diff --git a/CMakeLists.txt b/CMakeLists.txt
index f8cee71c8..0abf1df7b 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -551,6 +551,49 @@ else()
     message(STATUS "Unknown architecture")
 endif()
 
+# clock_gettime came in POSIX.1b (1993)
+# CLOCK_MONOTONIC came in POSIX.1-2001 / SUSv3 as optional
+# posix_memalign came in POSIX.1-2001 / SUSv3
+# M_PI is an XSI extension since POSIX.1-2001 / SUSv3, came in XPG1 (1985)
+add_compile_definitions(_XOPEN_SOURCE=600)
+
+# Somehow in OpenBSD whenever POSIX conformance is specified
+# some string functions rely on locale_t availability,
+# which was introduced in POSIX.1-2008, forcing us to go higher
+IF (CMAKE_SYSTEM_NAME MATCHES "OpenBSD")
+    remove_definitions(-D_XOPEN_SOURCE=600)
+    add_compile_definitions(_XOPEN_SOURCE=700)
+ENDIF()
+
+# Data types, macros and functions related to controlling CPU affinity and
+# some memory allocation are available on Linux through GNU extensions in libc
+IF (CMAKE_SYSTEM_NAME MATCHES "Linux")
+    add_compile_definitions(_GNU_SOURCE)
+ENDIF()
+
+# RLIMIT_MEMLOCK came in BSD, is not specified in POSIX.1,
+# and on macOS its availability depends on enabling Darwin extensions
+# similarly on DragonFly, enabling BSD extensions is necessary
+IF (CMAKE_SYSTEM_NAME MATCHES "Darwin")
+    add_compile_definitions(_DARWIN_C_SOURCE)
+ENDIF()
+IF (CMAKE_SYSTEM_NAME MATCHES "DragonFly")
+    add_compile_definitions(_DARWIN_C_SOURCE)
+ENDIF()
+
+# alloca is a non-standard interface that is not visible on BSDs when
+# POSIX conformance is specified, but not all of them provide a clean way
+# to enable it in such cases
+IF (CMAKE_SYSTEM_NAME MATCHES "FreeBSD")
+    add_compile_definitions(__BSD_VISIBLE)
+ENDIF()
+IF (CMAKE_SYSTEM_NAME MATCHES "NetBSD")
+    add_compile_definitions(_NETBSD_SOURCE)
+ENDIF()
+IF (CMAKE_SYSTEM_NAME MATCHES "OpenBSD")
+    add_compile_definitions(_BSD_SOURCE)
+ENDIF()
+
 #
 # libraries
 #
diff --git a/Makefile b/Makefile
index 86e36ba52..a774dc50f 100644
--- a/Makefile
+++ b/Makefile
@@ -106,6 +106,56 @@ MK_CFLAGS   = $(OPT) -std=c11   -fPIC
 MK_CXXFLAGS = $(OPT) -std=c++11 -fPIC
 MK_LDFLAGS  =
 
+# clock_gettime came in POSIX.1b (1993)
+# CLOCK_MONOTONIC came in POSIX.1-2001 / SUSv3 as optional
+# posix_memalign came in POSIX.1-2001 / SUSv3
+# M_PI is an XSI extension since POSIX.1-2001 / SUSv3, came in XPG1 (1985)
+MK_CFLAGS   += -D_XOPEN_SOURCE=600
+MK_CXXFLAGS += -D_XOPEN_SOURCE=600
+
+# Somehow in OpenBSD whenever POSIX conformance is specified
+# some string functions rely on locale_t availability,
+# which was introduced in POSIX.1-2008, forcing us to go higher
+ifeq ($(UNAME_S),OpenBSD)
+	MK_CFLAGS   += -U_XOPEN_SOURCE -D_XOPEN_SOURCE=700
+	MK_CXXFLAGS += -U_XOPEN_SOURCE -D_XOPEN_SOURCE=700
+endif
+
+# Data types, macros and functions related to controlling CPU affinity and
+# some memory allocation are available on Linux through GNU extensions in libc
+ifeq ($(UNAME_S),Linux)
+	MK_CFLAGS   += -D_GNU_SOURCE
+	MK_CXXFLAGS += -D_GNU_SOURCE
+endif
+
+# RLIMIT_MEMLOCK came in BSD, is not specified in POSIX.1,
+# and on macOS its availability depends on enabling Darwin extensions
+# similarly on DragonFly, enabling BSD extensions is necessary
+ifeq ($(UNAME_S),Darwin)
+	MK_CFLAGS   += -D_DARWIN_C_SOURCE
+	MK_CXXFLAGS += -D_DARWIN_C_SOURCE
+endif
+ifeq ($(UNAME_S),DragonFly)
+	MK_CFLAGS   += -D__BSD_VISIBLE
+	MK_CXXFLAGS += -D__BSD_VISIBLE
+endif
+
+# alloca is a non-standard interface that is not visible on BSDs when
+# POSIX conformance is specified, but not all of them provide a clean way
+# to enable it in such cases
+ifeq ($(UNAME_S),FreeBSD)
+	MK_CFLAGS   += -D__BSD_VISIBLE
+	MK_CXXFLAGS += -D__BSD_VISIBLE
+endif
+ifeq ($(UNAME_S),NetBSD)
+	MK_CFLAGS   += -D_NETBSD_SOURCE
+	MK_CXXFLAGS += -D_NETBSD_SOURCE
+endif
+ifeq ($(UNAME_S),OpenBSD)
+	MK_CFLAGS   += -D_BSD_SOURCE
+	MK_CXXFLAGS += -D_BSD_SOURCE
+endif
+
 ifdef LLAMA_DEBUG
 	MK_CFLAGS   += -O0 -g
 	MK_CXXFLAGS += -O0 -g
diff --git a/examples/beam-search/beam-search.cpp b/examples/beam-search/beam-search.cpp
index 4d021434b..6b31aea78 100644
--- a/examples/beam-search/beam-search.cpp
+++ b/examples/beam-search/beam-search.cpp
@@ -1,7 +1,3 @@
-#ifndef _GNU_SOURCE
-#define _GNU_SOURCE
-#endif
-
 #include "common.h"
 #include "llama.h"
 #include "build-info.h"
diff --git a/examples/embd-input/embd-input-lib.cpp b/examples/embd-input/embd-input-lib.cpp
index 87aac3479..ef12212ba 100644
--- a/examples/embd-input/embd-input-lib.cpp
+++ b/examples/embd-input/embd-input-lib.cpp
@@ -1,8 +1,3 @@
-// Defines sigaction on msys:
-#ifndef _GNU_SOURCE
-#define _GNU_SOURCE
-#endif
-
 #include "embd-input.h"
 
 #include <cassert>
diff --git a/examples/main/main.cpp b/examples/main/main.cpp
index c9ca7719b..be030fffb 100644
--- a/examples/main/main.cpp
+++ b/examples/main/main.cpp
@@ -1,8 +1,3 @@
-// Defines sigaction on msys:
-#ifndef _GNU_SOURCE
-#define _GNU_SOURCE
-#endif
-
 #include "common.h"
 
 #include "console.h"
diff --git a/examples/simple/simple.cpp b/examples/simple/simple.cpp
index 4ee85faca..ba5de0cc6 100644
--- a/examples/simple/simple.cpp
+++ b/examples/simple/simple.cpp
@@ -1,7 +1,3 @@
-#ifndef _GNU_SOURCE
-#define _GNU_SOURCE
-#endif
-
 #include "build-info.h"
 
 #include "common.h"
diff --git a/examples/speculative/speculative.cpp b/examples/speculative/speculative.cpp
index c6211ac79..822d7b529 100644
--- a/examples/speculative/speculative.cpp
+++ b/examples/speculative/speculative.cpp
@@ -1,7 +1,3 @@
-#ifndef _GNU_SOURCE
-#define _GNU_SOURCE
-#endif
-
 #include "build-info.h"
 
 #include "common.h"
diff --git a/ggml-alloc.c b/ggml-alloc.c
index e2ac891d1..a1f6e7bf4 100644
--- a/ggml-alloc.c
+++ b/ggml-alloc.c
@@ -1,8 +1,3 @@
-// defines MAP_ANONYMOUS
-#ifndef _GNU_SOURCE
-#define _GNU_SOURCE
-#endif
-
 #include "ggml-alloc.h"
 #include "ggml.h"
 #include <assert.h>
diff --git a/ggml.c b/ggml.c
index a4b9781d5..d5ca0101a 100644
--- a/ggml.c
+++ b/ggml.c
@@ -1,4 +1,3 @@
-#define _GNU_SOURCE // Defines CLOCK_MONOTONIC on Linux
 #define _CRT_SECURE_NO_DEPRECATE // Disables ridiculous "unsafe" warnigns on Windows
 
 #include "ggml.h"
diff --git a/llama.cpp b/llama.cpp
index cab7156f4..3f1190221 100644
--- a/llama.cpp
+++ b/llama.cpp
@@ -1,8 +1,3 @@
-// Defines fileno on msys:
-#ifndef _GNU_SOURCE
-#define _GNU_SOURCE
-#endif
-
 #include "llama.h"
 
 #include "ggml.h"