diff --git a/gemi.c b/gemi.c
index 68ef463..40a52bc 100644
--- a/gemi.c
+++ b/gemi.c
@@ -1,3 +1,4 @@
+#define _XOPEN_SOURCE /* getopt */
 #include <stdio.h>
 #include <stdlib.h>
 #include <signal.h>
diff --git a/mmap_file.c b/mmap_file.c
index 2873f96..9cd1a34 100644
--- a/mmap_file.c
+++ b/mmap_file.c
@@ -1,3 +1,4 @@
+#define _DEFAULT_SOURCE /* MAP_ANONYMOUS */
 #include <assert.h>
 #include <errno.h>
 #include <stdio.h>
@@ -11,6 +12,9 @@
 
 #include "mmap_file.h"
 
+const int PROT_MEM = PROT_WRITE | PROT_READ;
+const int MAP_MEM = MAP_ANONYMOUS | MAP_PRIVATE;
+
 bool allocate_mmap(struct mmap_file *f)
 {
     f->data = mmap(NULL, f->size, f->prot, f->flags, -1, 0);
diff --git a/mmap_file.h b/mmap_file.h
index 264967d..52dc8d6 100644
--- a/mmap_file.h
+++ b/mmap_file.h
@@ -10,8 +10,8 @@
 #define ERROR_MMAP(file) ((file).data == MAP_FAILED || (file).data == NULL)
 
 // definitions for memory backed mappings
-#define PROT_MEM (PROT_WRITE | PROT_READ)
-#define MAP_MEM (MAP_ANONYMOUS | MAP_PRIVATE)
+extern const int PROT_MEM;
+extern const int MAP_MEM;
 
 // 128KiB
 #define OUTPUT_FILE_SIZE (128 * 1024)
diff --git a/purr.c b/purr.c
index 5fe8913..e4c29a3 100644
--- a/purr.c
+++ b/purr.c
@@ -1,7 +1,9 @@
+#define _XOPEN_SOURCE /* getopt */
+
 #ifdef HAVE_PROG_INVOCATION
-#define _GNU_SOURCE
+#define _GNU_SOURCE /* program_invocation_short_name */
 #include <errno.h>
-#endif
+#endif /* HAVE_PROG_INVOCATION */
 
 #include <signal.h>
 #include <stdbool.h>
diff --git a/urls.c b/urls.c
index d491006..669e232 100644
--- a/urls.c
+++ b/urls.c
@@ -1,3 +1,4 @@
+#define _POSIX_C_SOURCE 200112L /* addrinfo */
 #include <string.h>
 #include <unistd.h>
 #include <stdlib.h>