Duplicate symbol 'foo' error after GCC 10 default changes to -fno-common

Confirmed facts

Starting with GCC 10, the compiler changed the default handling of tentative definitions from -fcommon to -fno-common. This makes each tentative definition a strong symbol, so when two translation units contain a tentative definition of the same global variable (e.g., foo) the linker reports a duplicate‑symbol error. GCC 11, 12, 13, and the released GCC 14 series have retained -fno-common as the default; there has been no official announcement to revert to -fcommon.

Likely explanation

If you are seeing duplicate symbol 'foo' with GCC ≥10, the cause is the stricter default. The linker error will disappear if either the compilation is forced to use -fcommon (restoring the old merging behavior) or the source is changed so that only one translation unit provides a definition while the others declare it extern.

Steps to resolve

1. Check your GCC version: gcc --version. If it is 10 or newer, the default is -fno-common.
2. Determine whether you can modify the source code to give foo a single definition.
   • If yes: in exactly one .c/.cpp file write int foo = 0; (or appropriate initializer) and in all other files declare extern int foo;. Rebuild; the error should vanish without any extra flags.
   • If no: add -fcommon to the compile command for all translation units (e.g., CFLAGS += -fcommon in a Makefile) or adjust your build system to pass this flag globally. This restores the pre‑GCC 10 merging behavior.
3. For projects that must support both older and newer GCC versions, make the flag conditional:

   # Example Makefile snippet
   ifeq ($(findstring 10,$(shell gcc -dumpversion)),)
       # GCC <10 already defaults to -fcommon; keep it explicit
       CFLAGS += -fcommon
   else
       # GCC ≥10 needs -fcommon to regain the old behavior
       CFLAGS += -fcommon
   endif
   

   (Replace the condition with a proper version test if needed.)
4. Verify the fix: rebuild and run nm -C *.o or objdump -t *.o to confirm that foo now has a single strong symbol (type D or B) or, when using -fcommon, a common symbol (type C) that the linker merges.

Please let me know if you are able to modify the source code to provide a single definition for foo. Your answer will determine whether the source‑change route or the -fcommon flag is the recommended path.