What is the need of empty braces '{ }' at the end of array of structs?
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 */};
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.
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.