Meaning of -fcommon and -fno-common

[David Given]

Amittai Aviram wrote:
> The GCC manual explains the -fcommon and -fno-common flag with reference to traditional Unix practice:  "In C code, controls the placement of uninitialized global variables. Unix C compilers have traditionally permitted multiple definitions of such variables in different compilation units by placing the variables in a common block."  What exactly is a "common block" in this context?  Where is it in the file and--assuming it's an ELF executable to be run on a Linux system--what segment (virtual address range) would the OS use for it, as opposed to the data section?  Thanks!

Variables in common blocks get folded into each other. So:

---file 1---
int i;
---file 2---
int i;
---end---

...will refer to the *same* global variable. After linking they end up
in the data segment just like any other global variable. They're not
really recommended these days due to principle of least surprise; people
are much better off using explicit declarations with extern instead.

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