https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85678
Bug ID: 85678
Summary: -fno-common should be default
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c
Assignee: unassigned at gcc dot gnu.org
Reporter: david at westcontrol dot com
Target Milestone: ---
The use of a "common" section to match up tentative object definitions in
different translation units in a program is contrary to the C standards
(section 6.9p5 in C99 and C11, section 6.7 in C90), gives a high risk of error,
may lead to poorer quality code, and provides no benefits except backwards
compatibility with ancient code.
Unix C compilers have traditionally allowed "int x;" in file a.c to be
allocated by the linker to the same storage as "int x;" in file b.c when these
two C files are linked together. gcc supports this if "-fcommon" is enabled,
which is the default on most targets. But it is contrary to the C (and C++)
standards that say there shall be exactly /one/ definition of each object. And
it is very easy to have errors in code by accidentally having duplicate names
in different files (if the programmer has not used "static"), perhaps even of
different types. With "-fno-common", such errors are caught at link time.
Also, data in common sections will hinder some types of optimisation, such as
"-fsection-anchors".
Surely it is time to make "-fno-common" the default, at least when a modern C
standard is specified indicating that the code is modern? People who need the
old behaviour can always get it with "-fcommon".
