Why can't C compilers rearrange struct members to eliminate alignment padding? [duplicate]

Solution 1:

There are multiple reasons why the C compiler cannot automatically reorder the fields:

• The C compiler doesn't know whether the struct represents the memory structure of objects beyond the current compilation unit (for example: a foreign library, a file on disc, network data, CPU page tables, ...). In such a case the binary structure of data is also defined in a place inaccessible to the compiler, so reordering the struct fields would create a data type that is inconsistent with the other definitions. For example, the header of a file in a ZIP file contains multiple misaligned 32-bit fields. Reordering the fields would make it impossible for C code to directly read or write the header (assuming the ZIP implementation would like to access the data directly):

  struct __attribute__((__packed__)) LocalFileHeader {
      uint32_t signature;
      uint16_t minVersion, flag, method, modTime, modDate;
      uint32_t crc32, compressedSize, uncompressedSize;
      uint16_t nameLength, extraLength;
  };
  

  The packed attribute prevents the compiler from aligning the fields according to their natural alignment, and it has no relation to the problem of field ordering. It would be possible to reorder the fields of LocalFileHeader so that the structure has both minimal size and has all fields aligned to their natural alignment. However, the compiler cannot choose to reorder the fields because it does not know that the struct is actually defined by the ZIP file specification.

• C is an unsafe language. The C compiler doesn't know whether the data will be accessed via a different type than the one seen by the compiler, for example:

  struct S {
      char a;
      int b;
      char c;
  };
  
  struct S_head {
      char a;
  };
  
  struct S_ext {
      char a;
      int b;
      char c;
      int d;
      char e;
  };
  
  struct S s;
  struct S_head *head = (struct S_head*)&s;
  fn1(head);
  
  struct S_ext ext;
  struct S *sp = (struct S*)&ext;
  fn2(sp);
  

  This is a widely used low-level programming pattern, especially if the header contains the type ID of data located just beyond the header.

• If a struct type is embedded in another struct type, it is impossible to inline the inner struct:

  struct S {
      char a;
      int b;
      char c, d, e;
  };
  
  struct T {
      char a;
      struct S s; // Cannot inline S into T, 's' has to be compact in memory
      char b;
  };
  

  This also means that moving some fields from S to a separate struct disables some optimizations:

  // Cannot fully optimize S
  struct BC { int b; char c; };
  struct S {
      char a;
      struct BC bc;
      char d, e;
  };
  

• Because most C compilers are optimizing compilers, reordering struct fields would require new optimizations to be implemented. It is questionable whether those optimizations would be able to do better than what programmers are able to write. Designing data structures by hand is much less time consuming than other compiler tasks such as register allocation, function inlining, constant folding, transformation of a switch statement into binary search, etc. Thus the benefits to be gained by allowing the compiler to optimize data structures appear to be less tangible than traditional compiler optimizations.

Solution 2:

C is designed and intended to make it possible to write non-portable hardware and format dependent code in a high level language. Rearrangement of structure contents behind the back of the programmer would destroy that ability.

Observe this actual code from NetBSD's ip.h:

/*
 * Structure of an internet header, naked of options.
 */
struct ip {
#if BYTE_ORDER == LITTLE_ENDIAN
    unsigned int ip_hl:4,       /* header length */
             ip_v:4;        /* version */
#endif
#if BYTE_ORDER == BIG_ENDIAN
    unsigned int ip_v:4,        /* version */
             ip_hl:4;       /* header length */
#endif
    u_int8_t  ip_tos;       /* type of service */
    u_int16_t ip_len;       /* total length */
    u_int16_t ip_id;        /* identification */
    u_int16_t ip_off;       /* fragment offset field */
    u_int8_t  ip_ttl;       /* time to live */
    u_int8_t  ip_p;         /* protocol */
    u_int16_t ip_sum;       /* checksum */
    struct    in_addr ip_src, ip_dst; /* source and dest address */
} __packed;

That structure is identical in layout to the header of an IP datagram. It is used to directly interpret blobs of memory blatted in by an ethernet controller as IP datagram headers. Imagine if the compiler arbitrarily re-arranged the contents out from under the author -- it would be a disaster.

And yes, it isn't precisely portable (and there's even a non-portable gcc directive given there via the __packed macro) but that's not the point. C is specifically designed to make it possible to write non-portable high level code for driving hardware. That's its function in life.