Why uninitialized global variable is weak symbol?

Question

It seems uninitialized global variable is treated as weak symbol in Gcc. What is the reason behind this?

Answer 1

Is this what you meant?

weak.c

#include 

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).