Why uninitialized global variable is weak symbol?

gcc, in C mode:

Uninitialised globals which are not declared extern are treated as "common" symbols, not weak symbols.

Common symbols are merged at link time so that they all refer to the same storage; if more than one object attempts to initialise such a symbol, you will get a link-time error. (If they aren't explicitly initialised anywhere, they will be placed in the BSS, i.e. initialised to 0.)

gcc, in C++ mode:

Not the same - it doesn't do the common symbols thing. "Uninitialised" globals which are not declared extern are implicitly initialised to a default value (0 for simple types, or default constructor).

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In either case, a weak symbol allows an initialised symbol to be overridden by a non-weak initialised symbol of the same name at link time.

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To illustrate (concentrating on the C case here), I'll use 4 variants of a main program, which are all the same except for the way that global is declared:

1. main_init.c:

   #include <stdio.h>
   
   int global = 999;
   
   int main(void) { printf("%d\n", global); return 0; }
   

2. main_uninit.c, which omits the initialisation:

   #include <stdio.h>
   
   int global;
   
   int main(void) { printf("%d\n", global); return 0; }
   

3. main_uninit_extern.c, which adds the extern keyword:

   #include <stdio.h>
   
   extern int global;
   
   int main(void) { printf("%d\n", global); return 0; }
   

4. main_init_weak.c, which initialises global and declares it to be a weak symbol:

   #include <stdio.h>
   
   int global __attribute__((weak)) = 999;
   
   int main(void) { printf("%d\n", global); return 0; }
   

and another_def.c which initialises the same global:

int global = 1234;

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Using main_uninit.c on its own gives 0:

$ gcc -o test main_uninit.c && ./test
0

but when another_def.c is included as well, global is explicitly initialised and we get the expected result:

$ gcc -o test main_uninit.c another_def.c && ./test
1234

(Note that this case fails instead if you're using C++.)

If we try with both main_init.c and another.def.c instead, we have 2 initialisations of global, which won't work:

$ gcc -o test main_init.c another_def.c && ./test
/tmp/cc5DQeaz.o:(.data+0x0): multiple definition of `global'
/tmp/ccgyz6rL.o:(.data+0x0): first defined here
collect2: ld returned 1 exit status

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main_uninit_extern.c on its own won't work at all - the extern keyword causes the symbol to be an ordinary external reference rather than a common symbol, so the linker complains:

$ gcc -o test main_uninit_extern.c && ./test
/tmp/ccqdYUIr.o: In function `main':
main_uninit_extern.c:(.text+0x12): undefined reference to `global'
collect2: ld returned 1 exit status

It works fine once the initialisation from another_def.c is included:

$ gcc -o test main_uninit_extern.c another_def.c && ./test
1234

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Using main_init_weak.c on its own gives the value we initialised the weak symbol to (999), as there is nothing to override it:

$ gcc -o test main_init_weak.c && ./test
999

But pulling in the other definition from another_def.c does work in this case, because the strong definition there overrides the weak definition in main_init_weak.c:

$ gcc -o test main_init_weak.c another_def.c && ./test
1234

The question is based on an incorrect premise. Uninitialized global variables are not weak symbols.

Apparently the question is referring to the ability to define the same uninitialized object with external linkage in multiple translation units. Formally, it is not allowed - it is an error in both C and C++. However, at least in C is recognized by C99 standard as "common extension" of the language, implemented in many real-life compilers

J.5 Common extensions

J.5.11 Multiple external definitions

1 There may be more than one external definition for the identifier of an object, with or without the explicit use of the keyword extern; if the definitions disagree, or more than one is initialized, the behavior is undefined (6.9.2).

Note, that contrary to the popular belief, C language explicitly prohibits introducing multiple definitions of entities with external linkage in the program, just like C++ does.

6.9 External definitions

5 An external definition is an external declaration that is also a definition of a function (other than an inline definition) or an object. If an identifier declared with external linkage is used in an expression (other than as part of the operand of a sizeof operator whose result is an integer constant), somewhere in the entire program there shall be exactly one external definition for the identifier; otherwise, there shall be no more than one.

However, the extension allowing this has been pretty popular with many C compilers, of which GCC just happens to be one.

Is this what you meant?

weak.c

#include <stdio.h>

int weak; /* global, weak, zero */

int main(void) {
  printf("weak value is %d.\n", weak);
  return 0;
}

strong.c

int weak = 42; /* global, strong, 42 */

Sample run

$ gcc weak.c
$ ./a.out
weak value is 0.
$ gcc weak.c strong.c
$ ./a.out
weak value is 42.

The int weak; in weak.c is a declaration, not a definition. Or you may say it's a tentative definition. The real definition is in strong.c when that object file is linked in the final program or in weak.c otherwise. This is a common extension, one that gcc uses (thanks Andrey).