diff --git a/libc/include/sys/mman.h b/libc/include/sys/mman.h
index 97c7022c..c7433b19 100644
--- a/libc/include/sys/mman.h
+++ b/libc/include/sys/mman.h
@@ -10,6 +10,10 @@
 
 #define MAP_FAILED (void*)-1
 
+#define MS_SYNC 1
+#define MS_ASYNC 2
+#define MS_INVALIDATE 4
+
 #ifdef __cplusplus
 extern "C"
 {
@@ -21,6 +25,9 @@ extern "C"
     /* Remove a memory mapping. */
     int munmap(void* addr, size_t len);
 
+    /* Write modified shared memory back to its associated file. */
+    int msync(void* addr, size_t len, int flags);
+
 #ifdef __cplusplus
 }
 #endif
diff --git a/libc/include/unistd.h b/libc/include/unistd.h
index af3b60da..5293b839 100644
--- a/libc/include/unistd.h
+++ b/libc/include/unistd.h
@@ -18,6 +18,9 @@
 
 #define _POSIX_CHOWN_RESTRICTED 200112L
 #define _POSIX_SHELL 200112L
+#define _POSIX_MAPPED_FILES 200112L
+#define _POSIX_SYNCHRONIZED_IO 200112L
+#define _POSIX_SAVED_IDS 200112L
 #define _POSIX_VDISABLE (-2)
 
 #ifdef __cplusplus
diff --git a/libc/src/sys/mman.cpp b/libc/src/sys/mman.cpp
index 2f855366..9fdc7a2d 100644
--- a/libc/src/sys/mman.cpp
+++ b/libc/src/sys/mman.cpp
@@ -19,4 +19,10 @@ extern "C"
         long rc = syscall(SYS_munmap, addr, len);
         __errno_return(rc, int);
     }
+
+    int msync(void* addr, size_t len, int)
+    {
+        long rc = syscall(SYS_msync, addr, len);
+        __errno_return(rc, int);
+    }
 }