Let's Build Video Player

This is the first article in the series of the articles where we are going to build video player - player. The idea is simple, we will create a minimal video player program and then extend the functionalities of it via "extension articles". Let's see how things will go! I will update the following list as I publish the article(s) in this series.

1. Let's Build Video Player

tl;dc: Refer this gist to get the final version of player.c code.

You need following:

1. GCC - sudo apt install build-essential
2. GTK4 - sudo apt install libgtk-4-dev
3. GStreamer & plugins - sudo apt install libgstreamer1.0-dev gstreamer1.0-plugins-base gstreamer1.0-plugins-good

Go ahead and create a file with name player.c and write the following stub code.

#include <gtk/gtk.h>

static void activate(GtkApplication* app, gpointer data) {
    GtkWidget* window = gtk_application_window_new(app);
    gtk_window_set_title(GTK_WINDOW(window), "Player");
    gtk_window_set_default_size(GTK_WINDOW(window), 600, 800);
    
    gtk_window_present(GTK_WINDOW(window));
}

int main(int argc, char** argv) {
    GtkApplication* app = gtk_application_new("dev.marichi.player", G_APPLICATION_DEFAULT_FLAGS);
    g_signal_connect(app, "activate", G_CALLBACK(activate), NULL);
    int status = g_application_run(G_APPLICATION(app), argc, argv);
    
    return status;
}

If you need brief explanation on this code, refer the Let's Build Terminal Pt. 1 article. Although, it is from "Let's Build Terminal" series but the first part is generic enough to understand such a code.

Use following command to compile and run the program.

gcc player.c `pkg-config --cflags --libs gtk4` && ./a.out

Let's add the header bar. The plan is to have a "Open" button in header bar that allow us to select the video file to open and play.

We can create a header bar using gtk_header_bar_new() function and then give a title to it using gtk_header_bar_set_title_widget(). Finally, we can use gtk_window_set_titlebar() function to set the header bar as title bar of the window.

#include <gtk/gtk.h>

static void activate(GtkApplication* app, gpointer data) {
    GtkWidget* window = gtk_application_window_new(app);
    gtk_window_set_title(GTK_WINDOW(window), "Player");
    gtk_window_set_default_size(GTK_WINDOW(window), 600, 800);
    
    // Header bar.
    GtkWidget* header_bar = gtk_header_bar_new();
    gtk_header_bar_set_title_widget(GTK_HEADER_BAR(header_bar), gtk_label_new("Player"));
    
    gtk_window_set_titlebar(GTK_WINDOW(window), header_bar);
    
    gtk_window_present(GTK_WINDOW(window));
}

int main(int argc, char** argv) {
    GtkApplication* app = gtk_application_new("dev.marichi.player", G_APPLICATION_DEFAULT_FLAGS);
    g_signal_connect(app, "activate", G_CALLBACK(activate), NULL);
    int status = g_application_run(G_APPLICATION(app), argc, argv);
    
    return status;
}

Now, go ahead and create a button using gtk_button_new_with_label() function and then pack it to the header bar.

#include <gtk/gtk.h>

static void activate(GtkApplication* app, gpointer data) {
    GtkWidget* window = gtk_application_window_new(app);
    gtk_window_set_title(GTK_WINDOW(window), "Player");
    gtk_window_set_default_size(GTK_WINDOW(window), 600, 800);
    
    // Header bar.
    GtkWidget* header_bar = gtk_header_bar_new();
    gtk_header_bar_set_title_widget(GTK_HEADER_BAR(header_bar), gtk_label_new("Player"));
    
    // Button.
    GtkWidget* button = gtk_button_new_with_label("Open");
    gtk_header_bar_pack_end(GTK_HEADER_BAR(header_bar), button);
    
    gtk_window_set_titlebar(GTK_WINDOW(window), header_bar);
    
    gtk_window_present(GTK_WINDOW(window));
}

int main(int argc, char** argv) {
    GtkApplication* app = gtk_application_new("dev.marichi.player", G_APPLICATION_DEFAULT_FLAGS);
    g_signal_connect(app, "activate", G_CALLBACK(activate), NULL);
    int status = g_application_run(G_APPLICATION(app), argc, argv);
    
    return status;
}

Run the program and it should produce output like this.

When user click on the button, it should open a file dialog to select the video. We need to pass the window instance as user data because it is needed to load the video into the window.

#include <gtk/gtk.h>

static void _on_open_clicked(GtkWidget* button, gpointer data) {

}

static void activate(GtkApplication* app, gpointer data) {
    GtkWidget* window = gtk_application_window_new(app);
    gtk_window_set_title(GTK_WINDOW(window), "Player");
    gtk_window_set_default_size(GTK_WINDOW(window), 600, 800);
    
    // Header bar.
    GtkWidget* header_bar = gtk_header_bar_new();
    gtk_header_bar_set_title_widget(GTK_HEADER_BAR(header_bar), gtk_label_new("Player"));
    
    // Button.
    GtkWidget* button = gtk_button_new_with_label("Open");
    g_signal_connect(button, "clicked", G_CALLBACK(_on_open_clicked), window);
    gtk_header_bar_pack_end(GTK_HEADER_BAR(header_bar), button);
    
    gtk_window_set_titlebar(GTK_WINDOW(window), header_bar);
    
    gtk_window_present(GTK_WINDOW(window));
}

int main(int argc, char** argv) {
    GtkApplication* app = gtk_application_new("dev.marichi.player", G_APPLICATION_DEFAULT_FLAGS);
    g_signal_connect(app, "activate", G_CALLBACK(activate), NULL);
    int status = g_application_run(G_APPLICATION(app), argc, argv);
    
    return status;
}

Let's cast and save the window passed as user data and create a file dialog using gtk_file_dialog_new() function.

#include <gtk/gtk.h>

static void _on_open_clicked(GtkWidget* button, gpointer data) {
    GtkWindow* window = GTK_WINDOW(data);
    
    GtkFileDialog* file_dialog = gtk_file_dialog_new();
}

static void activate(GtkApplication* app, gpointer data) {
    GtkWidget* window = gtk_application_window_new(app);
    gtk_window_set_title(GTK_WINDOW(window), "Player");
    gtk_window_set_default_size(GTK_WINDOW(window), 600, 800);
    
    // Header bar.
    GtkWidget* header_bar = gtk_header_bar_new();
    gtk_header_bar_set_title_widget(GTK_HEADER_BAR(header_bar), gtk_label_new("Player"));
    
    // Button.
    GtkWidget* button = gtk_button_new_with_label("Open");
    g_signal_connect(button, "clicked", G_CALLBACK(_on_open_clicked), window);
    gtk_header_bar_pack_end(GTK_HEADER_BAR(header_bar), button);
    
    gtk_window_set_titlebar(GTK_WINDOW(window), header_bar);
    
    gtk_window_present(GTK_WINDOW(window));
}

int main(int argc, char** argv) {
    GtkApplication* app = gtk_application_new("dev.marichi.player", G_APPLICATION_DEFAULT_FLAGS);
    g_signal_connect(app, "activate", G_CALLBACK(activate), NULL);
    int status = g_application_run(G_APPLICATION(app), argc, argv);
    
    return status;
}

In order to actually see the file dialog, we need to call the gtk_file_dialog_open() function. We need to pass 5 arguments - dialog itself, window as parent, NULL as cancellable operation, callback function, and user data.

#include <gtk/gtk.h>

static void _on_open_clicked(GtkWidget* button, gpointer data) {
    GtkWindow* window = GTK_WINDOW(data);
    
    GtkFileDialog* file_dialog = gtk_file_dialog_new();
    
    gtk_file_dialog_open(file_dialog, window, NULL, _on_file_opened, window);
}

static void activate(GtkApplication* app, gpointer data) {
    GtkWidget* window = gtk_application_window_new(app);
    gtk_window_set_title(GTK_WINDOW(window), "Player");
    gtk_window_set_default_size(GTK_WINDOW(window), 600, 800);
    
    // Header bar.
    GtkWidget* header_bar = gtk_header_bar_new();
    gtk_header_bar_set_title_widget(GTK_HEADER_BAR(header_bar), gtk_label_new("Player"));
    
    // Button.
    GtkWidget* button = gtk_button_new_with_label("Open");
    g_signal_connect(button, "clicked", G_CALLBACK(_on_open_clicked), window);
    gtk_header_bar_pack_end(GTK_HEADER_BAR(header_bar), button);
    
    gtk_window_set_titlebar(GTK_WINDOW(window), header_bar);
    
    gtk_window_present(GTK_WINDOW(window));
}

int main(int argc, char** argv) {
    GtkApplication* app = gtk_application_new("dev.marichi.player", G_APPLICATION_DEFAULT_FLAGS);
    g_signal_connect(app, "activate", G_CALLBACK(activate), NULL);
    int status = g_application_run(G_APPLICATION(app), argc, argv);
    
    return status;
}

We need to define the _on_file_opened() function. But is a asnyc callback. Due to this it accepts these parameters - source object that has started asnyc operation (file dialog) of type GObject*, result of the operation of type GAsyncResult*, and user data.

#include <gtk/gtk.h>

static void _on_file_opened(GObject* source_object, GAsyncResult* res, gpointer data) {

}

static void _on_open_clicked(GtkWidget* button, gpointer data) {
    GtkWindow* window = GTK_WINDOW(data);
    
    GtkFileDialog* file_dialog = gtk_file_dialog_new();
    
    gtk_file_dialog_open(file_dialog, window, NULL, _on_file_opened, window);
}

static void activate(GtkApplication* app, gpointer data) {
    GtkWidget* window = gtk_application_window_new(app);
    gtk_window_set_title(GTK_WINDOW(window), "Player");
    gtk_window_set_default_size(GTK_WINDOW(window), 600, 800);
    
    // Header bar.
    GtkWidget* header_bar = gtk_header_bar_new();
    gtk_header_bar_set_title_widget(GTK_HEADER_BAR(header_bar), gtk_label_new("Player"));
    
    // Button.
    GtkWidget* button = gtk_button_new_with_label("Open");
    g_signal_connect(button, "clicked", G_CALLBACK(_on_open_clicked), window);
    gtk_header_bar_pack_end(GTK_HEADER_BAR(header_bar), button);
    
    gtk_window_set_titlebar(GTK_WINDOW(window), header_bar);
    
    gtk_window_present(GTK_WINDOW(window));
}

int main(int argc, char** argv) {
    GtkApplication* app = gtk_application_new("dev.marichi.player", G_APPLICATION_DEFAULT_FLAGS);
    g_signal_connect(app, "activate", G_CALLBACK(activate), NULL);
    int status = g_application_run(G_APPLICATION(app), argc, argv);
    
    return status;
}

Again, let's get the values of dialog and window from the function parameters.

#include <gtk/gtk.h>

static void _on_file_opened(GObject* source_object, GAsyncResult* res, gpointer data) {
    GtkFileDialog* file_dialog = GTK_FILE_DIALOG(source_object);
    GtkWindow* window = GTK_WINDOW(data);
}

static void _on_open_clicked(GtkWidget* button, gpointer data) {
    GtkWindow* window = GTK_WINDOW(data);
    
    GtkFileDialog* file_dialog = gtk_file_dialog_new();
    
    gtk_file_dialog_open(file_dialog, window, NULL, _on_file_opened, window);
}

static void activate(GtkApplication* app, gpointer data) {
    GtkWidget* window = gtk_application_window_new(app);
    gtk_window_set_title(GTK_WINDOW(window), "Player");
    gtk_window_set_default_size(GTK_WINDOW(window), 600, 800);
    
    // Header bar.
    GtkWidget* header_bar = gtk_header_bar_new();
    gtk_header_bar_set_title_widget(GTK_HEADER_BAR(header_bar), gtk_label_new("Player"));
    
    // Button.
    GtkWidget* button = gtk_button_new_with_label("Open");
    g_signal_connect(button, "clicked", G_CALLBACK(_on_open_clicked), window);
    gtk_header_bar_pack_end(GTK_HEADER_BAR(header_bar), button);
    
    gtk_window_set_titlebar(GTK_WINDOW(window), header_bar);
    
    gtk_window_present(GTK_WINDOW(window));
}

int main(int argc, char** argv) {
    GtkApplication* app = gtk_application_new("dev.marichi.player", G_APPLICATION_DEFAULT_FLAGS);
    g_signal_connect(app, "activate", G_CALLBACK(activate), NULL);
    int status = g_application_run(G_APPLICATION(app), argc, argv);
    
    return status;
}

We need to call the gtk_file_dialog_open_finish() function. This function finish the gtk_file_dialog_open() function and return either the selected file or an error.

#include <gtk/gtk.h>

static void _on_file_opened(GObject* source_object, GAsyncResult* res, gpointer data) {
    GtkFileDialog* file_dialog = GTK_FILE_DIALOG(source_object);
    GtkWindow* window = GTK_WINDOW(data);
    
    GFile* file = NULL;
    GError* error = NULL;
    
    file = gtk_file_dialog_open_finish(file_dialog, res, &error);
}

static void _on_open_clicked(GtkWidget* button, gpointer data) {
    GtkWindow* window = GTK_WINDOW(data);
    
    GtkFileDialog* file_dialog = gtk_file_dialog_new();
    
    gtk_file_dialog_open(file_dialog, window, NULL, _on_file_opened, window);
}

static void activate(GtkApplication* app, gpointer data) {
    GtkWidget* window = gtk_application_window_new(app);
    gtk_window_set_title(GTK_WINDOW(window), "Player");
    gtk_window_set_default_size(GTK_WINDOW(window), 600, 800);
    
    // Header bar.
    GtkWidget* header_bar = gtk_header_bar_new();
    gtk_header_bar_set_title_widget(GTK_HEADER_BAR(header_bar), gtk_label_new("Player"));
    
    // Button.
    GtkWidget* button = gtk_button_new_with_label("Open");
    g_signal_connect(button, "clicked", G_CALLBACK(_on_open_clicked), window);
    gtk_header_bar_pack_end(GTK_HEADER_BAR(header_bar), button);
    
    gtk_window_set_titlebar(GTK_WINDOW(window), header_bar);
    
    gtk_window_present(GTK_WINDOW(window));
}

int main(int argc, char** argv) {
    GtkApplication* app = gtk_application_new("dev.marichi.player", G_APPLICATION_DEFAULT_FLAGS);
    g_signal_connect(app, "activate", G_CALLBACK(activate), NULL);
    int status = g_application_run(G_APPLICATION(app), argc, argv);
    
    return status;
}

If we get the error, we need to stop and free the error memory using g_error_free(). We can also print the error message using g_printerr() function.

#include <gtk/gtk.h>

static void _on_file_opened(GObject* source_object, GAsyncResult* res, gpointer data) {
    GtkFileDialog* file_dialog = GTK_FILE_DIALOG(source_object);
    GtkWindow* window = GTK_WINDOW(data);
    
    GFile* file = NULL;
    GError* error = NULL;
    
    file = gtk_file_dialog_open_finish(file_dialog, res, &error);
    
    if (error) {
        g_printerr("Error opening file %s\n", error->message);
        g_error_free(error);
        return;
    }
}

static void _on_open_clicked(GtkWidget* button, gpointer data) {
    GtkWindow* window = GTK_WINDOW(data);
    
    GtkFileDialog* file_dialog = gtk_file_dialog_new();
    
    gtk_file_dialog_open(file_dialog, window, NULL, _on_file_opened, window);
}

static void activate(GtkApplication* app, gpointer data) {
    GtkWidget* window = gtk_application_window_new(app);
    gtk_window_set_title(GTK_WINDOW(window), "Player");
    gtk_window_set_default_size(GTK_WINDOW(window), 600, 800);
    
    // Header bar.
    GtkWidget* header_bar = gtk_header_bar_new();
    gtk_header_bar_set_title_widget(GTK_HEADER_BAR(header_bar), gtk_label_new("Player"));
    
    // Button.
    GtkWidget* button = gtk_button_new_with_label("Open");
    g_signal_connect(button, "clicked", G_CALLBACK(_on_open_clicked), window);
    gtk_header_bar_pack_end(GTK_HEADER_BAR(header_bar), button);
    
    gtk_window_set_titlebar(GTK_WINDOW(window), header_bar);
    
    gtk_window_present(GTK_WINDOW(window));
}

int main(int argc, char** argv) {
    GtkApplication* app = gtk_application_new("dev.marichi.player", G_APPLICATION_DEFAULT_FLAGS);
    g_signal_connect(app, "activate", G_CALLBACK(activate), NULL);
    int status = g_application_run(G_APPLICATION(app), argc, argv);
    
    return status;
}

But, if we have file, we can proceed with playing the video on the window. Let's create a video widget using gtk_video_new() function and then set it as child of the window.

#include <gtk/gtk.h>

static void _on_file_opened(GObject* source_object, GAsyncResult* res, gpointer data) {
    GtkFileDialog* file_dialog = GTK_FILE_DIALOG(source_object);
    GtkWindow* window = GTK_WINDOW(data);
    
    GFile* file = NULL;
    GError* error = NULL;
    
    file = gtk_file_dialog_open_finish(file_dialog, res, &error);
    
    if (error) {
        g_printerr("Error opening file %s\n", error->message);
        g_error_free(error);
        return;
    }
    
    if (file) {
        GtkWidget* video = gtk_video_new();
        
        gtk_window_set_child(window, video);
    }
}

static void _on_open_clicked(GtkWidget* button, gpointer data) {
    GtkWindow* window = GTK_WINDOW(data);
    
    GtkFileDialog* file_dialog = gtk_file_dialog_new();
    
    gtk_file_dialog_open(file_dialog, window, NULL, _on_file_opened, window);
}

static void activate(GtkApplication* app, gpointer data) {
    GtkWidget* window = gtk_application_window_new(app);
    gtk_window_set_title(GTK_WINDOW(window), "Player");
    gtk_window_set_default_size(GTK_WINDOW(window), 600, 800);
    
    // Header bar.
    GtkWidget* header_bar = gtk_header_bar_new();
    gtk_header_bar_set_title_widget(GTK_HEADER_BAR(header_bar), gtk_label_new("Player"));
    
    // Button.
    GtkWidget* button = gtk_button_new_with_label("Open");
    g_signal_connect(button, "clicked", G_CALLBACK(_on_open_clicked), window);
    gtk_header_bar_pack_end(GTK_HEADER_BAR(header_bar), button);
    
    gtk_window_set_titlebar(GTK_WINDOW(window), header_bar);
    
    gtk_window_present(GTK_WINDOW(window));
}

int main(int argc, char** argv) {
    GtkApplication* app = gtk_application_new("dev.marichi.player", G_APPLICATION_DEFAULT_FLAGS);
    g_signal_connect(app, "activate", G_CALLBACK(activate), NULL);
    int status = g_application_run(G_APPLICATION(app), argc, argv);
    
    return status;
}

Let's create a media stream from the given file using gtk_media_fil_new_from_file() and set this steam to the video using gtk_video_set_media_stream(). Don't forget to unref the file object.

#include <gtk/gtk.h>

static void _on_file_opened(GObject* source_object, GAsyncResult* res, gpointer data) {
    GtkFileDialog* file_dialog = GTK_FILE_DIALOG(source_object);
    GtkWindow* window = GTK_WINDOW(data);
    
    GFile* file = NULL;
    GError* error = NULL;
    
    file = gtk_file_dialog_open_finish(file_dialog, res, &error);
    
    if (error) {
        g_printerr("Error opening file %s\n", error->message);
        g_error_free(error);
        return;
    }
    
    if (file) {
        GtkWidget* video = gtk_video_new();
        
        GtkMediaStream* media = GTK_MEDIA_STREAM(gtk_media_file_new_for_file(file));
        gtk_video_set_media_stream(GTK_VIDEO(video), media);
        
        gtk_window_set_child(window, video);

        g_object_unref(file);
    }
}

static void _on_open_clicked(GtkWidget* button, gpointer data) {
    GtkWindow* window = GTK_WINDOW(data);
    
    GtkFileDialog* file_dialog = gtk_file_dialog_new();
    
    gtk_file_dialog_open(file_dialog, window, NULL, _on_file_opened, window);
}

static void activate(GtkApplication* app, gpointer data) {
    GtkWidget* window = gtk_application_window_new(app);
    gtk_window_set_title(GTK_WINDOW(window), "Player");
    gtk_window_set_default_size(GTK_WINDOW(window), 600, 800);
    
    // Header bar.
    GtkWidget* header_bar = gtk_header_bar_new();
    gtk_header_bar_set_title_widget(GTK_HEADER_BAR(header_bar), gtk_label_new("Player"));
    
    // Button.
    GtkWidget* button = gtk_button_new_with_label("Open");
    g_signal_connect(button, "clicked", G_CALLBACK(_on_open_clicked), window);
    gtk_header_bar_pack_end(GTK_HEADER_BAR(header_bar), button);
    
    gtk_window_set_titlebar(GTK_WINDOW(window), header_bar);
    
    gtk_window_present(GTK_WINDOW(window));
}

int main(int argc, char** argv) {
    GtkApplication* app = gtk_application_new("dev.marichi.player", G_APPLICATION_DEFAULT_FLAGS);
    g_signal_connect(app, "activate", G_CALLBACK(activate), NULL);
    int status = g_application_run(G_APPLICATION(app), argc, argv);
    
    return status;
}

And with these changes, we now have the video player. Go ahead and run the program to confirm the video player!

Yay!

TIL: #include literally copy-paste the content of it

I mean I already knew about this point already but I was still thinking that there will be some magic in-between. It can't be that simple - copy-paste! But, the I watched this video by antirez.

---

Create a file with name print.c and write the following code - yes, just a printf() statement.

printf("hello, world\n");

Create another file with name hello.c and write the following code.

#include <stdio.h>

int main(void) {
    #include "print.c"

    return 0;
}

Now, compile and run the hello.c program (without including print.c in compilation).

gcc hello.c && ./a.out 
hello, world

It prints the output - hello, world as #include directive copy-paste the content of print.c file.

If you need proof then use -E compilation option. It'll print the preprocessed output. Let's also pass -P option to pretty-print the output.

gcc -E –P hello.c

The output is big. So, I truncated to only last few lines.

// ...
// ...
extern void flockfile (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__)) __attribute__ ((__nonnull__ (1)));
extern int ftrylockfile (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__)) __attribute__ ((__nonnull__ (1)));
extern void funlockfile (FILE *__stream) __attribute__ ((__nothrow__ , __leaf__)) __attribute__ ((__nonnull__ (1)));
extern int __uflow (FILE *);
extern int __overflow (FILE *, int);

int main(void) {
printf("hello, world\n");
 return 0;
}

There it is!

TIL: puts() is faster than printf()

Create a file with name hello.c and write the following code.

#include <stdio.h>

int main(void) {
    printf("hello, world\n");

    return 0;
}

We can pass option -S to generate assembly code of this hello.c program using GCC.

gcc –S hello.c

The command generates the hello.s file with following code.

	.file	"hello.c"
	.text
	.section	.rodata
.LC0:
	.string	"hello, world"
	.text
	.globl	main
	.type	main, @function
main:
.LFB0:
	.cfi_startproc
	pushq	%rbp
	.cfi_def_cfa_offset 16
	.cfi_offset 6, -16
	movq	%rsp, %rbp
	.cfi_def_cfa_register 6
	leaq	.LC0(%rip), %rax
	movq	%rax, %rdi
	call	puts@PLT
	movl	$0, %eax
	popq	%rbp
	.cfi_def_cfa 7, 8
	ret
	.cfi_endproc
.LFE0:
	.size	main, .-main
	.ident	"GCC: (Debian 14.2.0-19) 14.2.0"
	.section	.note.GNU-stack,"",@progbits

The above code is generated using GCC v14.2.0. As of writing, in the above, you won't be able to find printf() function. Instead, as you can see in the highlighted (bold) lines, that it use puts() function and \n is removed from string "hello, world" as puts() function add \n at the end.

This is the part of optimization done by GCC because we are not formatting the string with any expression in printf() function. We are just printing the string. If that is the case, GCC is smart enough to replace the printf() call with puts() and remove \n from the string at the end if present. If you really want to keep printf() function, you need to pass -fno-builtin-printf option.

gcc -S -fno-builtin-printf hello.c

Here is the output.

	.file	"hello.c"
	.text
	.section	.rodata
.LC0:
	.string	"hello, world\n"
	.text
	.globl	main
	.type	main, @function
main:
.LFB0:
	.cfi_startproc
	pushq	%rbp
	.cfi_def_cfa_offset 16
	.cfi_offset 6, -16
	movq	%rsp, %rbp
	.cfi_def_cfa_register 6
	leaq	.LC0(%rip), %rax
	movq	%rax, %rdi
	movl	$0, %eax
	call	printf@PLT
	movl	$0, %eax
	popq	%rbp
	.cfi_def_cfa 7, 8
	ret
	.cfi_endproc
.LFE0:
	.size	main, .-main
	.ident	"GCC: (Debian 14.2.0-19) 14.2.0"
	.section	.note.GNU-stack,"",@progbits

Furthermore, the optimization shouldn't happen if we do formatting.

#include <stdio.h>

int main(void) {
    int x = 42;
    printf("hello, world %d\n", x);

    return 0;
}

For you: Generate the assembly code and confirm!

Let's Build Image Viewer

This is the first article in the series of the articles where are are going to build Image viewer - imager. The idea is simple, we will create a minimal image viewer program and then extend the functionalities of it via "extension articles". Let's see how things will go! I'll update the following list as I publish the article(s) in this series.

1. Let's Build Image Viewer

tl;dc: Refer this gist to get the final version of imager.c code.

Go ahead and create a file with name imager.c and write the following stub code.

#include <gtk/gtk.h>

static void activate(GtkApplication* app, gpointer data) {
    GtkWidget* window = gtk_application_window_new(app);
    gtk_window_set_title(GTK_WINDOW(window), "Imager");
    gtk_window_set_default_size(GTK_WINDOW(window), 600, 800);
    
    gtk_window_present(GTK_WINDOW(window));
}

int main(int argc, char** argv) {
    GtkApplication* app = gtk_application_new("dev.marichi.imager", G_APPLICATION_DEFAULT_FLAGS);
    g_signal_connect(app, "activate", G_CALLBACK(activate), NULL);
    int status = g_application_run(G_APPLICATION(app), argc, argv);
    
    return status;
}

If you need brief explanation on this code, refer the Let's Build Terminal Pt. 1 article. Although, it is from "Let's Build Terminal" series but the first part is generic enough to understand such a code.

Use following command to compile and run the program.

gcc imager.c `pkg-config --cflags --libs gtk4` && ./a.out

Let's add the header bar. The plan is to have a "Open" button in header bar that allow us to select the image file to open and view.

We can create a header bar using gtk_header_bar_new() function and then give a title to it using gtk_header_bar_set_title_widget(). Finally, we can use gtk_window_set_titlebar() function to set the header bar as title bar of the window.

#include <gtk/gtk.h>

static void activate(GtkApplication* app, gpointer data) {
    GtkWidget* window = gtk_application_window_new(app);
    gtk_window_set_title(GTK_WINDOW(window), "Imager");
    gtk_window_set_default_size(GTK_WINDOW(window), 600, 800);
    
    // Header bar.
    GtkWidget* header_bar = gtk_header_bar_new();
    gtk_header_bar_set_title_widget(GTK_HEADER_BAR(header_bar), gtk_label_new("Imager"));
    
    gtk_window_set_titlebar(GTK_WINDOW(window), header_bar);
    
    gtk_window_present(GTK_WINDOW(window));
}

int main(int argc, char** argv) {
    GtkApplication* app = gtk_application_new("dev.marichi.imager", G_APPLICATION_DEFAULT_FLAGS);
    g_signal_connect(app, "activate", G_CALLBACK(activate), NULL);
    int status = g_application_run(G_APPLICATION(app), argc, argv);
    
    return status;
}

Now, go ahead and create a button using gtk_button_new_with_label() function and then pack it to the header bar.

#include <gtk/gtk.h>

static void activate(GtkApplication* app, gpointer data) {
    GtkWidget* window = gtk_application_window_new(app);
    gtk_window_set_title(GTK_WINDOW(window), "Imager");
    gtk_window_set_default_size(GTK_WINDOW(window), 600, 800);
    
    // Header bar.
    GtkWidget* header_bar = gtk_header_bar_new();
    gtk_header_bar_set_title_widget(GTK_HEADER_BAR(header_bar), gtk_label_new("Imager"));
    
    // Button.
    GtkWidget* button = gtk_button_new_with_label("Open");
    gtk_header_bar_pack_end(GTK_HEADER_BAR(header_bar), button);
    
    gtk_window_set_titlebar(GTK_WINDOW(window), header_bar);
    
    gtk_window_present(GTK_WINDOW(window));
}

int main(int argc, char** argv) {
    GtkApplication* app = gtk_application_new("dev.marichi.imager", G_APPLICATION_DEFAULT_FLAGS);
    g_signal_connect(app, "activate", G_CALLBACK(activate), NULL);
    int status = g_application_run(G_APPLICATION(app), argc, argv);
    
    return status;
}

Run the program and it should produce output like this.

When user click on button, it should open a file dialog to select the image. We need to pass the window instance as user data because it is needed to load the image into the window.

#include <gtk/gtk.h>

static void _on_open_clicked(GtkWidget* button, gpointer data) {

}

static void activate(GtkApplication* app, gpointer data) {
    GtkWidget* window = gtk_application_window_new(app);
    gtk_window_set_title(GTK_WINDOW(window), "Imager");
    gtk_window_set_default_size(GTK_WINDOW(window), 600, 800);
    
    // Header bar.
    GtkWidget* header_bar = gtk_header_bar_new();
    gtk_header_bar_set_title_widget(GTK_HEADER_BAR(header_bar), gtk_label_new("Imager"));
    
    // Button.
    GtkWidget* button = gtk_button_new_with_label("Open");
    g_signal_connect(button, "clicked", G_CALLBACK(_on_open_clicked), window);
    gtk_header_bar_pack_end(GTK_HEADER_BAR(header_bar), button);
    
    gtk_window_set_titlebar(GTK_WINDOW(window), header_bar);
    
    gtk_window_present(GTK_WINDOW(window));
}

int main(int argc, char** argv) {
    GtkApplication* app = gtk_application_new("dev.marichi.imager", G_APPLICATION_DEFAULT_FLAGS);
    g_signal_connect(app, "activate", G_CALLBACK(activate), NULL);
    int status = g_application_run(G_APPLICATION(app), argc, argv);
    
    return status;
}

Let's cast and save the window passed as user data and create a file dialog using gtk_file_dialog_new() function.

#include <gtk/gtk.h>

static void _on_open_clicked(GtkWidget* button, gpointer data) {
    GtkWindow* window = GTK_WINDOW(data);
    
    GtkFileDialog* file_dialog = gtk_file_dialog_new();
}

static void activate(GtkApplication* app, gpointer data) {
    GtkWidget* window = gtk_application_window_new(app);
    gtk_window_set_title(GTK_WINDOW(window), "Imager");
    gtk_window_set_default_size(GTK_WINDOW(window), 600, 800);
    
    // Header bar.
    GtkWidget* header_bar = gtk_header_bar_new();
    gtk_header_bar_set_title_widget(GTK_HEADER_BAR(header_bar), gtk_label_new("Imager"));
    
    // Button.
    GtkWidget* button = gtk_button_new_with_label("Open");
    g_signal_connect(button, "clicked", G_CALLBACK(_on_open_clicked), window);
    gtk_header_bar_pack_end(GTK_HEADER_BAR(header_bar), button);
    
    gtk_window_set_titlebar(GTK_WINDOW(window), header_bar);
    
    gtk_window_present(GTK_WINDOW(window));
}

int main(int argc, char** argv) {
    GtkApplication* app = gtk_application_new("dev.marichi.imager", G_APPLICATION_DEFAULT_FLAGS);
    g_signal_connect(app, "activate", G_CALLBACK(activate), NULL);
    int status = g_application_run(G_APPLICATION(app), argc, argv);
    
    return status;
}

In order to actually see the file dialog, we need to call the gtk_file_dialog_open() function. We need to pass 5 arguments - dialog itself, window as parent, NULL as cancellable operation, callback function, and user data.

#include <gtk/gtk.h>

static void _on_open_clicked(GtkWidget* button, gpointer data) {
    GtkWindow* window = GTK_WINDOW(data);
    
    GtkFileDialog* file_dialog = gtk_file_dialog_new();
    
    gtk_file_dialog_open(file_dialog, window, NULL, _on_file_opened, window);
}

static void activate(GtkApplication* app, gpointer data) {
    GtkWidget* window = gtk_application_window_new(app);
    gtk_window_set_title(GTK_WINDOW(window), "Imager");
    gtk_window_set_default_size(GTK_WINDOW(window), 600, 800);
    
    // Header bar.
    GtkWidget* header_bar = gtk_header_bar_new();
    gtk_header_bar_set_title_widget(GTK_HEADER_BAR(header_bar), gtk_label_new("Imager"));
    
    // Button.
    GtkWidget* button = gtk_button_new_with_label("Open");
    g_signal_connect(button, "clicked", G_CALLBACK(_on_open_clicked), window);
    gtk_header_bar_pack_end(GTK_HEADER_BAR(header_bar), button);
    
    gtk_window_set_titlebar(GTK_WINDOW(window), header_bar);
    
    gtk_window_present(GTK_WINDOW(window));
}

int main(int argc, char** argv) {
    GtkApplication* app = gtk_application_new("dev.marichi.imager", G_APPLICATION_DEFAULT_FLAGS);
    g_signal_connect(app, "activate", G_CALLBACK(activate), NULL);
    int status = g_application_run(G_APPLICATION(app), argc, argv);
    
    return status;
}

We need to define the _on_file_opened() function. But is a asnyc callback. Due to this it accepts these parameters - source object that has started asnyc operation (file dialog) of type GObject*, result of the operation of type GAsyncResult*, and user data.

#include <gtk/gtk.h>

static void _on_file_opened(GObject* source_object, GAsyncResult* res, gpointer data) {

}

static void _on_open_clicked(GtkWidget* button, gpointer data) {
    GtkWindow* window = GTK_WINDOW(data);
    
    GtkFileDialog* file_dialog = gtk_file_dialog_new();
    
    gtk_file_dialog_open(file_dialog, window, NULL, _on_file_opened, window);
}

static void activate(GtkApplication* app, gpointer data) {
    GtkWidget* window = gtk_application_window_new(app);
    gtk_window_set_title(GTK_WINDOW(window), "Imager");
    gtk_window_set_default_size(GTK_WINDOW(window), 600, 800);
    
    // Header bar.
    GtkWidget* header_bar = gtk_header_bar_new();
    gtk_header_bar_set_title_widget(GTK_HEADER_BAR(header_bar), gtk_label_new("Imager"));
    
    // Button.
    GtkWidget* button = gtk_button_new_with_label("Open");
    g_signal_connect(button, "clicked", G_CALLBACK(_on_open_clicked), window);
    gtk_header_bar_pack_end(GTK_HEADER_BAR(header_bar), button);
    
    gtk_window_set_titlebar(GTK_WINDOW(window), header_bar);
    
    gtk_window_present(GTK_WINDOW(window));
}

int main(int argc, char** argv) {
    GtkApplication* app = gtk_application_new("dev.marichi.imager", G_APPLICATION_DEFAULT_FLAGS);
    g_signal_connect(app, "activate", G_CALLBACK(activate), NULL);
    int status = g_application_run(G_APPLICATION(app), argc, argv);
    
    return status;
}

Again, let's get the values of dialog and window from the function parameters.

#include <gtk/gtk.h>

static void _on_file_opened(GObject* source_object, GAsyncResult* res, gpointer data) {
    GtkFileDialog* file_dialog = GTK_FILE_DIALOG(source_object);
    GtkWindow* window = GTK_WINDOW(data);
}

static void _on_open_clicked(GtkWidget* button, gpointer data) {
    GtkWindow* window = GTK_WINDOW(data);
    
    GtkFileDialog* file_dialog = gtk_file_dialog_new();
    
    gtk_file_dialog_open(file_dialog, window, NULL, _on_file_opened, window);
}

static void activate(GtkApplication* app, gpointer data) {
    GtkWidget* window = gtk_application_window_new(app);
    gtk_window_set_title(GTK_WINDOW(window), "Imager");
    gtk_window_set_default_size(GTK_WINDOW(window), 600, 800);
    
    // Header bar.
    GtkWidget* header_bar = gtk_header_bar_new();
    gtk_header_bar_set_title_widget(GTK_HEADER_BAR(header_bar), gtk_label_new("Imager"));
    
    // Button.
    GtkWidget* button = gtk_button_new_with_label("Open");
    g_signal_connect(button, "clicked", G_CALLBACK(_on_open_clicked), window);
    gtk_header_bar_pack_end(GTK_HEADER_BAR(header_bar), button);
    
    gtk_window_set_titlebar(GTK_WINDOW(window), header_bar);
    
    gtk_window_present(GTK_WINDOW(window));
}

int main(int argc, char** argv) {
    GtkApplication* app = gtk_application_new("dev.marichi.imager", G_APPLICATION_DEFAULT_FLAGS);
    g_signal_connect(app, "activate", G_CALLBACK(activate), NULL);
    int status = g_application_run(G_APPLICATION(app), argc, argv);
    
    return status;
}

We need to call the gtk_file_dialog_open_finish() function. This function finish the gtk_file_dialog_open() function and return either the selected file or an error.

#include <gtk/gtk.h>

static void _on_file_opened(GObject* source_object, GAsyncResult* res, gpointer data) {
    GtkFileDialog* file_dialog = GTK_FILE_DIALOG(source_object);
    GtkWindow* window = GTK_WINDOW(data);
    
    GFile* file = NULL;
    GError* error = NULL;
    
    file = gtk_file_dialog_open_finish(file_dialog, res, &error);
}

static void _on_open_clicked(GtkWidget* button, gpointer data) {
    GtkWindow* window = GTK_WINDOW(data);
    
    GtkFileDialog* file_dialog = gtk_file_dialog_new();
    
    gtk_file_dialog_open(file_dialog, window, NULL, _on_file_opened, window);
}

static void activate(GtkApplication* app, gpointer data) {
    GtkWidget* window = gtk_application_window_new(app);
    gtk_window_set_title(GTK_WINDOW(window), "Imager");
    gtk_window_set_default_size(GTK_WINDOW(window), 600, 800);
    
    // Header bar.
    GtkWidget* header_bar = gtk_header_bar_new();
    gtk_header_bar_set_title_widget(GTK_HEADER_BAR(header_bar), gtk_label_new("Imager"));
    
    // Button.
    GtkWidget* button = gtk_button_new_with_label("Open");
    g_signal_connect(button, "clicked", G_CALLBACK(_on_open_clicked), window);
    gtk_header_bar_pack_end(GTK_HEADER_BAR(header_bar), button);
    
    gtk_window_set_titlebar(GTK_WINDOW(window), header_bar);
    
    gtk_window_present(GTK_WINDOW(window));
}

int main(int argc, char** argv) {
    GtkApplication* app = gtk_application_new("dev.marichi.imager", G_APPLICATION_DEFAULT_FLAGS);
    g_signal_connect(app, "activate", G_CALLBACK(activate), NULL);
    int status = g_application_run(G_APPLICATION(app), argc, argv);
    
    return status;
}

If we get the error, we need to stop and free the error memory using g_error_free(). We can also print the error message using g_printerr() function.

#include <gtk/gtk.h>

static void _on_file_opened(GObject* source_object, GAsyncResult* res, gpointer data) {
    GtkFileDialog* file_dialog = GTK_FILE_DIALOG(source_object);
    GtkWindow* window = GTK_WINDOW(data);
    
    GFile* file = NULL;
    GError* error = NULL;
    
    file = gtk_file_dialog_open_finish(file_dialog, res, &error);
    
    if (error) {
        g_printerr("Error opening file %s\n", error->message);
        g_error_free(error);
        return;
    }
}

static void _on_open_clicked(GtkWidget* button, gpointer data) {
    GtkWindow* window = GTK_WINDOW(data);
    
    GtkFileDialog* file_dialog = gtk_file_dialog_new();
    
    gtk_file_dialog_open(file_dialog, window, NULL, _on_file_opened, window);
}

static void activate(GtkApplication* app, gpointer data) {
    GtkWidget* window = gtk_application_window_new(app);
    gtk_window_set_title(GTK_WINDOW(window), "Imager");
    gtk_window_set_default_size(GTK_WINDOW(window), 600, 800);
    
    // Header bar.
    GtkWidget* header_bar = gtk_header_bar_new();
    gtk_header_bar_set_title_widget(GTK_HEADER_BAR(header_bar), gtk_label_new("Imager"));
    
    // Button.
    GtkWidget* button = gtk_button_new_with_label("Open");
    g_signal_connect(button, "clicked", G_CALLBACK(_on_open_clicked), window);
    gtk_header_bar_pack_end(GTK_HEADER_BAR(header_bar), button);
    
    gtk_window_set_titlebar(GTK_WINDOW(window), header_bar);
    
    gtk_window_present(GTK_WINDOW(window));
}

int main(int argc, char** argv) {
    GtkApplication* app = gtk_application_new("dev.marichi.imager", G_APPLICATION_DEFAULT_FLAGS);
    g_signal_connect(app, "activate", G_CALLBACK(activate), NULL);
    int status = g_application_run(G_APPLICATION(app), argc, argv);
    
    return status;
}

But, if we have file, we can proceed with rendering the image on the window. Let's create a picture widget using gtk_picture_new() function and then set it as child of the window.

#include <gtk/gtk.h>

static void _on_file_opened(GObject* source_object, GAsyncResult* res, gpointer data) {
    GtkFileDialog* file_dialog = GTK_FILE_DIALOG(source_object);
    GtkWindow* window = GTK_WINDOW(data);
    
    GFile* file = NULL;
    GError* error = NULL;
    
    file = gtk_file_dialog_open_finish(file_dialog, res, &error);
    
    if (error) {
        g_printerr("Error opening file %s\n", error->message);
        g_error_free(error);
        return;
    }
    
    if (file) {
        GtkWidget* picture = gtk_picture_new();
        
        gtk_window_set_child(window, picture);
    }
}

static void _on_open_clicked(GtkWidget* button, gpointer data) {
    GtkWindow* window = GTK_WINDOW(data);
    
    GtkFileDialog* file_dialog = gtk_file_dialog_new();
    
    gtk_file_dialog_open(file_dialog, window, NULL, _on_file_opened, window);
}

static void activate(GtkApplication* app, gpointer data) {
    GtkWidget* window = gtk_application_window_new(app);
    gtk_window_set_title(GTK_WINDOW(window), "Imager");
    gtk_window_set_default_size(GTK_WINDOW(window), 600, 800);
    
    // Header bar.
    GtkWidget* header_bar = gtk_header_bar_new();
    gtk_header_bar_set_title_widget(GTK_HEADER_BAR(header_bar), gtk_label_new("Imager"));
    
    // Button.
    GtkWidget* button = gtk_button_new_with_label("Open");
    g_signal_connect(button, "clicked", G_CALLBACK(_on_open_clicked), window);
    gtk_header_bar_pack_end(GTK_HEADER_BAR(header_bar), button);
    
    gtk_window_set_titlebar(GTK_WINDOW(window), header_bar);
    
    gtk_window_present(GTK_WINDOW(window));
}

int main(int argc, char** argv) {
    GtkApplication* app = gtk_application_new("dev.marichi.imager", G_APPLICATION_DEFAULT_FLAGS);
    g_signal_connect(app, "activate", G_CALLBACK(activate), NULL);
    int status = g_application_run(G_APPLICATION(app), argc, argv);
    
    return status;
}

In order to actually render the picture in this widget, we need to use texture widget from the GDK library. We can create texture of selected file using gdk_texture_new_from_file() function and then use it with picture instance. The gdk_texture_new_from_file() function accepts file path and error struct. We can get the path of the selected file using g_file_get_path() function.

#include <gtk/gtk.h>

static void _on_file_opened(GObject* source_object, GAsyncResult* res, gpointer data) {
    GtkFileDialog* file_dialog = GTK_FILE_DIALOG(source_object);
    GtkWindow* window = GTK_WINDOW(data);
    
    GFile* file = NULL;
    GError* error = NULL;
    
    file = gtk_file_dialog_open_finish(file_dialog, res, &error);
    
    if (error) {
        g_printerr("Error opening file %s\n", error->message);
        g_error_free(error);
        return;
    }
    
    if (file) {
        char* path = g_file_get_path(file);
    
        GtkWidget* picture = gtk_picture_new();
        
        gtk_window_set_child(window, picture);
        
        GdkTexture* texture = gdk_texture_new_from_file(file, &error);
        
        if (error) {
        
        } else {
        
        }
    }
}

static void _on_open_clicked(GtkWidget* button, gpointer data) {
    GtkWindow* window = GTK_WINDOW(data);
    
    GtkFileDialog* file_dialog = gtk_file_dialog_new();
    
    gtk_file_dialog_open(file_dialog, window, NULL, _on_file_opened, window);
}

static void activate(GtkApplication* app, gpointer data) {
    GtkWidget* window = gtk_application_window_new(app);
    gtk_window_set_title(GTK_WINDOW(window), "Imager");
    gtk_window_set_default_size(GTK_WINDOW(window), 600, 800);
    
    // Header bar.
    GtkWidget* header_bar = gtk_header_bar_new();
    gtk_header_bar_set_title_widget(GTK_HEADER_BAR(header_bar), gtk_label_new("Imager"));
    
    // Button.
    GtkWidget* button = gtk_button_new_with_label("Open");
    g_signal_connect(button, "clicked", G_CALLBACK(_on_open_clicked), window);
    gtk_header_bar_pack_end(GTK_HEADER_BAR(header_bar), button);
    
    gtk_window_set_titlebar(GTK_WINDOW(window), header_bar);
    
    gtk_window_present(GTK_WINDOW(window));
}

int main(int argc, char** argv) {
    GtkApplication* app = gtk_application_new("dev.marichi.imager", G_APPLICATION_DEFAULT_FLAGS);
    g_signal_connect(app, "activate", G_CALLBACK(activate), NULL);
    int status = g_application_run(G_APPLICATION(app), argc, argv);
    
    return status;
}

Again, if we get the error, then we need to stop by printing the message and free the error memory.

#include <gtk/gtk.h>

static void _on_file_opened(GObject* source_object, GAsyncResult* res, gpointer data) {
    GtkFileDialog* file_dialog = GTK_FILE_DIALOG(source_object);
    GtkWindow* window = GTK_WINDOW(data);
    
    GFile* file = NULL;
    GError* error = NULL;
    
    file = gtk_file_dialog_open_finish(file_dialog, res, &error);
    
    if (error) {
        g_printerr("Error opening file %s\n", error->message);
        g_error_free(error);
        return;
    }
    
    if (file) {
        char* path = g_file_get_path(file);
    
        GtkWidget* picture = gtk_picture_new();
        
        gtk_window_set_child(window, picture);
        
        GdkTexture* texture = gdk_texture_new_from_file(file, &error);
        
        if (error) {
            g_printerr("Error loading image %s\n", error->message);
            g_error_free(error);
            return;
        } else {
        
        }
    }
}

static void _on_open_clicked(GtkWidget* button, gpointer data) {
    GtkWindow* window = GTK_WINDOW(data);
    
    GtkFileDialog* file_dialog = gtk_file_dialog_new();
    
    gtk_file_dialog_open(file_dialog, window, NULL, _on_file_opened, window);
}

static void activate(GtkApplication* app, gpointer data) {
    GtkWidget* window = gtk_application_window_new(app);
    gtk_window_set_title(GTK_WINDOW(window), "Imager");
    gtk_window_set_default_size(GTK_WINDOW(window), 600, 800);
    
    // Header bar.
    GtkWidget* header_bar = gtk_header_bar_new();
    gtk_header_bar_set_title_widget(GTK_HEADER_BAR(header_bar), gtk_label_new("Imager"));
    
    // Button.
    GtkWidget* button = gtk_button_new_with_label("Open");
    g_signal_connect(button, "clicked", G_CALLBACK(_on_open_clicked), window);
    gtk_header_bar_pack_end(GTK_HEADER_BAR(header_bar), button);
    
    gtk_window_set_titlebar(GTK_WINDOW(window), header_bar);
    
    gtk_window_present(GTK_WINDOW(window));
}

int main(int argc, char** argv) {
    GtkApplication* app = gtk_application_new("dev.marichi.imager", G_APPLICATION_DEFAULT_FLAGS);
    g_signal_connect(app, "activate", G_CALLBACK(activate), NULL);
    int status = g_application_run(G_APPLICATION(app), argc, argv);
    
    return status;
}

In else block, we'll call gtk_picture_set_paintable() to set the texture to make it printable by casting it using GDK_PIAINTABLE() macro. We also need to unref the texture as we are done with it.

#include <gtk/gtk.h>

static void _on_file_opened(GObject* source_object, GAsyncResult* res, gpointer data) {
    GtkFileDialog* file_dialog = GTK_FILE_DIALOG(source_object);
    GtkWindow* window = GTK_WINDOW(data);
    
    GFile* file = NULL;
    GError* error = NULL;
    
    file = gtk_file_dialog_open_finish(file_dialog, res, &error);
    
    if (error) {
        g_printerr("Error opening file %s\n", error->message);
        g_error_free(error);
        return;
    }
    
    if (file) {
        char* path = g_file_get_path(file);
    
        GtkWidget* picture = gtk_picture_new();
        
        gtk_window_set_child(window, picture);
        
        GdkTexture* texture = gdk_texture_new_from_file(file, &error);
        
        if (error) {
            g_printerr("Error loading image %s\n", error->message);
            g_error_free(error);
            return;
        } else {
            gtk_picture_set_paintable(GTK_PICTURE(picture), GDK_PAINTABLE(texture));
            g_object_unref(texture);
        }
    }
}

static void _on_open_clicked(GtkWidget* button, gpointer data) {
    GtkWindow* window = GTK_WINDOW(data);
    
    GtkFileDialog* file_dialog = gtk_file_dialog_new();
    
    gtk_file_dialog_open(file_dialog, window, NULL, _on_file_opened, window);
}

static void activate(GtkApplication* app, gpointer data) {
    GtkWidget* window = gtk_application_window_new(app);
    gtk_window_set_title(GTK_WINDOW(window), "Imager");
    gtk_window_set_default_size(GTK_WINDOW(window), 600, 800);
    
    // Header bar.
    GtkWidget* header_bar = gtk_header_bar_new();
    gtk_header_bar_set_title_widget(GTK_HEADER_BAR(header_bar), gtk_label_new("Imager"));
    
    // Button.
    GtkWidget* button = gtk_button_new_with_label("Open");
    g_signal_connect(button, "clicked", G_CALLBACK(_on_open_clicked), window);
    gtk_header_bar_pack_end(GTK_HEADER_BAR(header_bar), button);
    
    gtk_window_set_titlebar(GTK_WINDOW(window), header_bar);
    
    gtk_window_present(GTK_WINDOW(window));
}

int main(int argc, char** argv) {
    GtkApplication* app = gtk_application_new("dev.marichi.imager", G_APPLICATION_DEFAULT_FLAGS);
    g_signal_connect(app, "activate", G_CALLBACK(activate), NULL);
    int status = g_application_run(G_APPLICATION(app), argc, argv);
    
    return status;
}

Don't forget to free the path memory and unref the file object.

#include <gtk/gtk.h>

static void _on_file_opened(GObject* source_object, GAsyncResult* res, gpointer data) {
    GtkFileDialog* file_dialog = GTK_FILE_DIALOG(source_object);
    GtkWindow* window = GTK_WINDOW(data);
    
    GFile* file = NULL;
    GError* error = NULL;
    
    file = gtk_file_dialog_open_finish(file_dialog, res, &error);
    
    if (error) {
        g_printerr("Error opening file %s\n", error->message);
        g_error_free(error);
        return;
    }
    
    if (file) {
        char* path = g_file_get_path(file);
    
        GtkWidget* picture = gtk_picture_new();
        
        gtk_window_set_child(window, picture);
        
        GdkTexture* texture = gdk_texture_new_from_file(file, &error);
        
        if (error) {
            g_printerr("Error loading image %s\n", error->message);
            g_error_free(error);
            return;
        } else {
            gtk_picture_set_paintable(GTK_PICTURE(picture), GDK_PAINTABLE(texture));
            g_object_unref(texture);
        }
        
        g_free(path);
        g_object_unref(file);
    }
}

static void _on_open_clicked(GtkWidget* button, gpointer data) {
    GtkWindow* window = GTK_WINDOW(data);
    
    GtkFileDialog* file_dialog = gtk_file_dialog_new();
    
    gtk_file_dialog_open(file_dialog, window, NULL, _on_file_opened, window);
}

static void activate(GtkApplication* app, gpointer data) {
    GtkWidget* window = gtk_application_window_new(app);
    gtk_window_set_title(GTK_WINDOW(window), "Imager");
    gtk_window_set_default_size(GTK_WINDOW(window), 600, 800);
    
    // Header bar.
    GtkWidget* header_bar = gtk_header_bar_new();
    gtk_header_bar_set_title_widget(GTK_HEADER_BAR(header_bar), gtk_label_new("Imager"));
    
    // Button.
    GtkWidget* button = gtk_button_new_with_label("Open");
    g_signal_connect(button, "clicked", G_CALLBACK(_on_open_clicked), window);
    gtk_header_bar_pack_end(GTK_HEADER_BAR(header_bar), button);
    
    gtk_window_set_titlebar(GTK_WINDOW(window), header_bar);
    
    gtk_window_present(GTK_WINDOW(window));
}

int main(int argc, char** argv) {
    GtkApplication* app = gtk_application_new("dev.marichi.imager", G_APPLICATION_DEFAULT_FLAGS);
    g_signal_connect(app, "activate", G_CALLBACK(activate), NULL);
    int status = g_application_run(G_APPLICATION(app), argc, argv);
    
    return status;
}

And with these changes, we now have the image viewer. Go ahead and run the program to confirm the image viewer!

I've modified this article a bit and created another article as Let's Build Video Player.

Yay!