带位域​​和char的fwrite / fread结构*

Question

I am testing a custom structure where there is bitfield and a unsigned char * (that will be allocated later).

Here is the structure :

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

The problem is when I tried to save this struct inside a file in hex.

For example :

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 ?
}

The same problem goes to fread :

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

So , how can I write/read struct like this ?

Thanks for your help

PS for fread, this could work if there wasn't these bitfields: How to fread() structs?

Answer 1

there is no way to get an address of a bitfield. The usual way to handle it is to either use a temporary variable on read/write sides or just to save the whole struct as a single entity. With temp vars it looks like the following:

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;

or for the whole struct:

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