brk/sbrk demo

break.c

@@ -0,0 +1,57 @@
+#include <stdint.h>
+#include <stdio.h>
+#include <sys/mman.h> // mmap and friends
+#include <unistd.h>
+
+int main(void) {
+    /*
+     * See: "man 3 sysconf" or
+     * https://www.man7.org/linux/man-pages/man3/sysconf.3.html
+     *
+     * See: "man 3 getauxval" or
+     * https://www.man7.org/linux/man-pages/man3/getauxval.3.html
+     */
+    long page_size =
+        sysconf(_SC_PAGESIZE); // or _SC_PAGE_SIZE (POSIX allows both)
+    if (page_size == -1) {
+        perror("sysconf");
+        return 1;
+    }
+
+    printf("Page size: %ld bytes\n", page_size);
+
+    /*
+     * sbrk():
+     * Increase or decrease the end of accessible data space by DELTA bytes.
+     * If successful, returns the address the previous end of data space
+     * (i.e. the beginning of the new space, if DELTA > 0); returns (void \*) -1
+     * for errors (with errno set).
+     */
+    void *first = sbrk(0);
+    void *second = sbrk(4096);
+    void *third = sbrk(0);
+
+    printf("First: %p\n", first);
+    printf("Second: %p\n", second);
+    printf("Third: %p\n", third);
+
+    /*
+     * mmap, munmap - map or unmap files or devices into memory
+     *
+     * mmap()  creates a new mapping in the virtual address space of the
+     * calling process.  The starting address for the new mapping is specified
+     * in addr. The length argument specifies the  length  of  the  mapping
+     * (which  must  be greater than 0).
+     */
+
+    uint8_t *first_mmap = mmap(NULL, page_size, PROT_READ | PROT_WRITE,
+                               MAP_PRIVATE | MAP_ANON, -1, 0);
+
+    uint8_t *second_mmap = mmap(NULL, page_size, PROT_READ | PROT_WRITE,
+                                MAP_PRIVATE | MAP_ANON, -1, 0);
+
+    printf("First mmap: %p\n", first_mmap);
+    printf("Second mmap: %p\n", second_mmap);
+
+    return 0;
+}