utils.c: include <signal.h> for siginfo_t

NEWS.md

@@ -30,6 +30,8 @@
 
 2. `set()` now automatically pre-allocates new column slots if needed, similar to what `:=` already does, [#1831](https://github.com/Rdatatable/data.table/issues/1831) [#4100](https://github.com/Rdatatable/data.table/issues/4100). Thanks to @zachokeeffe and @tyner for the report and @ben-schwen for the fix.
 
+3. Fixed compilation failure like "error: unknown type name 'siginfo_t'" in v1.18.0 in some strict environments, e.g., FreeBSD, where the header file declaring the POSIX function `waitid` does not transitively include the header file defining the `siginfo_t` type, [#7516](https://github.com/rdatatable/data.table/issues/7516). Thanks to @jszhao for the report and @aitap for the fix.
+
 ## data.table [v1.18.0](https://github.com/Rdatatable/data.table/milestone/37?closed=1)  23 December 2025
 
 ### BREAKING CHANGE

src/utils.c

@@ -1,6 +1,10 @@
 #ifndef _WIN32
-#  define _POSIX_C_SOURCE 200809L // required for POSIX (not standard C) features in is_direct_child e.g. 'siginfo_t'
-#  include <sys/wait.h>
+#  if !defined(_POSIX_C_SOURCE) || _POSIX_C_SOURCE < 200809L
+#    undef _POSIX_C_SOURCE
+#    define _POSIX_C_SOURCE 200809L // required for POSIX (not standard C) features in is_direct_child e.g. 'siginfo_t'
+#  endif
+#  include <signal.h> // siginfo_t
+#  include <sys/wait.h> // waitid
 #endif
 
 #include "data.table.h"