NGINX Unit - universal web app server - a lightweight and versatile open source server that simplifies the application stack by natively executing application code across eight different programming language runtimes.
Declaring a 0-sized array (e.g 'char arr[0];') as the last member of a
structure is a GNU extension that was used to implement flexible array
members (FAMs) before they were standardised in C99 as simply '[]'.
The GNU extension itself was introduced to work around a hack of
declaring 1-sized arrays to mean a variable-length object. The advantage
of the 0-sized (and true FAMs) is that they don't count towards the size
of the structure.
Unit already declares some true FAMs, but it also declared some 0-sized
arrays.
Converting these 0-sized arrays to true FAMs is not only good for
consistency but will also allow better compiler checks now (as in a C99
FAM must be the last member of a structure and the compiler will warn
otherwise) and in the future as doing this fixes a bunch of warnings
(treated as errors in Unit by default) when compiled with
(Note -Warray-bounds is enabled by -Wall and -Wstrict-flex-arrays seems
to also be enabled via -Wall -Wextra, the -02 is required to make
-fstrict-flex-arrays more effective, =3 is the default on at least GCC
14)
such as
CC build/src/nxt_upstream.o
src/nxt_upstream.c: In function ‘nxt_upstreams_create’:
src/nxt_upstream.c:56:18: error: array subscript i is outside array bounds of ‘nxt_upstream_t[0]’ {aka ‘struct nxt_upstream_s[]’} [-Werror=array-bounds=]
56 | string = nxt_str_dup(mp, &upstreams->upstream[i].name, &name);
| ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from src/nxt_upstream.c:9:
src/nxt_upstream.h:55:48: note: while referencing ‘upstream’
55 | nxt_upstream_t upstream[0];
| ^~~~~~~~
Making our flexible array members proper C99 FAMs and ensuring any >0
sized trailing arrays in structures are really normal arrays will allow
to enable various compiler options (such as the above and more) that
will help keep our array usage safe.
Changing 0-sized arrays to FAMs should have no effect on structure
layouts/sizes (they both have a size of 0, although doing a sizeof() on
a FAM will result in a compiler error).
Looking at pahole(1) output for the nxt_http_route_ruleset_t structure
for the [0] and [] cases...
ac000
commented
4 months ago
Declaring a 0-sized array (e.g 'char arr[0];') as the last member of a structure is a GNU extension that was used to implement flexible array members (FAMs) before they were standardised in C99 as simply '[]'.
The GNU extension itself was introduced to work around a hack of declaring 1-sized arrays to mean a variable-length object. The advantage of the 0-sized (and true FAMs) is that they don't count towards the size of the structure.
Unit already declares some true FAMs, but it also declared some 0-sized arrays.
Converting these 0-sized arrays to true FAMs is not only good for consistency but will also allow better compiler checks now (as in a C99 FAM must be the last member of a structure and the compiler will warn otherwise) and in the future as doing this fixes a bunch of warnings (treated as errors in Unit by default) when compiled with
(Note -Warray-bounds is enabled by -Wall and -Wstrict-flex-arrays seems to also be enabled via -Wall -Wextra, the -02 is required to make -fstrict-flex-arrays more effective, =3 is the default on at least GCC 14)
such as
Making our flexible array members proper C99 FAMs and ensuring any >0 sized trailing arrays in structures are really normal arrays will allow to enable various compiler options (such as the above and more) that will help keep our array usage safe.
Changing 0-sized arrays to FAMs should have no effect on structure layouts/sizes (they both have a size of 0, although doing a sizeof() on a FAM will result in a compiler error).
Looking at pahole(1) output for the nxt_http_route_ruleset_t structure for the [0] and [] cases...
Also checking with the size(1) command on the effected object files shows no changes to their sizes