Add pointer chapter

1 files changed, 77 insertions, 3 deletions

diff --git a/md/notes/undefined_c/titles.md b/md/notes/undefined_c/titles.md
index 0108449..a09f0a7 100644
--- a/md/notes/undefined_c/titles.md
+++ b/md/notes/undefined_c/titles.md
@@ -3,8 +3,9 @@ keywords:c,linux,asm
 
 # Undefined C
 
-There is possible to piece of code inside online c compiler like https://www.onlinegdb.com/online_c_compiler
-Or run locally. With base check is done with gcc compiler.
+There is possible to run piece of code inside online c compiler like https://www.onlinegdb.com/online_c_compiler
+Or run locally. With base check is done with gcc compiler. There are many small tricks around running C code
+in practice that aren't covered in any generic tutorials.
 
 ## Compile
 
@@ -318,8 +319,80 @@ http://main.lv/writeup/c_macro_tricks.md
 
 https://jadlevesque.github.io/PPMP-Iceberg/
 
-### Signed/Unsigned
+
 ### Pointers
+
+One the C most loved feature is pointers, they allow to access addresses without any sanity check
+and they dont have any lifetime, so anything is possible with those.
+
+Pointer contains address which is interpreted according of pointer type
+
+```c
+int c;
+int ptr=&c;
+```
+
+Go over array of chars
+```c
+#include <stdio.h>
+#include <stdlib.h>
+
+int main() {
+	char s[]="asd";
+	char *c=&s;
+	while (*c != 0) {
+	    printf("NExt char %c addr %016x\n",*c,c);
+	    c++;
+	}
+}
+```
+Go over array of ints
+```c
+    int i=0;
+    int arr[] = {9,7,5,3,1};
+	int *ptr = arr;
+	while (i<5) {
+	    printf("Number value %d addr %016x\n",*ptr, ptr);
+	    ptr++;
+	    i++;
+	} 
+```
+
+Pointer arithmetics like +1 will move to next address that is offset of type size.
+As example below structure size is 12, and increment of pointer to that structure
+increment address to sizeof structure. And yes address is pointing to not mapped memory, so it 
+will segfault if accessed. 
+
+```c
+struct size12 {
+    int a,b,c;
+}
+
+int main() {
+    struct size12 *s=0;
+    s++;
+    printf("%016x\n",s);
+    s++;
+    printf("%016x\n",s);
+}
+```
+
+Double pointers are pointers to pointers
+
+```c
+#include <stdio.h>
+
+int main(int argc, char **argv) {
+	char *arg = argv[0];
+    printf("Program name %s\n",arg);
+}
+```
+
+#### How to shoot the leg
+Run pointer in while loop incrementing pointer. It will stop only when segfaults.
+
+Dont initialize pointer and it will have random value.
+
 ### Endianess 
 ### Static binary
 ### Dynamic binary
@@ -327,6 +400,7 @@ https://jadlevesque.github.io/PPMP-Iceberg/
 ### Compiler flags
 ### Allocate memory
 ### stdin,stdout,stderr
+### Signed/Unsigned