Question

When I’m looking the Linux kernel code, found the below code:

   struct thread_info {
    struct task_struct  *task;
    struct exec_domain  *exec_domain;
    unsigned long       flags;
    __u32           status;
    __u32           cpu;
    int         preempt_count;
    mm_segment_t        addr_limit;
    struct restart_block    restart_block;
    void __user     *sysenter_return;
    unsigned long           previous_esp;
    __u8            supervisor_stack[0];
};

Notice that the last variable “supervisor_stack”, it is a zero length array, what is the usage of it? Thanks in advance!

Answer 1

It’s the pre-C99 version of a flexible array member, offered by GCC as an extension.

The C99 way is to define the flexible array member with empty brackets,

__u8  supervisor_stack[];

It’s used to store data whose amount is not constant contiguous to the struct. Memory is allocated in the form

struct foo *ptr = malloc(sizeof *ptr + whatever_is_needed);

In paragraph 18 of 6.7.2.1, the standard (draft N1570) describes them:

As a special case, the last element of a structure with more than one named member may
have an incomplete array type; this is called a flexible array member. In most situations,
the flexible array member is ignored. In particular, the size of the structure is as if the
flexible array member were omitted except that it may have more trailing padding than
the omission would imply. However, when a . (or ->) operator has a left operand that is
(a pointer to) a structure with a flexible array member and the right operand names that
member, it behaves as if that member were replaced with the longest array (with the same
element type) that would not make the structure larger than the object being accessed; the
offset of the array shall remain that of the flexible array member, even if this would differ
from that of the replacement array. If this array would have no elements, it behaves as if
it had one element but the behavior is undefined if any attempt is made to access that
element or to generate a pointer one past it.