multiple definition of `variable` ... first defined here

Randomly I decided to compile some old projects I worked on and I was surprised to see a few compilation errors in an assembler I wrote years back. As it has been many years since I last touched most of the projects I looked at, I was pleased to see the compiler catching obvious mistakes I had made in the past. Though this did come to a surprise as to why the compiler I used years ago never complained such obvious mistakes. The solution and reason for the last compilation error was not immediate to me:

$ make
gcc -o assembler assembler.c symbol_table.c parser.c  -fsanitize=address -lasan
/usr/bin/ld: /tmp/cc1MoBol.o:(.bss+0x0): multiple definition of `table'; /tmp/cc0B4XxW.o:(.bss+0x0): first defined here
/usr/bin/ld: /tmp/cc1MoBol.o:(.bss+0x81): multiple definition of `__odr_asan.table'; /tmp/cc0B4XxW.o:(.bss+0x40): first defined here

At first I thought I may had made a stupid mistake and defined the struct called table twice but all I could find was symbol_table.h, the file that declared the variable, being included by assembler.c and parser.c. This led to the conclusion there must have been a compiler behavioral change between GCC 9 and GCC 14. After a quick googling and going through going through the Release Notes, it turns out that starting from GCC 10, GCC now defaults to -fno-common:

GCC now defaults to -fno-common. As a result, global variable accesses are more efficient on various targets. In C, global variables with multiple tentative definitions now result in linker errors. With -fcommon such definitions are silently merged during linking.

In the Porting to GCC 10 webpage, the developers of GCC notes:

A common mistake in C is omitting extern when declaring a global variable in a header file. If the header is included by several files it results in multiple definitions of the same variable

To resolve this issue, one can either silently ignore their mistake and compile with -fcommon or to correctly declare the global variable with the extern keyword.