diff --git a/src/common/Makefile b/src/common/Makefile
index 485daae7f..ec7ed48ea 100644
--- a/src/common/Makefile
+++ b/src/common/Makefile
@@ -1,5 +1,5 @@
 CC = gcc
-CFLAGS = -g -Wall -Wno-typedef-redefinition --std=c99 -D_XOPEN_SOURCE=500 -D_POSIX_C_SOURCE=200809L -fPIC -I. -Ithirdparty -Ithirdparty/ae
+CFLAGS = -g -Wall -Wextra -Werror=implicit-function-declaration -Wno-typedef-redefinition -Wno-sign-compare -Wno-unused-parameter -Wno-type-limits -Wno-missing-field-initializers --std=c99 -D_XOPEN_SOURCE=500 -D_POSIX_C_SOURCE=200809L -fPIC -I. -Ithirdparty -Ithirdparty/ae
 BUILD = build
 
 all: hiredis redis $(BUILD)/libcommon.a
diff --git a/src/global_scheduler/Makefile b/src/global_scheduler/Makefile
index b98240be2..da45947f1 100644
--- a/src/global_scheduler/Makefile
+++ b/src/global_scheduler/Makefile
@@ -1,5 +1,5 @@
 CC = gcc
-CFLAGS = -g -Wall --std=c99 -D_XOPEN_SOURCE=500 -D_POSIX_C_SOURCE=200809L -I. -I../common -I../common/thirdparty -I../common/state
+CFLAGS = -g -Wall -Wextra -Werror=implicit-function-declaration -Wno-sign-compare -Wno-unused-parameter -Wno-type-limits -Wno-missing-field-initializers --std=c99 -D_XOPEN_SOURCE=500 -D_POSIX_C_SOURCE=200809L -I. -I../common -I../common/thirdparty -I../common/state
 BUILD = build
 
 all: $(BUILD)/global_scheduler
diff --git a/src/photon/Makefile b/src/photon/Makefile
index 9d7db8eb7..e0334bd27 100644
--- a/src/photon/Makefile
+++ b/src/photon/Makefile
@@ -1,5 +1,5 @@
 CC = gcc
-CFLAGS = -g -Wall --std=c99 -D_XOPEN_SOURCE=500 -D_POSIX_C_SOURCE=200809L -I.. -I../common -I../common/thirdparty -fPIC
+CFLAGS = -g -Wall -Wextra -Werror=implicit-function-declaration -Wno-sign-compare -Wno-unused-parameter -Wno-type-limits -Wno-missing-field-initializers --std=c99 -D_XOPEN_SOURCE=500 -D_POSIX_C_SOURCE=200809L -I.. -I../common -I../common/thirdparty -fPIC
 BUILD = build
 
 all: $(BUILD)/photon_scheduler $(BUILD)/photon_client.a
diff --git a/src/plasma/Makefile b/src/plasma/Makefile
index 1e33f376c..f0dfe7497 100644
--- a/src/plasma/Makefile
+++ b/src/plasma/Makefile
@@ -1,5 +1,5 @@
 CC = gcc
-CFLAGS = -g -Wall --std=c99 -D_XOPEN_SOURCE=500 -D_POSIX_C_SOURCE=200809L -I. -I../common -I../common/thirdparty
+CFLAGS = -g -Wall -Wextra -Werror=implicit-function-declaration -Wno-sign-compare -Wno-unused-parameter -Wno-type-limits -Wno-missing-field-initializers --std=c99 -D_XOPEN_SOURCE=500 -D_POSIX_C_SOURCE=200809L -I. -I../common -I../common/thirdparty
 TEST_CFLAGS = -DPLASMA_TEST=1 -I.
 BUILD = build
 
diff --git a/src/plasma/plasma_client.c b/src/plasma/plasma_client.c
index c0222a408..22408d5e1 100644
--- a/src/plasma/plasma_client.c
+++ b/src/plasma/plasma_client.c
@@ -25,6 +25,11 @@
 #define NUM_CONNECT_ATTEMPTS 50
 #define CONNECT_TIMEOUT 100
 
+#ifndef _WIN32
+/* This function is actually not declared in standard POSIX, so declare it. */
+extern int usleep(useconds_t usec);
+#endif
+
 typedef struct {
   /** Key that uniquely identifies the  memory mapped file. In practice, we
    *  take the numerical value of the file descriptor in the object store. */
diff --git a/src/plasma/test/manager_tests.c b/src/plasma/test/manager_tests.c
index f33e6d7b1..3da35a9c0 100644
--- a/src/plasma/test/manager_tests.c
+++ b/src/plasma/test/manager_tests.c
@@ -15,6 +15,11 @@
 #include "plasma_client.h"
 #include "plasma_manager.h"
 
+#ifndef _WIN32
+/* This function is actually not declared in standard POSIX, so declare it. */
+extern int usleep(useconds_t usec);
+#endif
+
 SUITE(plasma_manager_tests);
 
 const char *manager_addr = "127.0.0.1";