Bizarre use of conditional operator in Linux

Question

In the 3.0.4 Linux kernel, mm/filemap.c has this line of code:

retval = retval ?: desc.error; 

I've tried compiling a similar minimal test case with gcc -Wall and don't get any warnings; the behavior seems identical to:

retval = retval ? retval : desc.error; 

Looking at the C99 standard, I can't figure out what formally describes this behavior. Why is this OK?

Answer 1

As several others have said, this is a GCC extension, not part of any standard. You'll get a warning for it if you use the -pedantic switch.

The point of this extension is not really visible in this case, but imagine if instead it was

retval = foo() ?: desc.error; 

With the extension, foo() is called only once. Without it, you have to introduce a temporary variable to avoid calling foo() twice.

Answer 2

It's a gcc extension. x ?: y is equivalent to x ? x : y --- see http://gcc.gnu.org/onlinedocs/gcc/Conditionals.html#Conditionals.

Yes, I think it's evil too.