2 files changed, 3 insertions, 1 deletions
diff --git a/config.mk b/config.mk
index 6f9acdb..494927c 100644
--- a/config.mk
+++ b/config.mk
@@ -4,3 +4,5 @@ MANPREFIX = $(PREFIX)/share/man
 CPPFLAGS = -D_DEFAULT_SOURCE -D_BSD_SOURCE -D_XOPEN_SOURCE=700 -D_GNU_SOURCE
 CFLAGS   = -std=c99 -Wall -Wextra -pedantic -O2 $(CPPFLAGS)
 LDFLAGS  = -s -lrt
+
+# Add -DSEMUN_ALREADY_DEFINED to CPPFLAGS if `union semun` is already defined by libc
diff --git a/libbus.c b/libbus.c
index 0a84353..5abb4e5 100644
--- a/libbus.c
+++ b/libbus.c
@@ -196,7 +196,7 @@
 
 
 
-#ifdef _SEM_SEMUN_UNDEFINED
+#ifndef SEMUN_ALREADY_DEFINED
 union semun {
 	int val;
 	struct semid_ds *buf;