From 82967c6024f20f66a5ecf072cfe3028674ab8be2 Mon Sep 17 00:00:00 2001
From: Remzi Arpaci-Dusseau <remzi.arpacidusseau@gmail.com>
Date: Tue, 10 Aug 2021 13:08:35 -0500
Subject: [PATCH] initial cut of mmap'd stack code

---
 file-intro/pstack.c | 70 +++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 70 insertions(+)
 create mode 100644 file-intro/pstack.c

diff --git a/file-intro/pstack.c b/file-intro/pstack.c
new file mode 100644
index 0000000..b8e4f58
--- /dev/null
+++ b/file-intro/pstack.c
@@ -0,0 +1,70 @@
+// Persistent stack example of how mmap() naturally creates a software abstraction of persistent memory.
+// Author: Terence Kelly <tpklly@acm.org>
+
+// Note: Use 'dd' to make the initial (empty) image, e.g., 
+//   dd if=/dev/zero of=ps.img bs=32 count=1
+// makes a 32-byte empty file (the first integer being the size of the stack,
+// the remaining integers being values)
+
+#include <assert.h>
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <unistd.h>
+#include <sys/mman.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+
+int open_or_die(const char *path, int oflag) {
+    int fd = open(path, oflag);
+    assert(fd > 0);
+    return fd;
+}
+
+int fstat_or_die(int fildes, struct stat *buf) {
+    int rc = fstat(fildes, buf);
+    assert(rc == 0);
+    return rc;
+}
+
+void *mmap_or_die(void *addr, size_t len, int prot, int flags, int fd, off_t offset) {
+    void *p = mmap(addr, len, prot, flags, fd, offset);
+    assert(p != MAP_FAILED);
+    return p;
+}
+
+typedef struct {
+    size_t n;
+    int stack[]; // zero-length per C99
+} pstack_t; 
+
+int main(int argc, char *argv[]) {
+    int fd, rc;
+    struct stat s;
+    size_t file_size;
+    pstack_t *p;
+    
+    fd = open_or_die("ps.img", O_RDWR);
+    rc = fstat_or_die(fd, &s);
+
+    file_size = (size_t) s.st_size;
+    assert(file_size % sizeof(int) == 0);
+
+    p = mmap_or_die(NULL, file_size, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
+
+    for (int i = 1; i < argc; i++)
+	if (strcmp(argv[i], "pop") == 0) {  
+	    if (p->n > 0)  // stack not empty
+		printf("%d\n", p->stack[--p->n]);
+	    else
+		printf("stack empty\n");
+	} else
+	    if (sizeof(pstack_t) + p->n * sizeof(int) < file_size)  // stack not full
+		p->stack[p->n++] = atoi(argv[i]);
+	    else
+		printf("stack full: %d not added\n", atoi(argv[i]));
+
+    close(fd);
+    return 0;
+}