this is a known issue and I submitted a patch to Allegro github last November
https://github.com/liballeg/allegro5/commit/1d1cc0542c711afe15d4563bbd67f57554ed8d0f

you might want to review the 4.2 branch on Github as there are a
couple other fixes for compilation issues in there

On Mon, Jun 21, 2021 at 5:08 PM Paul Dufresne (dufresnep@zoho.com)
[via djgpp@delorie.com] <djgpp@delorie.com> wrote:
>
> While compiler kraptor game for DJGPP, I seems to get the same problem as:
> https://www.allegro.cc/forums/thread/617748
>
> I tried to recompile the game with: -fgnu89-inline but I believe the library itself
> would have to be recompile if using gcc newer than version 2.x.
>
> Well, I have some doubts because I would have believe people would have
> discovered it before me.
>
> Here is an extract of errors I get:
>
> ld: obj/djgpp/captura.o:captura.c:(.text+0x110): multiple definition of `is_same_bitmap'; obj/djgpp/bomba.o:bomba.c:(.text+0x110): first defined here
> ld: obj/djgpp/captura.o:captura.c:(.text+0x160): multiple definition of `is_linear_bitmap'; obj/djgpp/bomba.o:bomba.c:(.text+0x160): first defined here
> ld: obj/djgpp/captura.o:captura.c:(.text+0x180): multiple definition of `is_planar_bitmap'; obj/djgpp/bomba.o:bomba.c:(.text+0x180): first defined here
> ld: obj/djgpp/captura.o:captura.c:(.text+0x1a0): multiple definition of `is_memory_bitmap'; obj/djgpp/bomba.o:bomba.c:(.text+0x1a0): first defined here
> ld: obj/djgpp/captura.o:captura.c:(.text+0x1c0): multiple definition of `is_screen_bitmap'; obj/djgpp/bomba.o:bomba.c:(.text+0x1c0): first defined here
> ld: obj/djgpp/captura.o:captura.c:(.text+0x210): multiple definition of `is_video_bitmap'; obj/djgpp/bomba.o:bomba.c:(.text+0x210): first defined here
> ld: obj/djgpp/captura.o:captura.c:(.text+0x220): multiple definition of `is_system_bitmap'; obj/djgpp/bomba.o:bomba.c:(.text+0x220): first defined here
> ld: obj/djgpp/captura.o:captura.c:(.text+0x240): multiple definition of `is_sub_bitmap'; obj/djgpp/bomba.o:bomba.c:(.text+0x240): first defined here
> ld: obj/djgpp/captura.o:captura.c:(.text+0x260): multiple definition of `acquire_bitmap'; obj/djgpp/bomba.o:bomba.c:(.text+0x260): first defined here
> ld: obj/djgpp/captura.o:captura.c:(.text+0x280): multiple definition of `release_bitmap'; obj/djgpp/bomba.o:bomba.c:(.text+0x280): first defined here
> ld: obj/djgpp/captura.o:captura.c:(.text+0x2a0): multiple definition of `acquire_screen'; obj/djgpp/bomba.o:bomba.c:(.text+0x2a0): first defined here
> ld: obj/djgpp/captura.o:captura.c:(.text+0x2d0): multiple definition of `release_screen'; obj/djgpp/bomba.o:bomba.c:(.text+0x2d0): first defined here
> ld: obj/djgpp/captura.o:captura.c:(.text+0x300): multiple definition of `is_inside_bitmap'; obj/djgpp/bomba.o:bomba.c:(.text+0x300): first defined here
> ld: obj/djgpp/captura.o:captura.c:(.text+0x370): multiple definition of `get_clip_rect'; obj/djgpp/bomba.o:bomba.c:(.text+0x370): first defined here
> ld: obj/djgpp/captura.o:captura.c:(.text+0x3a0): multiple definition of `set_clip_state'; obj/djgpp/bomba.o:bomba.c:(.text+0x3a0): first defined here
> ld: obj/djgpp/captura.o:captura.c:(.text+0x3b0): multiple definition of `get_clip_state'; obj/djgpp/bomba.o:bomba.c:(.text+0x3b0): first defined here
> ld: obj/djgpp/captura.o:captura.c:(.text+0x3c0): multiple definition of `makecol15'; obj/djgpp/bomba.o:bomba.c:(.text+0x3c0): first defined here
> ld: obj/djgpp/captura.o:captura.c:(.text+0x400): multiple definition of `makecol16'; obj/djgpp/bomba.o:bomba.c:(.text+0x400): first defined here
> ...
> ld: obj/djgpp/combo.o:combo.c:(.text+0x56c): multiple definition of `getg15'; obj/djgpp/bomba.o:bomba.c:(.text+0x560): first defined here
> ld: obj/djgpp/combo.o:combo.c:(.text+0x589): multiple definition of `getb15'; obj/djgpp/bomba.o:bomba.c:(.text+0x580): first defined here
> ld: obj/djgpp/combo.o:combo.c:(.text+0x5a6): multiple definition of `getr16'; obj/djgpp/bomba.o:bomba.c:(.text+0x5a0): first defined here
> ld: obj/djgpp/combo.o:combo.c:(.text+0x5c3): multiple definition of `getg16'; obj/djgpp/bomba.o:bomba.c:(.text+0x5c0): first defined here
> ld: obj/djgpp/combo.o:combo.c:(.text+0x5e0): multiple definition of `getb16'; obj/djgpp/bomba.o:bomba.c:(.text+0x5e0): first defined here
> ld: obj/djgpp/combo.o:combo.c:(.text+0x5fd): multiple definition of `getr24'; obj/djgpp/bomba.o:bomba.c:(.text+0x600): first defined here
> ld: obj/djgpp/combo.o:combo.c:(.text+0x615): multiple definition of `getg24'; obj/djgpp/bomba.o:bomba.c:(.text+0x620): first defined here
> ld: obj/djgpp/combo.o:combo.c:(.text+0x62d): multiple definition of `getb24'; obj/djgpp/bomba.o:bomba.c:(.text+0x640): first defined here
> ld: obj/djgpp/combo.o:combo.c:(.text+0x645): multiple definition of `getr32'; obj/djgpp/bomba.o:bomba.c:(.text+0x660): first defined here
> ld: obj/djgpp/combo.o:combo.c:(.text+0x65d): multiple definition of `getg32'; obj/djgpp/bomba.o:bomba.c:(.text+0x680): first defined here
> ld: obj/djgpp/combo.o:combo.c:(.text+0x675): multiple definition of `getb32'; obj/djgpp/bomba.o:bomba.c:(.text+0x6a0): first defined here
> ld: obj/djgpp/combo.o:combo.c:(.text+0x68d): multiple definition of `geta32'; obj/djgpp/bomba.o:bomba.c:(.text+0x6c0): first defined here
> ld: obj/djgpp/combo.o:combo.c:(.text+0x6a5): multiple definition of `getpixel'; obj/djgpp/bomba.o:bomba.c:(.text+0x6e0): first defined here
> ld: obj/djgpp/combo.o:combo.c:(.text+0x6c9): multiple definition of `putpixel'; obj/djgpp/bomba.o:bomba.c:(.text+0x6f0): first defined here
> ld: obj/djgpp/combo.o:combo.c:(.text+0x6ee): multiple definition of `_allegro_vline'; obj/djgpp/bomba.o:bomba.c:(.text+0x700): first defined here
> ld: obj/djgpp/combo.o:combo.c:(.text+0x719): multiple definition of `_allegro_hline'; obj/djgpp/bomba.o:bomba.c:(.text+0x710): first defined here
> ld: obj/djgpp/combo.o:combo.c:(.text+0x744): multiple definition of `line'; obj/djgpp/bomba.o:bomba.c:(.text+0x720): first defined here
> ld: obj/djgpp/combo.o:combo.c:(.text+0x772): multiple definition of `fastline'; obj/djgpp/bomba.o:bomba.c:(.text+0x730): first defined here
> ld: obj/djgpp/combo.o:combo.c:(.text+0x7a0): multiple definition of `rectfill'; obj/djgpp/bomba.o:bomba.c:(.text+0x740): first defined here
> ld: obj/djgpp/combo.o:combo.c:(.text+0x7ce): multiple definition of `triangle'; obj/djgpp/bomba.o:bomba.c:(.text+0x750): first defined here
> ld: obj/djgpp/combo.o:combo.c:(.text+0x7ff): multiple definition of `polygon'; obj/djgpp/bomba.o:bomba.c:(.text+0x760): first defined here
> ld: obj/djgpp/combo.o:combo.c:(.text+0x827): multiple definition of `rect'; obj/djgpp/bomba.o:bomba.c:(.text+0x780): first defined here
> ld: obj/djgpp/combo.o:combo.c:(.text+0x858): multiple definition of `circle'; obj/djgpp/bomba.o:bomba.c:(.text+0x7a0): first defined here
> ld: obj/djgpp/combo.o:combo.c:(.text+0x886): multiple definition of `circlefill'; obj/djgpp/bomba.o:bomba.c:(.text+0x7c0): first defined here
> ld: obj/djgpp/combo.o:combo.c:(.text+0x8b4): multiple definition of `ellipse'; obj/djgpp/bomba.o:bomba.c:(.text+0x7e0): first defined here
> ld: obj/djgpp/combo.o:combo.c:(.text+0x8e5): multiple definition of `ellipsefill'; obj/djgpp/bomba.o:bomba.c:(.text+0x800): first defined here
> ld: obj/djgpp/combo.o:combo.c:(.text+0x916): multiple definition of `arc'; obj/djgpp/bomba.o:bomba.c:(.text+0x820): first defined here
> ld: obj/djgpp/combo.o:combo.c:(.text+0x94a): multiple definition of `spline'; obj/djgpp/bomba.o:bomba.c:(.text+0x840): first defined here
> ...
> ld: obj/djgpp/partic.o:partic.c:(.text+0x719): multiple definition of `_allegro_hline'; obj/djgpp/bomba.o:bomba.c:(.text+0x710): first defined here
> ld: obj/djgpp/partic.o:partic.c:(.text+0x744): multiple definition of `line'; obj/djgpp/bomba.o:bomba.c:(.text+0x720): first defined here
> ld: obj/djgpp/partic.o:partic.c:(.text+0x772): multiple definition of `fastline'; obj/djgpp/bomba.o:bomba.c:(.text+0x730): first defined here
> ld: obj/djgpp/partic.o:partic.c:(.text+0x7a0): multiple definition of `rectfill'; obj/djgpp/bomba.o:bomba.c:(.text+0x740): first defined here
> ld: obj/djgpp/partic.o:partic.c:(.text+0x7ce): multiple definition of `triangle'; obj/djgpp/bomba.o:bomba.c:(.text+0x750): first defined here
> ld: obj/djgpp/partic.o:partic.c:(.text+0x7ff): multiple definition of `polygon'; obj/djgpp/bomba.o:bomba.c:(.text+0x760): first defined here
> ld: obj/djgpp/partic.o:partic.c:(.text+0x827): multiple definition of `rect'; obj/djgpp/bomba.o:bomba.c:(.text+0x780): first defined here
> ld: obj/djgpp/partic.o:partic.c:(.text+0x858): multiple definition of `circle'; obj/djgpp/bomba.o:bomba.c:(.text+0x7a0): first defined here
> ld: obj/djgpp/partic.o:partic.c:(.text+0x886): multiple definition of `circlefill'; obj/djgpp/bomba.o:bomba.c:(.text+0x7c0): first defined here
> ld: obj/djgpp/partic.o:partic.c:(.text+0x8b4): multiple definition of `ellipse'; obj/djgpp/bomba.o:bomba.c:(.text+0x7e0): first defined here
> ld: obj/djgpp/partic.o:partic.c:(.text+0x8e5): multiple definition of `ellipsefill'; obj/djgpp/bomba.o:bomba.c:(.text+0x800): first defined here
> ld: obj/djgpp/partic.o:partic.c:(.text+0x916): multiple definition of `arc'; obj/djgpp/bomba.o:bomba.c:(.text+0x820): first defined here
> ld: obj/djgpp/partic.o:partic.c:(.text+0x94a): multiple definition of `spline'; obj/djgpp/bomba.o:bomba.c:(.text+0x840): first defined here
> ld: obj/djgpp/partic.o:partic.c:(.text+0x972): multiple definition of `floodfill'; obj/djgpp/bomba.o:bomba.c:(.text+0x860): first defined here
> ld: obj/djgpp/partic.o:partic.c:(.text+0x99a): multiple definition of `polygon3d'; obj/djgpp/bomba.o:bomba.c:(.text+0x880): first defined here
> ld: obj/djgpp/partic.o:partic.c:(.text+0x9c8): multiple definition of `polygon3d_f'; obj/djgpp/bomba.o:bomba.c:(.text+0x8a0): first defined here
> ld: obj/djgpp/partic.o:partic.c:(.text+0x9f6): multiple definition of `triangle3d'; obj/djgpp/bomba.o:bomba.c:(.text+0x8c0): first defined here
> ld: obj/djgpp/partic.o:partic.c:(.text+0xa27): multiple definition of `triangle3d_f'; obj/djgpp/bomba.o:bomba.c:(.text+0x8e0): first defined here
> ld: obj/djgpp/partic.o:partic.c:(.text+0xa58): multiple definition of `quad3d'; obj/djgpp/bomba.o:bomba.c:(.text+0x900): first defined here
> ld: obj/djgpp/partic.o:partic.c:(.text+0xa8c): multiple definition of `quad3d_f'; obj/djgpp/bomba.o:bomba.c:(.text+0x920): first defined here
> ld: obj/djgpp/partic.o:partic.c:(.text+0xac0): multiple definition of `draw_sprite'; obj/djgpp/bomba.o:bomba.c:(.text+0x940): first defined here
> ld: obj/djgpp/partic.o:partic.c:(.text+0xb0e): multiple definition of `draw_sprite_v_flip'; obj/djgpp/bomba.o:bomba.c:(.text+0x990): first defined here
> ld: obj/djgpp/partic.o:partic.c:(.text+0xb33): multiple definition of `draw_sprite_h_flip'; obj/djgpp/bomba.o:bomba.c:(.text+0x9a0): first defined here
> ld: obj/djgpp/partic.o:partic.c:(.text+0xb58): multiple definition of `draw_sprite_vh_flip'; obj/djgpp/bomba.o:bomba.c:(.text+0x9b0): first defined here
> ld: obj/djgpp/partic.o:partic.c:(.text+0xb7d): multiple definition of `draw_trans_sprite'; obj/djgpp/bomba.o:bomba.c:(.text+0x9c0): first defined here
> ld: obj/djgpp/partic.o:partic.c:(.text+0xbcb): multiple definition of `draw_lit_sprite'; obj/djgpp/bomba.o:bomba.c:(.text+0xa10): first defined here
> ld: obj/djgpp/partic.o:partic.c:(.text+0xbf6): multiple definition of `draw_gouraud_sprite'; obj/djgpp/bomba.o:bomba.c:(.text+0xa20): first defined here
> ld: obj/djgpp/partic.o:partic.c:(.text+0xc2a): multiple definition of `draw_character_ex'; obj/djgpp/bomba.o:bomba.c:(.text+0xa40): first defined here
> ld: obj/djgpp/partic.o:partic.c:(.text+0xc58): multiple definition of `rotate_sprite'; obj/djgpp/bomba.o:bomba.c:(.text+0xa50): first defined here
> ld: obj/djgpp/partic.o:partic.c:(.text+0xccc): multiple definition of `rotate_sprite_v_flip'; obj/djgpp/bomba.o:bomba.c:(.text+0xaa0): first defined here
> ld: obj/djgpp/partic.o:partic.c:(.text+0xd40): multiple definition of `rotate_scaled_sprite'; obj/djgpp/bomba.o:bomba.c:(.text+0xaf0): first defined here
> ld: obj/djgpp/partic.o:partic.c:(.text+0xdcc): multiple definition of `rotate_scaled_sprite_v_flip'; obj/djgpp/bomba.o:bomba.c:(.text+0xb60): first defined here
> ld: obj/djgpp/partic.o:partic.c:(.text+0xe58): multiple definition of `pivot_sprite'; obj/djgpp/bomba.o:bomba.c:(.text+0xbd0): first defined here
> ld: obj/djgpp/partic.o:partic.c:(.text+0xeab): multiple definition of `pivot_sprite_v_flip'; obj/djgpp/bomba.o:bomba.c:(.text+0xc20): first defined her
>

Hello,

This is a change in behaviour of GCC-10: last change described in
https://gcc.gnu.org/gcc-10/changes.html#c

Using -fcommon is certainly not the correct correction (but could be a
workaround for old programs with a lot of occurence of this problem),
but adding extern or static depending on if the variable should be
shared or not is better.

Regards,

Damien

On 27/06/2021 00:20, Paul Dufresne (dufresnep@zoho.com) [via
djgpp@delorie.com] wrote:
> I now think that adding "external" before declarations in .h is the
> correct solution.
> If they were not mark static already, it is that the value is expected
> to be shared (being global).
> The solution is *I now think* to add external, and have the real
> declaration in a unique place, like in main.c.
>

Am 27.06.2021 um 18:07 schrieb Damien Guibouret:

> Using -fcommon is certainly not the correct correction (but could be a
> workaround for old programs with a lot of occurence of this problem),
> but adding extern or static depending on if the variable should be
> shared or not is better.

Adding 'static' into a header is hardly ever the right solution.

The rules of thumb are: never write 'extern' in a *.c file, and never
'static' in a *.h file.