diff --git a/configure b/configure
index ff59f1422d8..7029e036269 100755
--- a/configure
+++ b/configure
@@ -7590,10 +7590,10 @@ $as_echo "#define PROFILE_PID_DIR 1" >>confdefs.h
   fi
 fi
 
-# On Solaris, we need this #define to get POSIX-conforming versions
-# of many interfaces (sigwait, getpwuid_r, ...).
+# On Solaris, we need these #defines to get POSIX-conforming versions
+# of many interfaces (sigwait, getpwuid_r, shmdt, ...).
 if test "$PORTNAME" = "solaris"; then
-  CPPFLAGS="$CPPFLAGS -D_POSIX_PTHREAD_SEMANTICS"
+  CPPFLAGS="$CPPFLAGS -D_POSIX_C_SOURCE=200112L -D_POSIX_PTHREAD_SEMANTICS"
 fi
 
 # We already have this in Makefile.win32, but configure needs it too
diff --git a/configure.ac b/configure.ac
index 21817009642..afc229e729b 100644
--- a/configure.ac
+++ b/configure.ac
@@ -693,10 +693,10 @@ if test "$enable_profiling" = yes && test "$ac_cv_prog_cc_g" = yes; then
   fi
 fi
 
-# On Solaris, we need this #define to get POSIX-conforming versions
-# of many interfaces (sigwait, getpwuid_r, ...).
+# On Solaris, we need these #defines to get POSIX-conforming versions
+# of many interfaces (sigwait, getpwuid_r, shmdt, ...).
 if test "$PORTNAME" = "solaris"; then
-  CPPFLAGS="$CPPFLAGS -D_POSIX_PTHREAD_SEMANTICS"
+  CPPFLAGS="$CPPFLAGS -D_POSIX_C_SOURCE=200112L -D_POSIX_PTHREAD_SEMANTICS"
 fi
 
 # We already have this in Makefile.win32, but configure needs it too
diff --git a/meson.build b/meson.build
index 451c3f6d851..ec5cd9259df 100644
--- a/meson.build
+++ b/meson.build
@@ -261,7 +261,9 @@ elif host_system == 'openbsd'
 elif host_system == 'sunos'
   portname = 'solaris'
   export_fmt = '-Wl,-M@0@'
-  cppflags += '-D_POSIX_PTHREAD_SEMANTICS'
+  # We need these #defines to get POSIX-conforming versions
+  # of many interfaces (sigwait, getpwuid_r, shmdt, ...).
+  cppflags += [ '-D_POSIX_C_SOURCE=200112L', '-D_POSIX_PTHREAD_SEMANTICS' ]
 
 elif host_system == 'windows'
   portname = 'win32'