Source: http://stackoverflow.com/questions/4255193/declaring-zero-size-vector
This is called a flexible array member, and in C99 is written as char bar[];, and in C89 was written as char bar[1];, and which some compilers would let you write as char bar[0];. Basically, you only use pointers to the structure, and allocate them all with an amount of extra space at the end:
const size_t i = sizeof("Hello, world!");
struct foo *p = malloc(offsetof(struct foo, bar) + i);
memcpy(p->bar, "Hello, world!", i);
// initialize other members of p
printf("%s\n", p->bar);
That way, p->bar stores a string whose size isn't limited by an array declaration, but which is still all done in the same allocation as the rest of the struct (rather than needing the member to be a char * and need two mallocs and two frees to set it up).
好處：連續（虛擬）內存空間以及僅需一次的free操作。