something broken in al_lock_bitmap

[rmbeer]

This's the source code with undefined behavior:

` /* This example displays a picture on the screen, with support for

• command-line parameters, multi-screen, screen-orientation and

• zooming. */

  include

  include

  include <allegro5/allegro.h>

  include <allegro5/allegro_image.h>

  ALLEGRO_DISPLAY display; ALLEGRO_BITMAPgzz;

  void Renderx(){ static int szz=0; al_clear_to_color(al_map_rgb(0,0,0)); int px,py; charzdat,p; ALLEGRO_LOCKED_REGIONzzz; zzz=al_lock_bitmap(gzz,ALLEGRO_LOCK_WRITEONLY,ALLEGRO_PIXEL_FORMAT_RGBA_8888); //zzz=al_lock_bitmap(gzz,ALLEGRO_LOCK_WRITEONLY,ALLEGRO_PIXEL_FORMAT_RGB_888); if(!zzz){printf("fallofallo\n");return -1;} zdat=(char*)zzz->data;p=0; //printf("%d / %d\n",zzz->pixel_size,zzz->pitch); for(px=0;px<240;px++){p=0; //for(px=0;px<480;px++){p=0; //for(py=0;py<320;py++){ for(py=0;py<szz;py++){ //zdat[p]=(px+py)&255;zdat[p+1]=0;zdat[p+2]=0;zdat[p+3]=0;p+=4; //dont show nothing zdat[p]=0;zdat[p+1]=(px+py)&255;zdat[p+2]=0;zdat[p+3]=255;p+=4; //show gradient white //zdat[p]=0;zdat[p+1]=0;zdat[p+2]=(px+py)&255;zdat[p+3]=0;p+=4; //show gradient purple //zdat[p]=0;zdat[p+1]=0;zdat[p+2]=0;zdat[p+3]=(px+py)&255;p+=4; //show gradient black }zdat+=zzz->pitch; }szz++;if(szz>640)szz=640; al_unlock_bitmap(gzz); al_set_target_backbuffer(display); }

  int main(int argc, char *argv) { const char filename; ALLEGRO_BITMAP bitmap; ALLEGRO_TIMER timer; ALLEGRO_EVENT_QUEUE *queue; bool redraw = true; double zoom = 1; double t0; double t1;

  if (!al_init()) { printf("Could not init Allegro.\n"); return -1; }

  /* The second parameter to the process can optionally specify what

  • adapter to use. */ if (argc > 2) { al_set_new_display_adapter(atoi(argv[2])); }

    /* Allegro requires installing drivers for all input devices before

  • they can be used. */ al_install_mouse(); al_install_keyboard();

    /* Initialize the image addon. Requires the allegro_image addon

  • library. */ al_init_image_addon();

    // Create a new display that we can render the image to. display = al_create_display(640, 480); if (!display) { printf("Error creating display\n"); return -2; }

    al_set_window_title(display, "Pruebas");

    // Create a timer that fires 30 times a second. timer = al_create_timer(1.0 / 15); queue = al_create_event_queue(); al_register_event_source(queue, al_get_keyboard_event_source()); al_register_event_source(queue, al_get_display_event_source(display)); al_register_event_source(queue, al_get_timer_event_source(timer)); al_start_timer(timer); // Start the timer al_set_blender(ALLEGRO_ADD,ALLEGRO_ALPHA,ALLEGRO_INVERSE_ALPHA); gzz=al_create_bitmap(640,480); if(!gzz){printf("fallofallo22\n");return -1;} al_set_target_bitmap(gzz); Renderx(); // Primary 'game' loop. while (1) { ALLEGRO_EVENT event; al_wait_for_event(queue, &event); // Wait for and get an event. if (event.type == ALLEGRO_EVENT_DISPLAY_ORIENTATION) { int o = event.display.orientation; if (o == ALLEGRO_DISPLAY_ORIENTATION_0_DEGREES) { printf("0 degrees\n"); } else if (o == ALLEGRO_DISPLAY_ORIENTATION_90_DEGREES) { printf("90 degrees\n"); } else if (o == ALLEGRO_DISPLAY_ORIENTATION_180_DEGREES) { printf("180 degrees\n"); } else if (o == ALLEGRO_DISPLAY_ORIENTATION_270_DEGREES) { printf("270 degrees\n"); } else if (o == ALLEGRO_DISPLAY_ORIENTATION_FACE_UP) { printf("Face up\n"); } else if (o == ALLEGRO_DISPLAY_ORIENTATION_FACE_DOWN) { printf("Face down\n"); } } if (event.type == ALLEGRO_EVENT_DISPLAY_CLOSE) break; /* Use keyboard to zoom image in and out.

    • 1: Reset zoom.
    • +: Zoom in 10%
    • -: Zoom out 10%

    • f: Zoom to width of window */ if (event.type == ALLEGRO_EVENT_KEY_CHAR) { if (event.keyboard.keycode == ALLEGRO_KEY_ESCAPE) break; // Break the loop and quite on escape key. if (event.keyboard.unichar == '1') zoom = 1; if (event.keyboard.unichar == '+') zoom = 1; if (event.keyboard.unichar == '-') zoom = 0; if (event.keyboard.unichar == 'f') zoom = (double)al_get_display_width(display) / al_get_bitmap_width(bitmap); }

      // Trigger a redraw on the timer event if (event.type == ALLEGRO_EVENT_TIMER) redraw = true;

      // Redraw, but only if the event queue is empty if (redraw && al_is_event_queue_empty(queue)) { redraw = false; al_set_target_backbuffer(display); // Clear so we don't get trippy artifacts left after zoom. al_clear_to_color(al_map_rgb_f(0, 0, zoom)); al_draw_bitmap(gzz,0,0,0); al_flip_display(); Renderx(); } }

    return 0; }

  / vim: set sts=4 sw=4 et: / `

It's based in ex_bitmap.c

• Something are wrong with ALLEGRO_PIXEL_FORMAT_RGBA_8888 and ALLEGRO_PIXEL_FORMAT_RGB_888, both are a size of 4 bytes x pixel, and give strange behavior if you change the code for 4 or 3 bytes.
• al_set_blender affect al_lock_bitmap, this are undocumented.
• Don't have sense how are drawing in the bitmap, seem like fragmented.
• Where you render in all the bitmap, get segfault by overflow or double free.

[jeffythedragonslayer]

Hi! The formatting is broken up to double t1;

[allefant]

• both are a size of 4 bytes x pixel

According to https://liballeg.org/a5docs/trunk/graphics.html#allegro_pixel_format one of them is 4 bytes and one is 3 bytes.