Answer.

In C, structures (structs) can include other structures and various data types. However, the placement of fields within a structure can affect its size and alignment. The compiler adds extra bytes (padding) to comply with the alignment based on the largest field. This is important for performance and correct operation of the processor.

If a structure contains nested structures, alignment is applied recursively:

The size of the structure is always a multiple of the alignment of the largest field.
Nested structures may lead to unexpected padding, causing sizeof(struct) to be larger than the sum of sizeof(fields).

How to control size?

Sort fields by decreasing size.
Use alignment directives (e.g., #pragma pack in MSVC or the __attribute__((packed)) attribute in GCC/Clang).

Example:

#include <stdio.h>

struct Inner {
  char c;
  int x;
};

struct Outer {
  char a;
  struct Inner b;
  short s;
};

int main() {
  printf("sizeof(struct Inner) = %zu\n", sizeof(struct Inner));
  printf("sizeof(struct Outer) = %zu\n", sizeof(struct Outer));
  return 0;
}
// sizeof(struct Inner) = 8 (on 64-bit), sizeof(struct Outer) = 16 or 24

Using pack and packed reduces size by ignoring alignment, but this may decrease performance or cause errors on some platforms (ARM, DSP, etc.).

Trick question.

Question: Can you guarantee the same size of the same structure on all platforms?

Answer: No! The C standard does not guarantee the same alignment, byte order (endian), and may "add" padding between fields and at the end of the structure. For binary compatibility of structures, use manual alignment control and always explicitly test sizeof on each target platform.

Example:

struct Data {
  char c;
  int x;
};
// sizeof(Data) is usually 8 on 64-bit, 5 or 8 on 32-bit

History

When exchanging a structure between a microcontroller (ARM) and a PC through a binary protocol, the data received from ARM did not match expectations on PC — due to different padding and byte order. This led to incorrect parameter decoding, resulting in physical device control errors.

History

In a network protocol, packed structures were added to speed up data transmission, but it was not considered that on a platform without alignment, individual fields did not work (the processor did not support misaligned access). The result: hardware exceptions when working with network packets.

History

The server was working with an external file consisting of structure records. After a compiler update, the size of the structure changed (different alignment). Old data in the file stopped being read correctly, and part of the information became "corrupted."

Explain the mechanics of nested structures and struct alignment in C. How to control the size of a structure, and what problems may arise?

Answer.

How to control size?

Trick question.