variable length struct in gnuc

问题:

Is it able to define a variable length struct in gnuc to represent a object as follow:

field1: fixed 4bytes;
field2: length of field3
field3: variable length
field4: length of field5
field5: variable length
field6: fixed 8bytes
field7: fixed 1byte

I know in gnuc we can use the zero-size array to implement a variable length struct, e.g.

typedef struct varStruct{
  int foo1;
  int foo2[0];
}varStruct;

But the above usage requires the variable length field placed at the tail of the struct. What if they are in the middle?

回答1:

You can't have a struct with more than one variable array or a variable array in the middle. Think about it, how would the compiler know where field3 and field4 start if the length field2 is variable?

If field1 contains the length of the next two fields, you could read the members of the struct manually. Example code (read it as pseudocode):

#define PADDING 8 /* or perhaps sizeof(void *) */

typedef struct {
   int32_t field_len;
   char data[0];
} main_str_t;

typedef struct {
   int64_t one;
   int8_t another;
} tail_str_t;

....

main_str_t *data = get_data();
int32_t len = data->field_len;
int32_t padded_len = ((field_len + PADDING - 1) / PADDING) * PADDING;
char *field2 = data->data;
char *field3 = field2 + padded_len;
tail_str_t *tail = field3 + padded_len;

回答2:

Not possible, since the compiler (is responsible for and) will not be able to calculate offsets for field4 and field5.

回答3:

You'll need to do this with 3 structs and do a little address arithmetic at run-time:

struct1:
  field1: fixed 4bytes;
  field2: length of field3
  field3: variable length

struct2:
  field4: length of field5
  field5: variable length

struct3:
  field6: fixed 8bytes
  field7: fixed 1byte