start moving malloc and friends in

2026-01-23 02:05:57 +01:00 · 2019-08-11 00:00:00 +00:00
parent 975b15b814
commit d7a24ea200
25 changed files with 222 additions and 26 deletions
--- a/userland/posix/mmap_file.c
+++ b/userland/posix/mmap_file.c
@@ -0,0 +1,118 @@
+/* https://cirosantilli.com/linux-kernel-module-cheat#mmap
+ *
+ * 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>
+#include <fcntl.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <sys/mman.h>
+#include <sys/stat.h>
+#include <sys/types.h>
+#include <unistd.h>
+
+#include <lkmc.h>
+
+int main(void) {
+    char *filepath = LKMC_TMP_FILE;
+    enum Constexpr { NUMINTS = 4 };
+    size_t filesize = NUMINTS * sizeof(int);
+
+    int i;
+    int fd;
+    int result;
+    /* mmapped array of int's */
+    int *map;
+
+    /* Write to file with mmap. */
+    {
+        /* `O_WRONLY` is not sufficient when mmaping, need `O_RDWR`.*/
+        fd = open(filepath, O_RDWR | O_CREAT | O_TRUNC, S_IRUSR | S_IWUSR);
+        if (fd == -1) {
+            perror("open");
+            exit(EXIT_FAILURE);
+        }
+
+        /* Set fd position and write to it to strech the file. */
+        result = lseek(fd, filesize - 1, SEEK_SET);
+        if (result == -1) {
+            close(fd);
+            perror("lseek");
+            exit(EXIT_FAILURE);
+        }
+
+        /* Write something to the file to actually strech it. */
+        result = write(fd, "", 1);
+        if (result != 1) {
+            close(fd);
+            perror("write");
+            exit(EXIT_FAILURE);
+        }
+
+        /* Do the actual mapping call. */
+        map = mmap(NULL, filesize, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0);
+        if (map == MAP_FAILED) {
+            close(fd);
+            perror("mmap");
+            exit(EXIT_FAILURE);
+        }
+
+        /* Write int's to the file as if it were memory because MAP_SHARED was used. */
+        for (i = 0; i < NUMINTS; ++i) {
+            map[i] = i;
+        }
+
+        /* Free mmapped memory. */
+        if (munmap(map, filesize) == -1) {
+            perror("munmap");
+            exit(EXIT_FAILURE);
+        }
+
+        /* Un-mmaping doesn't close the file, so we still need to do that. */
+        if (close(fd) == -1) {
+            perror("close");
+            exit(EXIT_FAILURE);
+        }
+    }
+
+    /* Read result back in. */
+    {
+        fd = open(filepath, O_RDONLY, 0);
+        if (fd == -1) {
+            perror("open");
+            exit(EXIT_FAILURE);
+        }
+
+        map = mmap(0, filesize, PROT_READ, MAP_SHARED, fd, 0);
+        if (map == MAP_FAILED) {
+            close(fd);
+            perror("mmap");
+            exit(EXIT_FAILURE);
+        }
+
+        assert(map[1] == 1);
+
+        /* Segmentation fault because no `PROT_WRITE`: */
+        /*map[1] = 2;*/
+
+        if (munmap(map, filesize) == -1) {
+            perror("munmap");
+            exit(EXIT_FAILURE);
+        }
+
+        if (close(fd) == -1) {
+            perror("close");
+            exit(EXIT_FAILURE);
+        }
+    }
+
+    return EXIT_SUCCESS;
+}