct test), 1, f);