On Sun, Apr 11, 2010 at 4:11 AM, Otto Moerbeek <o...@drijf.net> wrote: > On Sun, Apr 11, 2010 at 03:49:26AM -0700, James Hartley wrote: > >> On Sat, Apr 10, 2010 at 9:14 PM, Jesus Sanchez <zexe...@gmail.com> wrote: >> >> > This is not really OpenBSD related but since it's a UNIX-like OS and >> > here are really experienced people coding in C I thought this was a good >> > place to ask. >> > >> >> Actually, not. Your questions are general C programming questions. A >> number of sites exist which thrive on questions like this. Sometimes, they >> actually give the right answers. >> >> >> > Back to a.c later than the 0x2211 >> > assignement I printed the variable and showed 0x11 (at that point i >> > realized the mistake). But I was just wondering where the h*ll went the >> > 0x22 bits on the memory?? >> >> >> Truncated. >> >> >> > I know the rigth thing is to declare the variable 'foo' on a header >> > file and include it in all my code... >> >> >> No, this is incorrect. It is a poor idea to declare variables within header >> files. This bad practice will lead to linker errors due to duplicate >> definitions. >> >> One solution is to define all global variables in a single *.c file & place >> extern statements to each of these global variables in a header file which >> can then be included as many times in as many places as you choose. Here, >> all global variables are defined only once which is required. > > You are mixing up the terms definition and declaration. > > In C, a declaration introduces a name and some or all properties of > that name. > > A definition does the same, but also reserves memory for the object. > > In global scape "int a" is a definition, while "extern int a" is a declaration. > > So a declaration of a variable in a header file is pefectly ok. A > definition, hoewever, is frowned upon. There exist some rules to allow > old code (where this idom is pretty comon) to compile and link, but > you'd better no introduce so called "commons" in new code. > > -Otto
The -fno-common option of gcc is helpful in catching this, once one knows what to look for. --patrick p.s., used in compiling OP's example code gives: b.o(.sbss+0x0): In function `overfoo': /tmp/b.c:3: multiple definition of `foo' a.o(.sbss+0x0):/tmp/a.c:10: first defined here ld: Warning: size of symbol `foo' changed from 1 in a.o to 4 in b.o
