Add endianness chapter

1 files changed, 121 insertions, 5 deletions

diff --git a/md/notes/undefined_c/titles.md b/md/notes/undefined_c/titles.md
index a09f0a7..014035b 100644
--- a/md/notes/undefined_c/titles.md
+++ b/md/notes/undefined_c/titles.md
@@ -393,20 +393,136 @@ Run pointer in while loop incrementing pointer. It will stop only when segfaults
 
 Dont initialize pointer and it will have random value.
 
+
+
+### Allocate memory
+
+From programs perspective memory allocation is adding address range to executable that can be addressed.
+
+malloc should be accompanied with free statement, otherwise it will have memory leaks.
+
+```c
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+int main() {
+    char *c = malloc(16);
+    memset(c,0,16);
+    int *arr = malloc(16*sizeof(int));
+    memset(arr,0,16*sizeof(int));
+    free(c);
+    free(arr);
+}
+```
+
+### Signed/Unsigned
+
+Signed and unsigned variables differ just in one bit interpretation. But they have different behavior on minimal and maximal values.
+
+
+```c
+#include <stdio.h>
+#include <limits.h>
+int main() 
+{
+    int i=INT_MAX;
+    unsigned int u=UINT_MAX;
+
+    printf("i=%d\n",i);
+    printf("u=%u\n",u);
+    
+    i++;
+    u++;
+    printf("i=%d\n",i);
+    printf("u=%u\n",u);
+    i=0;
+    u=0;
+    i--;
+    u--;
+    printf("i=%d\n",i);
+    printf("u=%u\n",u);
+
+}
+```
+
 ### Endianess 
+
+
+```c
+#include <stdlib.h>
+#include <stdio.h>
+#include <fcntl.h>
+#include <unistd.h>
+
+int main() {
+	int arr[4] = {0x00112233,0x44556677,0x8899AABB, 0xCCDDEEFF};
+	printf("%08x\n",arr[0]);
+	printf("%08x\n",arr[1]);
+	printf("%08x\n",arr[2]);
+	printf("%08x\n",arr[3]);
+
+	FILE *f = fopen("int.hex","w+");
+	fprintf(f,"%08x",arr[0]);
+	fprintf(f,"%08x",arr[1]);
+	fprintf(f,"%08x",arr[2]);
+	fprintf(f,"%08x",arr[3]);
+	fclose(f);
+
+	int fd=open("int.bin",O_CREAT|O_RDWR,S_IWUSR|S_IRUSR|S_IRGRP|S_IRWXO);
+	write(fd,arr,sizeof(arr));
+	close(fd);
+
+	int i;
+    fd = open("int.bin2",O_CREAT|O_RDWR,S_IWUSR|S_IRUSR|S_IRGRP|S_IRWXO);
+    for (i=0;i<4;i++) {
+    	uint32_t val = (arr[i]>>16) &0x0000ffff;
+    	val += (arr[i]<<16)&0xffff0000;
+    	write(fd,&val,sizeof(uint32_t));
+    }
+    close(fd);
+}
+```
+
+While saving formated values to file you will get what you expect
+```
+$ cat int.hex 
+00112233445566778899aabbccddeeff
+```
+
+Saving just memory dump of all values, will give you different result
+```
+$ hexdump int.bin 
+0000000 2233 0011 6677 4455 aabb 8899 eeff ccdd
+0000010
+```
+
+Need to swap 16bit pairs to look same as value memory dump
+```
+$ hexdump int.bin2
+0000000 0011 2233 4455 6677 8899 aabb ccdd eeff
+0000010
+``` 
+
+### Compiler flags
+
+Compiler have whole list of command line arguments that you can enable for different purposes, lets look into some of them
+https://gcc.gnu.org/onlinedocs/gcc/Option-Summary.html
+
+
+
+
 ### Static binary
 ### Dynamic binary
-### Styles
-### Compiler flags
-### Allocate memory
 ### stdin,stdout,stderr
-### Signed/Unsigned
+### Styles
+
 
 
 
 ## Basic usage
 
-### File manipulation
+### File manipulation with libc
 ### File manipulation with syscalls
 
 ## Base usage