diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 05cc1e445..ab3337b07 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -173,8 +173,8 @@ test-lin-dev-gcc-strict-cran:
     _R_S3_METHOD_LOOKUP_BASEENV_AFTER_GLOBALENV_: "FALSE" ## detects S3 method lookup found on search path #4777
     _R_S3_METHOD_LOOKUP_REPORT_SEARCH_PATH_USES_: "TRUE"
   script:
-    - echo 'CFLAGS=-g -O2 -flto=auto -fno-common -fopenmp -Wall -Wvla -pedantic -fstack-protector-strong -D_FORTIFY_SOURCE=2' > ~/.R/Makevars
-    - echo 'CXXFLAGS=-g -O2 -flto=auto -fno-common -fopenmp -Wall -Wvla -pedantic -fstack-protector-strong -D_FORTIFY_SOURCE=2' >> ~/.R/Makevars
+    - echo 'CFLAGS=-g -O2 -flto=auto -fno-common -fopenmp -Wall -Wvla -pedantic -fstack-protector-strong -D_FORTIFY_SOURCE=2 -D_POSIX_C_SOURCE=200809L' > ~/.R/Makevars
+    - echo 'CXXFLAGS=-g -O2 -flto=auto -fno-common -fopenmp -Wall -Wvla -pedantic -fstack-protector-strong -D_FORTIFY_SOURCE=2 -D_POSIX_C_SOURCE=200809L' >> ~/.R/Makevars
     - *install-deps
     - R CMD check --as-cran $(ls -1t data.table_*.tar.gz | head -n 1)
     - (! grep "warning:" data.table.Rcheck/00install.out)
diff --git a/NEWS.md b/NEWS.md
index 256c7450a..79d8b607a 100644
--- a/NEWS.md
+++ b/NEWS.md
@@ -36,6 +36,8 @@
 
 4. `sum(<int64 column>)` by group is correct with missing entries and GForce activated ([#7571](https://github.com/Rdatatable/data.table/issues/7571)). Thanks to @rweberc for the report and @manmita for the fix. The issue was caused by a faulty early `break` that spilled between groups, and resulted in silently incorrect results!
 
+5. 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
diff --git a/src/utils.c b/src/utils.c
index e11bc09d5..11ec0c1a7 100644
--- a/src/utils.c
+++ b/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"