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

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.