mmap anonymous

README.adoc

@@ -12535,11 +12535,26 @@ Programs under link:userland/c/[] are examples of https://en.wikipedia.org/wiki/
 
 Allocate memory! Vs using the stack: https://stackoverflow.com/questions/4584089/what-is-the-function-of-the-push-pop-instructions-used-on-registers-in-x86-ass/33583134#33583134
 
-* link:userland/c/malloc.c[]: `malloc` hello world: allocate two ints and use them.
+link:userland/c/malloc.c[]: `malloc` hello world: allocate two ints and use them.
-* link:userland/c/out_of_memory.c[]: test how much memory Linux lets us allocate
 
 LInux 5.1 / glibc 2.29 implements it with the <<mmap,`mmap` system call>>.
 
+===== malloc out o fmemory
+
+Test how much memory Linux lets us allocate:
+
+....
+./run --userland userland/c/out_of_memory.c
+....
+
+Source: link:userland/c/out_of_memory.c[]
+
+Outcome at c03d5d18ea971ae85d008101528d84c2ff25eb27 on Ubuntu 19.04 <<p51>> host: prints up to `0x1000000000` (64GiB).
+
+TODO dive into source code.
+
+Bibliography: https://stackoverflow.com/questions/2798330/maximum-memory-which-malloc-can-allocate
+
 ==== GCC C extensions
 
 ===== C empty struct
@@ -12638,19 +12653,36 @@ getconf -a
 
 ==== mmap
 
-The mmap system call allows advanced memory operations:
+The mmap system call allows advanced memory operations.
-
-* link:userland/posix/mmap_file.c[]: memory mapped file example
 
 mmap is notably used to implement the <<malloc,malloc ANSI C>> function, replacing the previously used break system call.
 
 Linux adds has several POSIX extension flags to it.
 
+[[mmap-map-anonymous]]
+===== mmap MAP_ANONYMOUS
+
+Basic `mmap` example, do the same as link:userland/c/malloc.c[], but with `mmap`.
+
+Example: userland/linux/mmap_anonymous.c[]
+
+In POSIX 7 mmap always maps to a file.
+
+If we add the MAP_ANONYMOUS Linux extension however, this is not required, and mmap can be used to allocate memory like malloc.
+
+===== mmap file
+
+Memory mapped file example: link:userland/posix/mmap_file.c[]
+
+The example creates a file, mmaps to it, writes to maped memory, and then closes the file.
+
+We then read the file and confirm it was written to.
+
 ===== brk
 
 Previously <<posix>>, but was deprecated in favor of <<malloc>>
 
-Example: link:userland/glibc/brk.c[]
+Example: link:userland/linux/brk.c[]
 
 The example allocates two ints and uses them, and then deallocates back.
 

userland/c/malloc.c

@@ -5,18 +5,27 @@
 #include <stdlib.h>
 
 int main(void) {
-    size_t bytes = sizeof(int) * 2;
+    int *is;
-    /* Allocate 2 ints. */
+    size_t nbytes = 2 * sizeof(*is);
-    int *is = malloc(bytes);
+
+    /* Allocate 2 ints. Note that unlike traditional stack arrays (non-VLA)
+     * this value does not have to be determined at compile time! */
+    is = malloc(nbytes);
+
     /* This can happen for example if we ask for too much memory. */
     if (is == NULL) {
         perror("malloc");
         exit(EXIT_FAILURE);
     }
+
+    /* Write to and read from the allocated memory. */
     is[0] = 1;
+    is[1] = 2;
     assert(is[0] == 1);
+    assert(is[1] == 2);
+
     /* Free the allocated memory. */
     free(is);
+
     return EXIT_SUCCESS;
 }
-

userland/glibc/README.adoc

@@ -1 +0,0 @@
-This directory contains glibc extensions to POSIX / ANSI C.

userland/glibc/build

@@ -1 +0,0 @@
-../build

userland/glibc/test

@@ -1 +0,0 @@
-../test

userland/linux/mmap_anonymous.c

@@ -0,0 +1,40 @@
+/* https://cirosantilli.com/linux-kernel-module-cheat#mmap-map-anonymous */
+
+#define _GNU_SOURCE
+#include <assert.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/mman.h>
+#include <unistd.h>
+
+int main(void) {
+    int *is;
+    size_t nbytes = 2 * sizeof(*is);
+
+    /* Allocate 2 ints. */
+    is = mmap(
+        NULL,
+        nbytes,
+        PROT_READ | PROT_WRITE,
+        MAP_SHARED | MAP_ANONYMOUS,
+        -1,
+        0
+    );
+
+    /* This can happen for example if we ask for too much memory. */
+    if (is == NULL) {
+        perror("mmap");
+        exit(EXIT_FAILURE);
+    }
+
+    /* Write to and read from the allocated memory. */
+    is[0] = 1;
+    is[1] = 2;
+    assert(is[0] == 1);
+    assert(is[1] == 2);
+
+    /* Free the allocated memory. */
+    munmap(is, nbytes);
+
+    return EXIT_SUCCESS;
+}

userland/posix/mmap_file.c

@@ -1,13 +1,4 @@
-/* https://cirosantilli.com/linux-kernel-module-cheat#mmap
+/* https://cirosantilli.com/linux-kernel-module-cheat#mmap-file */
- *
- * Example of mmap on files.
- *
- * Create a file, mmap to it, write to maped memory, close.
- *
- * Then read the file and confirm it was written to.
- *
- * Implemented in Linux by the mmap syscall.
- */
 
 #define _XOPEN_SOURCE 700
 #include <assert.h>