Lab 06 - Structs and enums

• A struct groups variables under one name.
• Members can be of different types.
• Access with the dot operator (.) or arrow (→) if you have a pointer.
• Structs can be copied by value (unlike arrays!).
• sizeof(struct) may include padding bytes - compiler aligns fields for efficiency.

struct Point {
    int x;
    int y;
};
 
struct Point p = {10, 20};
printf("%d %d", p.x, p.y);

• Use typedef for convenience:

typedef struct {
    char name[20];
    int age;
} Person;

• All members share the same memory.
• Size = size of the largest member.
• Only one field is valid at a time.
• Writing to one member overwrites all others.
• Common in embedded systems or binary data parsing.

union Number {
    int i;
    float f;
    char c;
};
 
union Number n;
n.f = 3.14;
printf("%f", n.f);

• Named integer constants for clarity.
• Default values start at 0, unless specified.
• Enum identifiers are just integers.
• You can assign any int, even if not in the enum (C does not check validity).

enum Color { RED, GREEN = 5, BLUE };
enum Color c = BLUE;
printf("%d\n", c);  // prints 6

• Compact representation of flags or small numbers inside structs.
• Specify number of bits per field.
• Bit-field ordering (left/right) is implementation-defined.
• Be careful mixing signed/unsigned types.
• Cannot take the address of a bit-field.

struct Status {
    unsigned int power : 1;
    unsigned int error : 1;
    unsigned int mode  : 2; // 0–3
};

• Runs before compilation - simple text substitution.
• Compilers can set macros directly from the command line
• Use -E to see the preprocessor phase output or -S for assembly
• Common uses:
  • #define macros
  • #ifdef, #ifndef, #endif for conditional compilation
  • #include for file inclusion
• Always parenthesize macro arguments:

      #define SQUARE(x) ((x)*(x))  // OK
      #define SQUARE(x) (x*x)  // WRONG

#define PI 3.14159
#define SQUARE(x) ((x)*(x))
#define DEBUG
 
#ifdef DEBUG
  printf("Debug mode on\n");
#endif

Create a program that defines:

enum Department { IT, HR, SALES };
struct Employee {
    char name[30];
    int age;
    enum Department dept;
};

• Read info for 3 employees from the user.
• Print them in a formatted table.
• Use a helper function const char* dept_to_str(enum Department d) to return department names.

Define a union for storing a 32-bit register as either:

• an unsigned int raw; value, or
• a struct of bit fields showing:

struct {
    unsigned int enable   : 1;
    unsigned int mode     : 2;
    unsigned int error    : 1;
    unsigned int reserved : 28;
};

Your program will:

• Allow user to enter a register value in hex (0x…)
• Print individual bit field values.

Use a macro for debug control:

#define DEBUG 1

• Wrap debug `printf()` statements in #ifdef DEBUG
• Print a message like:

#ifdef DEBUG
    printf("[DEBUG] Value = %d\n", x);
#endif

• Try compiling after commenting out `#define DEBUG` or modifying the code to print debug messages only when the compiler defines the macro.