c - Why should a struct's size reflect its alignment? -

according wikipedia:

the last member padded number of bytes required total size of structure should multiple of largest alignment of structure member

in understanding, means in following:

struct {   char *p;     // 8 bytes   char c;      // 1 byte }; struct b {   struct a;  // 16 bytes   char d;      // 1 bytes }; 

struct a have size of 16 bytes, , struct b have size of 24 bytes.

the common explanation arrays of a should have elements accessible @ address of array plus index times size of a.

but fail see why case. why not a has size 9 , b has size 10 (both 8 bytes alignment), , use special "array-storage" size when indexing array?

of course, we'd still store types in arrays in way compatible alignment (using 16 bytes store each b element). then, we'd compute element addresses taking account alignment, instead of considering size alone (the compiler can statically).

for example, store 64 objects in 1kb bytes array of b's, instead of 42.

in each translation unit of c, sizeof(t) same, regardless of context of t. proposal introduce @ least 2 values sizeof(t): 1 arrays of t , different 1 individual objects of t. introduces context-dependence sizeof operator. incompatible how c handles pointers, arrays, , addresses of objects.

consider following:

void zero_a(struct *a) { memset(a,0,sizeof(*a)); }  /* ... */ struct single; struct several[3]; struct b b;  b.d = 3; zero_a(&b.a); zero_a(&single); zero_a(several+1); 

under proposal, zero_a have know whether pointer passed pointed struct a in array context (where sizeof(*a) == 16) or struct a outside of array context (where sizeof(*a) == 9). standard c doesn't support this. if compiler guessed wrong, or information lost (eg: in round-trip through volatile struct *), zero_a(&single) invoke undefined behavior (by writing past bounds of single), , zero_a(&b.a) overwrite b.d , invoke undefined behavior.

tightly packing structs array relatively uncommon requirement, , adding context-dependence sizeof introduce lot of complications language, libraries, , abis. there times need this, , c gives tools need: memcpy , unions.