DS Log

In my blog, I delve into the world of programming web technologies, Linux, Unix-like, and graphic design using free tools on Linux.

Friday, January 2, 2026

What are those "ARG1 | ARG2" wierd arguments in C?

In C, a bit-mask argument refers to passing an integer value to a function where individual bits (or groups of bits) in that integer represent boolean flags or options. This technique allows you to combine multiple settings into a single argument using bitwise operations.

How It Works

Each option is typically defined as a power of two (i.e., a number with only one bit set), so that each option corresponds to a unique bit position.

#define OPTION_A (1 << 0)  // binary: 0001 (1)
#define OPTION_B (1 << 1)  // binary: 0010 (2)
#define OPTION_C (1 << 2)  // binary: 0100 (4)
#define OPTION_D (1 << 3)  // binary: 1000 (8)

Combining Options

You combine options using the bitwise OR (|) operator:

int flags = OPTION_A | OPTION_C;  // binary: 0101 (5)

This means both OPTION_A and OPTION_C are enabled.

Checking Options

Inside the function, you test whether a particular option is set using the bitwise AND (&) operator:

#include <stdio.h>

#define VERBOSE   (1 << 0)  // 1
#define UPPERCASE (1 << 1)  // 2
#define DRY_RUN   (1 << 2)  // 4

void process_text(const char* text, int flags) {
    if (flags & DRY_RUN) {
        printf("[DRY RUN] Would process: %s\n", text);
        return;
    }

    char buffer[100];
    if (flags & UPPERCASE) {
        // Simple uppercase conversion (for demo)
        for (int i = 0; text[i]; i++) {
            buffer[i] = (text[i] >= 'a' && text[i] <= 'z') ? text[i] - 32 : text[i];
        }
        buffer[strlen(text)] = '\0';
        text = buffer;
    }

    if (flags & VERBOSE) {
        printf("Processing text: %s\n", text);
    } else {
        printf("%s\n", text);
    }
}

int main() {
    process_text("hello", VERBOSE | UPPERCASE);  // Enables both options
    process_text("world", DRY_RUN);
    return 0;
}

How many flags can you stack?

It depends on the size (in bits) of the integer type you use:

Type	Bits	Max Distinct Flags
unsigned char / uint8_t	8	8
unsigned short / uint16_t	16	16
unsigned int / uint32_t	32	32
unsigned long long / uint64_t	64	64

Another 32bit example

#include <stdint.h>
#include <stdio.h>

// Mouse buttons
#define LMOUSEBUTTON    (1U << 0)  // Left mouse button
#define RMOUSEBUTTON    (1U << 1)  // Right mouse button
#define MMOUSEBUTTON    (1U << 2)  // Middle mouse button

// Keyboard modifier keys
#define SHIFTKEY        (1U << 3)
#define CTRLKEY         (1U << 4)
#define ALTKEY          (1U << 5)
#define SUPERKEY        (1U << 6)  // Windows / Command key

// Movement keys (WASD + arrows)
#define W_KEY           (1U << 7)
#define A_KEY           (1U << 8)
#define S_KEY           (1U << 9)
#define D_KEY           (1U << 10)
#define UP_ARROW        (1U << 11)
#define DOWN_ARROW      (1U << 12)
#define LEFT_ARROW      (1U << 13)
#define RIGHT_ARROW     (1U << 14)

// Action keys
#define SPACE_KEY       (1U << 15)
#define ENTER_KEY       (1U << 16)
#define ESC_KEY         (1U << 17)
#define TAB_KEY         (1U << 18)

// Common game keys
#define E_KEY           (1U << 19)
#define Q_KEY           (1U << 20)
#define R_KEY           (1U << 21)
#define F_KEY           (1U << 22)
#define MOUSE_WHEEL_UP    (1U << 23)
#define MOUSE_WHEEL_DOWN  (1U << 24)

// Extra (reserved for future use or custom actions)
#define CUSTOM_1        (1U << 25)
#define CUSTOM_2        (1U << 26)
#define CUSTOM_3        (1U << 27)
#define CUSTOM_4        (1U << 28)
// You can go up to (1U << 31) safely on 32-bit unsigned

// Optional: Combined masks
#define MODIFIERS_MASK (SHIFTKEY | CTRLKEY | ALTKEY | SUPERKEY)

#define MOVEMENT_MASK (W_KEY | A_KEY | S_KEY | D_KEY | \
                       UP_ARROW | DOWN_ARROW | LEFT_ARROW | RIGHT_ARROW)

// Helper function to check if a specific key/flag is pressed
static inline int is_key_pressed(uint32_t input_state, uint32_t key_flag)
{
    return (input_state & key_flag) != 0;
}

// Example: Check if running (W + SHIFT)
static inline int is_running(uint32_t input_state)
{
    return is_key_pressed(input_state, W_KEY) && is_key_pressed(input_state, SHIFTKEY);
}

int main(void)
{
    // Simulate an input state: Left mouse + Shift + W pressed
    uint32_t state = LMOUSEBUTTON | SHIFTKEY | W_KEY;

    if (is_key_pressed(state, LMOUSEBUTTON))
    {
        printf("Left mouse is pressed\n");
    }

    if (is_running(state))
    {
        printf("Player is running forward!\n");
    }

    // Additional example checks
    if (is_key_pressed(state, SHIFTKEY))
    {
        printf("Shift key is held\n");
    }

    return 0;
}

Notes:

posted by damirs @ January 02, 2026