问题
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?
回答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.
回答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.
回答3:
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.
回答4:
This is a gcc-specific extension to C and is not standard.
来源：https://stackoverflow.com/questions/7814694/bizarre-use-of-conditional-operator-in-linux