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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?
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Conditional Operators The logical AND and logical OR operators both take two operands. Each operand is a boolean expression (i.e., it evaluates to either true or false). The logical AND condition returns true if both operands are true, otherwise, it returns false.
Bash - Ternary Operator Bash or shell script example for Ternary Operator expression with examples. Bash programming does not have support for ternary operator syntax. The syntax is similar to if and else conditional expression. if expression is true, value1 is returned,else value2 is returned.
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.
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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.
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