--- a/riegeli/bytes/cfile_handle.cc
+++ b/riegeli/bytes/cfile_handle.cc
@@ -13,10 +13,12 @@
 // limitations under the License.
 
 // Make `O_CLOEXEC` available on Darwin.
-#if !defined(_XOPEN_SOURCE) || _XOPEN_SOURCE < 700
-#undef _XOPEN_SOURCE
-#define _XOPEN_SOURCE 700
-#endif
+#if !defined(__FreeBSD__)
+#if !defined(_XOPEN_SOURCE) || _XOPEN_SOURCE < 700
+#undef _XOPEN_SOURCE
+#define _XOPEN_SOURCE 700
+#endif
+#endif  /* !__FreeBSD__ */
 
 #include "riegeli/bytes/cfile_handle.h"
 

--- a/riegeli/bytes/cfile_reader.cc
+++ b/riegeli/bytes/cfile_reader.cc
@@ -15,10 +15,12 @@
 #ifndef _WIN32
 
 // Make `fseeko()` and `ftello()` available.
-#if !defined(_XOPEN_SOURCE) || _XOPEN_SOURCE < 500
-#undef _XOPEN_SOURCE
-#define _XOPEN_SOURCE 500
-#endif
+#if !defined(__FreeBSD__)
+#if !defined(_XOPEN_SOURCE) || _XOPEN_SOURCE < 500
+#undef _XOPEN_SOURCE
+#define _XOPEN_SOURCE 500
+#endif
+#endif  /* !__FreeBSD__ */
 
 // Make `off_t` 64-bit even on 32-bit systems.
 #undef _FILE_OFFSET_BITS

--- a/riegeli/bytes/cfile_writer.cc
+++ b/riegeli/bytes/cfile_writer.cc
@@ -15,10 +15,12 @@
 #ifndef _WIN32
 
 // Make `fseeko()` and `ftello()` available.
-#if !defined(_XOPEN_SOURCE) || _XOPEN_SOURCE < 500
-#undef _XOPEN_SOURCE
-#define _XOPEN_SOURCE 500
-#endif
+#if !defined(__FreeBSD__)
+#if !defined(_XOPEN_SOURCE) || _XOPEN_SOURCE < 500
+#undef _XOPEN_SOURCE
+#define _XOPEN_SOURCE 500
+#endif
+#endif  /* !__FreeBSD__ */
 
 // Make `off_t` 64-bit even on 32-bit systems.
 #undef _FILE_OFFSET_BITS

--- a/riegeli/bytes/fd_handle.cc
+++ b/riegeli/bytes/fd_handle.cc
@@ -13,10 +13,12 @@
 // limitations under the License.
 
 // Make `openat()` available.
-#if !defined(_POSIX_C_SOURCE) || _POSIX_C_SOURCE < 200809
-#undef _POSIX_C_SOURCE
-#define _POSIX_C_SOURCE 200809
-#endif
+#if !defined(__FreeBSD__)
+#if !defined(_POSIX_C_SOURCE) || _POSIX_C_SOURCE < 200809
+#undef _POSIX_C_SOURCE
+#define _POSIX_C_SOURCE 200809
+#endif
+#endif  /* !__FreeBSD__ */
 
 #include "riegeli/bytes/fd_handle.h"
 

--- a/riegeli/bytes/fd_internal.cc
+++ b/riegeli/bytes/fd_internal.cc
@@ -15,10 +15,12 @@
 #ifndef _WIN32
 
 // Make `readlink()` available, and make `O_CLOEXEC` available on Darwin.
-#if !defined(_XOPEN_SOURCE) || _XOPEN_SOURCE < 700
-#undef _XOPEN_SOURCE
-#define _XOPEN_SOURCE 700
-#endif
+#if !defined(__FreeBSD__)
+#if !defined(_XOPEN_SOURCE) || _XOPEN_SOURCE < 700
+#undef _XOPEN_SOURCE
+#define _XOPEN_SOURCE 700
+#endif
+#endif  /* !__FreeBSD__ */
 
 #endif
 

--- a/riegeli/bytes/fd_mmap_reader.cc
+++ b/riegeli/bytes/fd_mmap_reader.cc
@@ -15,10 +15,12 @@
 #ifndef _WIN32
 
 // Make `posix_fadvise()` available.
-#if !defined(_XOPEN_SOURCE) || _XOPEN_SOURCE < 600
-#undef _XOPEN_SOURCE
-#define _XOPEN_SOURCE 600
-#endif
+#if !defined(__FreeBSD__)
+#if !defined(_XOPEN_SOURCE) || _XOPEN_SOURCE < 600
+#undef _XOPEN_SOURCE
+#define _XOPEN_SOURCE 600
+#endif
+#endif  /* !__FreeBSD__ */
 
 // Make `off_t` 64-bit even on 32-bit systems.
 #undef _FILE_OFFSET_BITS

--- a/riegeli/bytes/fd_reader.cc
+++ b/riegeli/bytes/fd_reader.cc
@@ -15,10 +15,12 @@
 #ifndef _WIN32
 
 // Make `pread()` and `posix_fadvise()` available.
-#if !defined(_XOPEN_SOURCE) || _XOPEN_SOURCE < 600
-#undef _XOPEN_SOURCE
-#define _XOPEN_SOURCE 600
-#endif
+#if !defined(__FreeBSD__)
+#if !defined(_XOPEN_SOURCE) || _XOPEN_SOURCE < 600
+#undef _XOPEN_SOURCE
+#define _XOPEN_SOURCE 600
+#endif
+#endif  /* !__FreeBSD__ */
 
 // Make `off_t` 64-bit even on 32-bit systems.
 #undef _FILE_OFFSET_BITS

--- a/riegeli/bytes/fd_writer.cc
+++ b/riegeli/bytes/fd_writer.cc
@@ -15,10 +15,12 @@
 #ifndef _WIN32
 
 // Make `pwrite()` and `ftruncate()` available.
-#if !defined(_XOPEN_SOURCE) || _XOPEN_SOURCE < 500
-#undef _XOPEN_SOURCE
-#define _XOPEN_SOURCE 500
-#endif
+#if !defined(__FreeBSD__)
+#if !defined(_XOPEN_SOURCE) || _XOPEN_SOURCE < 500
+#undef _XOPEN_SOURCE
+#define _XOPEN_SOURCE 500
+#endif
+#endif  /* !__FreeBSD__ */
 
 // Make `off_t` 64-bit even on 32-bit systems.
 #undef _FILE_OFFSET_BITS