add a malloc example

it demonstrates that memory is only allocated if used.

Signed-off-by: Manuel Traut <manut@linutronix.de>

3 files changed, 69 insertions, 0 deletions

diff --git a/schulung_tools/malloc/Makefile b/schulung_tools/malloc/Makefile
new file mode 100644
index 0000000..b6160a8
--- /dev/null
+++ b/schulung_tools/malloc/Makefile
@@ -0,0 +1,7 @@
+malloc: malloc.c
+	$(CROSS_COMPILE)gcc -g -omalloc malloc.c
+
+clean:
+	rm -f malloc core
+
+.PHONY: clean
diff --git a/schulung_tools/malloc/README b/schulung_tools/malloc/README
new file mode 100644
index 0000000..77b18cd
--- /dev/null
+++ b/schulung_tools/malloc/README
@@ -0,0 +1,5 @@
+This program demonstrates that a processes is not actually assigned memory
+until it page-faults on the pages.
+
+WARNING: This program might fill your RAM and cause the Linux OOM Killer to
+         be invoked.
diff --git a/schulung_tools/malloc/malloc.c b/schulung_tools/malloc/malloc.c
new file mode 100644
index 0000000..a636426
--- /dev/null
+++ b/schulung_tools/malloc/malloc.c
@@ -0,0 +1,57 @@
+#include <stdio.h>
+#include <stdlib.h>
+#include <unistd.h>
+#include <string.h>
+
+#define NUM_OF_GBYTES 8
+
+/* if verbose is enabled forks are done for printing additional informations */
+#define VERBOSE 0
+
+#if VERBOSE
+/* forking is not always possible if memory is low */
+#define sys(cmd) if(system(cmd) == -1) printf("%s failed\n", cmd);
+#else
+#define sys(cmd) ;
+#endif
+
+#define MEMFREE "free -h"
+
+int main(void)
+{
+	char *p[NUM_OF_GBYTES];
+	int i;
+
+	printf("memory usage:\n");
+	sys(MEMFREE);
+	printf("\n\n");
+
+	for (i = 0; i < NUM_OF_GBYTES; i++) {
+		p[i] = malloc(1024 * 1024 * 1024);
+		printf("allocated 1GB: %p\n", p[i]);
+		sys(MEMFREE);
+	}
+
+	printf("\n\n");
+
+	for (i = 0; i < NUM_OF_GBYTES; i++) {
+		if (p[i]) {
+			printf("memsetting 1GB: %p\n", p[i]);
+			sys("date +%s.%N");
+			memset(p[i], 1, 1024 * 1024 * 1024);
+			sys("date +%s.%N");
+			sys(MEMFREE);
+		}
+	}
+
+	printf("\n\n");
+
+	for (i = 0; i < NUM_OF_GBYTES; i++) {
+		if (p[i]) {
+			printf("free 1GB: %p\n", p[i]);
+			free(p[i]);
+			sys(MEMFREE);
+		}
+	}
+	return 0;
+}