diff --git a/HISTORY b/HISTORY
index 3c30daa4..9a5f2c00 100644
--- a/HISTORY
+++ b/HISTORY
@@ -1,8 +1,9 @@
 SnapRAID HISTORY
 ================
 
-11.4 2020/05 (WorkInProgress)
+11.4 2020/05
 =============================
+ * Fix build errors due new gcc 10 default for -fno-common.
  * In fixing, if a parity is filtered out, don't attempt to recover its size,
    and proceed without it if missing.
  * Avoid unnecessary parity read when fixing the parity itself.
diff --git a/cmdline/io.c b/cmdline/io.c
index c71ba411..156d512f 100644
--- a/cmdline/io.c
+++ b/cmdline/io.c
@@ -19,6 +19,19 @@
 
 #include "io.h"
 
+void (*io_start)(struct snapraid_io* io,
+	block_off_t blockstart, block_off_t blockmax,
+	int (*block_is_enabled)(void* arg, block_off_t), void* blockarg) = 0;
+void (*io_stop)(struct snapraid_io* io) = 0;
+block_off_t (*io_read_next)(struct snapraid_io* io, void*** buffer) = 0;
+struct snapraid_task* (*io_data_read)(struct snapraid_io* io, unsigned* diskcur, unsigned* waiting_map, unsigned* waiting_mac) = 0;
+struct snapraid_task* (*io_parity_read)(struct snapraid_io* io, unsigned* levcur, unsigned* waiting_map, unsigned* waiting_mac) = 0;
+void (*io_parity_write)(struct snapraid_io* io, unsigned* levcur, unsigned* waiting_map, unsigned* waiting_mac) = 0;
+void (*io_write_preset)(struct snapraid_io* io, block_off_t blockcur, int skip) = 0;
+void (*io_write_next)(struct snapraid_io* io, block_off_t blockcur, int skip, int* writer_error) = 0;
+void (*io_refresh)(struct snapraid_io* io) = 0;
+
+
 /**
  * Get the next block position to process.
  */
diff --git a/cmdline/io.h b/cmdline/io.h
index 82ec7448..ea6644bb 100644
--- a/cmdline/io.h
+++ b/cmdline/io.h
@@ -313,14 +313,14 @@ void io_done(struct snapraid_io* io);
 /**
  * Start all the worker threads.
  */
-void (*io_start)(struct snapraid_io* io,
+extern void (*io_start)(struct snapraid_io* io,
 	block_off_t blockstart, block_off_t blockmax,
 	int (*block_is_enabled)(void* arg, block_off_t), void* blockarg);
 
 /**
  * Stop all the worker threads.
  */
-void (*io_stop)(struct snapraid_io* io);
+extern void (*io_stop)(struct snapraid_io* io);
 
 /**
  * Next read position.
@@ -332,7 +332,7 @@ void (*io_stop)(struct snapraid_io* io);
  * \param buffer The data buffers to use for this position.
  * \return The parity position.
  */
-block_off_t (*io_read_next)(struct snapraid_io* io, void*** buffer);
+extern block_off_t (*io_read_next)(struct snapraid_io* io, void*** buffer);
 
 /**
  * Read a data block.
@@ -343,7 +343,7 @@ block_off_t (*io_read_next)(struct snapraid_io* io, void*** buffer);
  * \param diskcur The position of the data block in the ::handle_map vector.
  * \return The completed task.
  */
-struct snapraid_task* (*io_data_read)(struct snapraid_io* io, unsigned* diskcur, unsigned* waiting_map, unsigned* waiting_mac);
+extern struct snapraid_task* (*io_data_read)(struct snapraid_io* io, unsigned* diskcur, unsigned* waiting_map, unsigned* waiting_mac);
 
 /**
  * Read a parity block.
@@ -354,7 +354,7 @@ struct snapraid_task* (*io_data_read)(struct snapraid_io* io, unsigned* diskcur,
  * \param levcur The position of the parity block in the ::parity_handle_map vector.
  * \return The completed task.
  */
-struct snapraid_task* (*io_parity_read)(struct snapraid_io* io, unsigned* levcur, unsigned* waiting_map, unsigned* waiting_mac);
+extern struct snapraid_task* (*io_parity_read)(struct snapraid_io* io, unsigned* levcur, unsigned* waiting_map, unsigned* waiting_mac);
 
 /**
  * Write of a parity block.
@@ -364,7 +364,7 @@ struct snapraid_task* (*io_parity_read)(struct snapraid_io* io, unsigned* levcur
  * \param io InputOutput context.
  * \param levcur The position of the parity block in the ::parity_handle_map vector.
  */
-void (*io_parity_write)(struct snapraid_io* io, unsigned* levcur, unsigned* waiting_map, unsigned* waiting_mac);
+extern void (*io_parity_write)(struct snapraid_io* io, unsigned* levcur, unsigned* waiting_map, unsigned* waiting_mac);
 
 /**
  * Preset the write position.
@@ -376,7 +376,7 @@ void (*io_parity_write)(struct snapraid_io* io, unsigned* levcur, unsigned* wait
  * \param blockcur The parity position to write.
  * \param skip Skip the writes, in case parity doesn't need to be updated.
  */
-void (*io_write_preset)(struct snapraid_io* io, block_off_t blockcur, int skip);
+extern void (*io_write_preset)(struct snapraid_io* io, block_off_t blockcur, int skip);
 
 /**
  * Next write position.
@@ -389,12 +389,12 @@ void (*io_write_preset)(struct snapraid_io* io, block_off_t blockcur, int skip);
  * \param skip Skip the writes, in case parity doesn't need to be updated.
  * \param writer_error Return the number of errors. Vector of IO_WRITER_ERROR_MAX elements.
  */
-void (*io_write_next)(struct snapraid_io* io, block_off_t blockcur, int skip, int* writer_error);
+extern void (*io_write_next)(struct snapraid_io* io, block_off_t blockcur, int skip, int* writer_error);
 
 /**
  * Refresh the number of cached blocks for all data and parity disks.
  */
-void (*io_refresh)(struct snapraid_io* io);
+extern void (*io_refresh)(struct snapraid_io* io);
 
 #endif
 
diff --git a/cmdline/portable.h b/cmdline/portable.h
index 19e6fe1d..a665a550 100644
--- a/cmdline/portable.h
+++ b/cmdline/portable.h
@@ -389,14 +389,14 @@ void os_clear(void);
  *
  * If no log file is selected, it's 0.
  */
-FILE* stdlog;
+extern FILE* stdlog;
 
 /**
  * Exit codes for testing.
  */
-int exit_success;
-int exit_failure;
-int exit_sync_needed;
+extern int exit_success;
+extern int exit_failure;
+extern int exit_sync_needed;
 #undef EXIT_SUCCESS
 #undef EXIT_FAILURE
 #define EXIT_SUCCESS exit_success
diff --git a/cmdline/stream.h b/cmdline/stream.h
index 35b941f6..6ec33912 100644
--- a/cmdline/stream.h
+++ b/cmdline/stream.h
@@ -28,7 +28,7 @@
  *
  * It's not a constant for testing purpose.
  */
-unsigned STREAM_SIZE;
+extern unsigned STREAM_SIZE;
 
 #define STREAM_STATE_READ 0 /**< The stream is in a normal state of read. */
 #define STREAM_STATE_WRITE 1 /**< The stream is in a normal state of write. */
diff --git a/cmdline/support.c b/cmdline/support.c
index 80dfa5c2..0bda1284 100644
--- a/cmdline/support.c
+++ b/cmdline/support.c
@@ -79,6 +79,7 @@ void lock_done(void)
 /* print */
 
 int msg_level = 0;
+FILE* stdlog = 0;
 
 /*
  * Note that in the following functions we always flush both
diff --git a/cmdline/support.h b/cmdline/support.h
index 5bde967a..e7e76b5f 100644
--- a/cmdline/support.h
+++ b/cmdline/support.h
@@ -415,7 +415,7 @@ int smartctl_flush(FILE* f, const char* file, const char* name);
  *
  * Ensure to change that before starting any thread.
  */
-int thread_cond_signal_outside;
+extern int thread_cond_signal_outside;
 
 /**
  * Thread wrappers to handle error conditions.
diff --git a/cmdline/util.h b/cmdline/util.h
index 3d7e3941..8717a14c 100644
--- a/cmdline/util.h
+++ b/cmdline/util.h
@@ -161,7 +161,7 @@ static inline uint32_t crc32c_plain(uint32_t crc, const unsigned char* ptr, unsi
 /**
  * Compute the CRC-32 (Castagnoli)
  */
-uint32_t (*crc32c)(uint32_t crc, const unsigned char* ptr, unsigned size);
+extern uint32_t (*crc32c)(uint32_t crc, const unsigned char* ptr, unsigned size);
 
 /**
  * Internal entry points for testing.
diff --git a/configure.ac b/configure.ac
index 267bebb7..f82b6c52 100644
--- a/configure.ac
+++ b/configure.ac
@@ -22,6 +22,8 @@ AC_CHECK_CC_OPT([-fno-omit-frame-pointer], CFLAGS="$CFLAGS -fno-omit-frame-point
 AC_CHECK_CC_OPT([-fno-inline-functions-called-once], CFLAGS="$CFLAGS -fno-inline-functions-called-once", [])
 AC_CHECK_CC_OPT([-fno-inline-small-functions], CFLAGS="$CFLAGS -fno-inline-small-functions", [])
 AC_CHECK_CC_OPT([-rdynamic], CFLAGS="$CFLAGS -rdynamic", [])
+# This the new default for gcc 10
+AC_CHECK_CC_OPT([-fno-common], CFLAGS="$CFLAGS -fno-common", [])
 
 dnl Options to improve the performance
 AC_CHECK_CC_OPT([-march=native], CFLAGS="$CFLAGS -march=native", [])