organise

1 files changed, 67 insertions, 0 deletions

diff --git a/2/homework/bytes_structure_2.71.c b/2/homework/bytes_structure_2.71.c
new file mode 100644
index 0000000..61ead1c
--- /dev/null
+++ b/2/homework/bytes_structure_2.71.c
@@ -0,0 +1,67 @@
+#include <stdio.h>
+#include <string.h>
+
+typedef unsigned packed_t;
+
+/*
+  3    2    1    0
+  byte byte byte byte
+  1000 1100 1110 1111
+  xbyte(..., 2);
+  >> 8
+  0000 0000 1000 1100
+  & 0xFF
+  0000 0000 0000 1100
+
+  0 <= x < 7 -> id
+  -8 < x <  0 -> minus
+*/
+int xbyte(packed_t word, int bytenume);
+int xbyte(packed_t word, int bytenum) {
+    int max_bytes = 3;
+    int max_bits = max_bytes << 3;
+    int shift_bits = (3 - bytenum) << 3;
+    return (int) word << shift_bits >> max_bits;
+}
+
+/*
+  Copy integer into buffer if space is available
+  WARNING: The following code is buggy
+
+  size_t is an unsigned. So one of the operands gets casted.
+  Either
+  int -> unsigned
+  or
+  unsigned -> int
+
+  apparently the result of unsigned - int or int - unsigned -> unsigned
+ */
+void buggy_copy_int(int val, void *buf, int maxbytes) {
+    /*
+      Check if maxbytes will cause problems when converted to unsigned.
+     */
+    if (maxbytes < 0) {
+        return;
+    }
+    if (maxbytes >= sizeof(val)) {
+        memcpy(buf, (void *) &val, sizeof(val));
+    }
+}
+
+int main() {
+    int maxbytes = 4;
+    char buf[maxbytes];
+    int i;
+    for (i=0; i<maxbytes; ++i) {
+        buf[i] = 'a';
+    }
+
+    buggy_copy_int(32, buf, maxbytes);
+    for (i=0; i<maxbytes; ++i) {
+        printf("%x\n", buf[i]);
+    }
+    return 0;
+
+    printf("%x\n", xbyte(0x0000FF00, 1));
+    return 0;
+}