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

This is a GCC extension called Conditionals with Omitted Operands. Omitting the middle operand has the effect of using the value of the conditional as the omitted operand without evaluating it again. It is safe to use even if the conditional is a macro.