c - prevent gcc from removing an unused variable

Question

In our source files we usually have a version string like that:

static const char srcvers[] = "VERSION/foo.c/1.01/09.04.15";

When that string isn't optimized away, it's quite useful in certain cases, as one can determine the version of each source file linked to an executable by simply calling strings a.out | grep VERSION.

Unfortunately it is optimized away by gcc (using '-O'). So my question is, is there a simple way (a compiler switch would be great) to make gcc keep that variable (its name is always the same) without switching off any other optimizations.

Edit

What, in my opinion, makes the question different from that one, is that I'm was hoping to find a solution for which I wouldn't have to touch thousands of source files.

Answer 1

You can use __attribute__((used)) gcc (also works in clang) specific (I see that the question is tagged gcc) attributes for this:

This attribute, attached to a function, means that code must be emitted for the function even if it appears that the function is not referenced. This is useful, for example, when the function is referenced only in inline assembly.

From https://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html

Demo:

$ cat a.c
static const char srcvers[] __attribute__((used)) = "VERSION/foo.c/1.01/09.04.15";
$ gcc -O3 -c a.c
$ strings a.o
VERSION/foo.c/1.01/09.04.15

You can use some #ifs and #defines to make this terser and also compile on compilers which don't support this extension.