EXP03-A. Do not assume the size of a structure is the sum of the of the sizes of its members

According to ISO/IEC 9899-1999,

There may be unnamed padding at the end of a structure or union.

This is often referred to as structure padding. Structure members are arranged in memory as they are declared in the program text. Padding is added to the structure to ensure the structure is properly aligned in memory.

Non-Compliant Code Example

In the example below, assuming the size of buf is equal sizeof(size_t) + (sizeof(char) * 50), which would equal 54 (assuming sizeof(size_t) is 4) is incorrect. The sizeof(buf) may actually evaluate to be 56 due to structure padding.

struct buffer {
    size_t size;
    char buffer[50];
};

int main(int argc, char *argv[]) {
  struct buffer buf;

  printf("%u",sizeof(buf));

  return 0;
}