diff --git a/content/notes/18-processes-sharing-and-threads.md b/content/notes/18-processes-sharing-and-threads.md
index 0f78899a7..160529eea 100644
--- a/content/notes/18-processes-sharing-and-threads.md
+++ b/content/notes/18-processes-sharing-and-threads.md
@@ -10,6 +10,30 @@ tags:
 ![linux memory space layout](https://i.imgur.com/vLlD03U.png)
 
 Can't assume all variables are initally zero. 
+OS decide which process maps who which physical address
+
+memory mapping `mmap` is a family of system calls: 
+- allow two processes to share some region of their memory space.
+- achieved by creating a memory region that can be shared by other processes
+
+![map diagram](https://i.imgur.com/IDk8AcN.png)
+
+![mmap examples](https://i.imgur.com/zKhFpsm.png)
+
+shared memory by two processes mapping to the same file or using fork() after mmap()
+``` c
+random_fd = open("/home/hzy/test/zero", O_RDWR);
+ptr = mmap(NULL, 10240, PROT_READ|PROT_WRITE,
+	MAP_SHARED | MAP_FILE, random_fd, 0);
+```
+
+private mapping is often ysed to set up new memory sections.
+``` c
+fd = open("mmap_test_file", O_RDWR); p
+tr = mmap(NULL, 10240, PROT_READ|PROT_WRITE, 
+	MAP_PRIVATE|MAP_FILE, fd, 0);
+```
+
 
 # Threads
 Lightweight process - shares everything except the stack