Add type_punning.c file

It compiles in both C AND C++!

c/hello_world.c

@@ -1,4 +1,7 @@
 /*
+
+This file is part of eRCaGuy_hello_world: https://github.com/ElectricRCAircraftGuy/eRCaGuy_hello_world
+
 hello_world
 
 Gabriel Staples

c/type_punning.c

@@ -0,0 +1,160 @@
+/*
+
+This file is part of eRCaGuy_hello_world: https://github.com/ElectricRCAircraftGuy/eRCaGuy_hello_world
+
+GS
+22 Apr. 2021
+
+Show various ways of type punning (reinterpreting multi-byte values as byte arrays) in C and C++.
+
+To compile and run (assuming you've already `cd`ed into this dir):
+1. In C:
+    mkdir -p bin && gcc -Wall -Wextra -Werror -O3 -std=c11 -save-temps=obj type_punning.c \
+    -o bin/type_punning && bin/type_punning
+2. In C++
+    mkdir -p bin && g++ -Wall -Wextra -Werror -O3 -std=c++17 -save-temps=obj type_punning.c \
+    -o bin/type_punning && bin/type_punning
+
+References:
+1. [my answer--READ THIS FOR A DETAILED EXPLANATION OF THE BELOW CODE!]
+   https://stackoverflow.com/questions/67206482/portability-of-using-union-for-conversion/67206563#67206563
+
+*/
+
+#include <stdint.h>
+#include <stdio.h>
+
+typedef union my_union_u
+{
+    uint32_t value;
+    /// A byte array large enough to hold the largest of any value in the union.
+    uint8_t bytes[sizeof(uint32_t)];
+} my_union_t;
+
+int main()
+{
+    printf("Type punning and ptr-based serialization demo\n");
+
+    // -------------------------------------------------------------------------
+
+    printf("TECHNIQUE 1: union-based type punning:\n");
+    {
+        my_union_t u;
+
+        // write to uint32_t value
+        u.value = 1234;
+
+        // read & print individual bytes from uint32_t value
+        printf("1st byte = 0x%02X\n", (u.bytes)[0]);
+        printf("2nd byte = 0x%02X\n", (u.bytes)[1]);
+        printf("3rd byte = 0x%02X\n", (u.bytes)[2]);
+        printf("4th byte = 0x%02X\n", (u.bytes)[3]);
+    }
+
+    // -------------------------------------------------------------------------
+
+    printf("TECHNIQUE 2: reading through raw pointers:\n");
+    {
+        uint32_t value = 1234;
+        uint8_t *bytes = (uint8_t *)&value;
+
+        // read & print individual bytes from uint32_t value
+        printf("1st byte = 0x%02X\n", bytes[0]);
+        printf("2nd byte = 0x%02X\n", bytes[1]);
+        printf("3rd byte = 0x%02X\n", bytes[2]);
+        printf("4th byte = 0x%02X\n", bytes[3]);
+    }
+
+    // -------------------------------------------------------------------------
+
+    printf("TECHNIQUE 3.1: use bit-masks and bit shifting:\n");
+    {
+        uint32_t value = 1234;
+
+        // read individual bytes from uint32_t value
+        uint8_t byte0 = (value >> 0)  & 0xff;
+        uint8_t byte1 = (value >> 8)  & 0xff;
+        uint8_t byte2 = (value >> 16) & 0xff;
+        uint8_t byte3 = (value >> 24) & 0xff;
+
+        // Print the bytes
+        printf("1st byte = 0x%02X\n", byte0);
+        printf("2nd byte = 0x%02X\n", byte1);
+        printf("3rd byte = 0x%02X\n", byte2);
+        printf("4th byte = 0x%02X\n", byte3);
+    }
+
+    // -------------------------------------------------------------------------
+
+    printf("TECHNIQUE 3.2: use a convenience macro to do bit-masks and "
+           "bit shifting:\n");
+    {
+        #define BYTE(value, byte_num) ((uint8_t)(((value) >> (8*(byte_num))) & 0xff))
+
+        uint32_t value = 1234;
+
+        // read individual bytes from uint32_t value
+
+        uint8_t byte0 = BYTE(value, 0);
+        uint8_t byte1 = BYTE(value, 1);
+        uint8_t byte2 = BYTE(value, 2);
+        uint8_t byte3 = BYTE(value, 3);
+
+        // OR
+
+        uint8_t bytes[] = {
+            BYTE(value, 0),
+            BYTE(value, 1),
+            BYTE(value, 2),
+            BYTE(value, 3),
+        };
+
+        // Print the bytes
+
+        printf("1st byte = 0x%02X\n", byte0);
+        printf("2nd byte = 0x%02X\n", byte1);
+        printf("3rd byte = 0x%02X\n", byte2);
+        printf("4th byte = 0x%02X\n", byte3);
+        printf("---------------\n");
+        printf("1st byte = 0x%02X\n", bytes[0]);
+        printf("2nd byte = 0x%02X\n", bytes[1]);
+        printf("3rd byte = 0x%02X\n", bytes[2]);
+        printf("4th byte = 0x%02X\n", bytes[3]);
+    }
+
+
+    return 0;
+}
+
+/*
+SAMPLE OUTPUT (ran on an x86-64 **little endian** Ubuntu 20.04 machine):
+
+    eRCaGuy_hello_world/c$ mkdir -p bin && gcc -Wall -Wextra -Werror -O3 -std=c11 -save-temps=obj type_punning.c     -o bin/type_punning && bin/type_punning
+    Type punning and ptr-based serialization demo
+    TECHNIQUE 1: union-based type punning:
+    1st byte = 0xD2
+    2nd byte = 0x04
+    3rd byte = 0x00
+    4th byte = 0x00
+    TECHNIQUE 2: reading through raw pointers:
+    1st byte = 0xD2
+    2nd byte = 0x04
+    3rd byte = 0x00
+    4th byte = 0x00
+    TECHNIQUE 3.1: use bit-masks and bit shifting:
+    1st byte = 0xD2
+    2nd byte = 0x04
+    3rd byte = 0x00
+    4th byte = 0x00
+    TECHNIQUE 3.2: use a convenience macro to do bit-masks and bit shifting:
+    1st byte = 0xD2
+    2nd byte = 0x04
+    3rd byte = 0x00
+    4th byte = 0x00
+    ---------------
+    1st byte = 0xD2
+    2nd byte = 0x04
+    3rd byte = 0x00
+    4th byte = 0x00
+
+*/