26

I hit some code in Linux kernel:

static struct ctl_table ip_ct_sysctl_table[] = {
    {
        .procname   = "ip_conntrack_max",
        .maxlen     = sizeof(int),
        .mode       = 0644,
        .proc_handler   = proc_dointvec,
    },
    // ...
    {
        .procname   = "ip_conntrack_log_invalid",
        .maxlen     = sizeof(unsigned int),
        .mode       = 0644,
        .proc_handler   = proc_dointvec_minmax,
        .extra1     = &log_invalid_proto_min,
        .extra2     = &log_invalid_proto_max,
    },
    { }
};

Here an array of structs ends with { }. For what purpose was it added?
Btw, a bit above this code there is another array of structs, but without empty braces at the end.
When should I use empty braces at the end of an array of structs?

4 Answers

15

This particular change was part of the sysctl net: Remove unused binary sysctl code commit by Eric W. Biederman, changing the initialization of the last element of the ip_ct_sysctl_table array from {0} to {} (and performs similar changes to many other array initializations).

The {0} pattern seems to have been around for much longer though, and both {0} or {} final element-initialization is commonly (in the Linux source code) explicitly referred to as Terminating entry, so it is likely a pattern present to allow consuming these arrays without knowing their lengths, terminating consumption when hitting the zero-initialized terminating entry. E.g. for the similar arrays in sound/aoa/fabrics/snd-aoa-fabric-layout.c the intent of the zero-initialization is even explicitly mentioned in a comment, e.g.:

static struct codec_connection toonie_connections[] = {
  {
      .connected = CC_SPEAKERS | CC_HEADPHONE,
      .codec_bit = 0,
  },
  {} /* terminate array by .connected == 0 */
};
10

You're probably familiar with zero-terminated strings. ctl_table ip_ct_sysctl_table[] is a zero-terminated array, i.e. the last array entry has all-zero members.

3

It's one zero initialized element at the end of the array in order to increase the number of elements of the array by one.

Consider this small demo:

#include <stdio.h>

struct Test
{
  int x;
  int y;
} arr[] =
{
    {1,2},
    {3,4},
//  {}
};

int main(void) {
    printf("%zu\n", sizeof(arr) / sizeof(arr[0]));
    return 0;
}

The size of the arr array will change if you uncomment the {} at the end of the array initialisation list.

Outputs:

With // {} (array has 2 elements)

2

With {} (array has 3 elements)

3

Further explanation:

The ip_ct_sysctl_table array is only used at one place, that is here:

in->ctl_table = kmemdup(ip_ct_sysctl_table,
                sizeof(ip_ct_sysctl_table),
                GFP_KERNEL);

The extra {} increases the total size ip_ct_sysctl_table.

3

What is the need of empty braces '{ }' at the end of array of structs?

To be clear: the "empty braces '{ }' at the end of array of structs" is not needed to satisfy C syntax requirements.

When should I use empty braces at the end of an array of structs?

When code wants a sentinel value.

It is sometimes useful for the program to have a final array element of all zeros - certainly to detect the end. The need comes from the application's use of array ctl_table ip_ct_sysctl_table[], not from a C language need.