Can I 'extend' a struct in C?

Evidently this feature has been added to C11, but alas I don't have access to a C compiler of recent vintage (>= GCC 4.6.2).

typedef struct foo {
  int a;
} foo;

typedef struct bar {
  struct foo;
  int b;
} bar;

int main() {
  bar b;
  b.a = 42;
  b.b = 99;
  return 0;
}

You can, using pointers, because a pointer to a structure object is guaranteed to point its first member. See e.g. this article.

#include <stdlib.h>
#include <stdio.h>

typedef struct foo_s {
    int a;
} foo;

typedef struct bar_s {
    foo super;
    int b;
} bar;

int fooGetA(foo *x) {
  return x->a;
}

void fooSetA(foo *x, int a) {
  x->a = a;
}

int main() {
  bar* derived = (bar*) calloc(1, sizeof(bar));
  fooSetA((foo*) derived, 5);
  derived->b = 3;
  printf("result: %d\n", fooGetA((foo*) derived));
  return 0;
}

If you ment

typedef struct foo_s {
    int a;
  } foo;

typedef struct bar_s {
 foo my_foo;
int b;
} bar;

so you can do:

bar b; b.my_foo.a = 3;

Otherwise, There's no way of doing it in C since the sizeof(bar_s) is detriment on compile time. It's not a good practice but you can save a void * ptr; pointer within bar_s, and another enum which describes the ptr type, and cast by the type.

i.e:

typedef enum internalType{
  INTERNAL_TYPE_FOO = 0,
}internalType_t;

typedef struct bar_s {
 internalType_t ptrType;
 void* ptr;
int b;
} bar;

and then:

bar b;  foo f;
b.ptrType = INTERNAL_TYPE_FOO;
b.ptr = &f;

and some where else in the code:

 if (b.ptrType == INTERNAL_TYPE_FOO) {
    foo* myFooPtr = (foo *)b.ptr;
 }

Not possible in C the way you did. But you can mimic inheritance having a foo member variable in bar.

typedef struct bar_s {
    foo obj;
    int b;
} bar;

bar b;
b.obj.a = 10;