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From: "Wiktor S." <wswiktorSP AT Mpoczta DOT fm>
Newsgroups: comp.os.msdos.djgpp
Subject: GCC 5.1.0 problem with <dos.h>
Date: Tue, 28 Apr 2015 01:22:58 +0200
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To: djgpp AT delorie DOT com
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Reply-To: djgpp AT delorie DOT com

Hello.
I found this problem when trying to compile the game of Quake, which 
compiles with no problems with GCC 4.9.2

Simply #include'ing <dos.h> in multiple .c files causes the following 
errors:

    C:\wiktor\myprogs\djtest>gcc -c a.c

    C:\wiktor\myprogs\djtest>gcc -c b.c

    C:\wiktor\myprogs\djtest>gcc a.o b.o -o a.exe
    b.o:b.c:(.text+0x0): multiple definition of `inportb'
    a.o:a.c:(.text+0x0): first defined here
    b.o:b.c:(.text+0x1b): multiple definition of `inportw'
    a.o:a.c:(.text+0x1b): first defined here
    b.o:b.c:(.text+0x39): multiple definition of `inportl'
    a.o:a.c:(.text+0x39): first defined here
    b.o:b.c:(.text+0x54): multiple definition of `outportb'
    a.o:a.c:(.text+0x54): first defined here
    b.o:b.c:(.text+0x71): multiple definition of `outportw'
    a.o:a.c:(.text+0x71): first defined here
    b.o:b.c:(.text+0x90): multiple definition of `outportl'
    a.o:a.c:(.text+0x90): first defined here
    collect2.exe: error: ld returned 1 exit status

The workaround is to use a linker option

    LDFLAGS=-Wl,--allow-multiple-definition


Example source:

======== a.c ========
#include <dos.h>

int main()
{

}

======== b.c ========
#include <dos.h>

=====================


-- 
Wiktor Sywula