Extending a struct in C -

i came across colleague's code looked this:

typedef struct {   int x; }a;  typedef struct b {   a;   int d; }b;  void fn(){   b *b;   ((a*)b)->x = 10; } 

his explanation since struct a first member of struct b, b->x same b->a.x , provides better readability.
makes sense, considered practice? , work across platforms? runs fine on gcc.

yes, work cross-platform, doesn't necessarily make idea.

as per iso c standard (all citations below c11), 6.7.2.1 structure , union specifiers /15, there not allowed padding before first element of structure

in addition, 6.2.7 compatible type , composite type states that:

two types have compatible type if types same

and undisputed a , a-within-b types identical.

this means memory accesses a fields same in both a , b types, more sensible b->a.x should using if have concerns maintainability in future.

and, though have worry strict type aliasing, don't believe applies here. is illegal alias pointers standard has specific exceptions.

6.5 expressions /7 states of exceptions, footnote:

the intent of list specify circumstances in object may or may not aliased.

the exceptions listed are:

• a type compatible effective type of object;
• some other exceptions need not concern here; and
• an aggregate or union type includes 1 of aforementioned types among members (including, recursively, member of subaggregate or contained union).

that, combined struct padding rules mentioned above, including phrase:

a pointer structure object, suitably converted, points initial member

seems indicate example allowed for. core point have remember here type of expression ((a*)b) a*, not b*. makes variables compatible purposes of unrestricted aliasing.

that's reading of relevant portions of standard, i've been wrong before ^(a), doubt in case.

so, if have genuine need this, work okay i'd documenting constraints in code very close structures not bitten in future.

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^(a) wife tell you, , without prompting :-)