Meaning of -fcommon and -fno-common

Meaning of -fcommon and -fno-common

[Amittai Aviram]

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!

Amittai

Amittai Aviram
PhD Student in Computer Science
Yale University
646 483 2639
amittai.aviram@yale.edu
http://www.amittai.com

Re: Meaning of -fcommon and -fno-common

[David Given]

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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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Re: Meaning of -fcommon and -fno-common

[Ian Lance Taylor]

Amittai Aviram <amittai.aviram@yale.edu> writes:

> 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!

Common symbols exist in object files, not executables.  The linker will
group the common symbols and put them in the data segment.

Ian