15.6 Packed Structures

In GNU C you can force a structure to be laid out with no gaps by adding __attribute__((packed)) after struct (or at the end of the structure type declaration). Here’s an example:

struct __attribute__((packed)) foo
{
  char a;
  int c;
  char b;
};

Without __attribute__((packed)), this structure occupies 12 bytes (as described in the previous section), assuming 4-byte alignment for int. With __attribute__((packed)), it is only 6 bytes long—the sum of the lengths of its fields.

Use of __attribute__((packed)) often results in fields that don’t have the normal alignment for their types. Taking the address of such a field can result in an invalid pointer because of its improper alignment. Dereferencing such a pointer can cause a SIGSEGV signal on a machine that doesn’t, in general, allow unaligned pointers.

See Attributes in Declarations.