I saw a program in C that had code like the following:
static void *arr[1] = {&& varOne,&& varTwo,&& varThree};
varOne: printf("One") ;
varTwo: printf("Two") ;
varThree: printf("Three") ;
I am confused about what the &&
does because there is nothing to the left of it. Does it evaluate as null by default? Or is this a special case?
Edit: Added some more information to make the question/code more clear for my question. Thank you all for the help. This was a case of the gcc specific extension.
It's a gcc-specific extension, a unary &&
operator that can be applied to a label name, yielding its address as a void*
value.
As part of the extension, goto *ptr;
is allowed where ptr
is an expression of type void*
.
It's documented here in the gcc manual.
You can get the address of a label defined in the current function (or a containing function) with the unary operator
&&
. The value has typevoid *
. This value is a constant and can be used wherever a constant of that type is valid. For example:void *ptr; /* ... */ ptr = &&foo;
To use these values, you need to be able to jump to one. This is done with the computed goto statement,
goto *exp;
. For example,goto *ptr;
Any expression of type
void *
is allowed.
As zwol points out in a comment, gcc uses &&
rather than the more obvious &
because a label and an object with the same name can be visible simultaneously, making &foo
potentially ambiguous if &
means "address of label". Label names occupy their own namespace (not in the C++ sense), and can appear only in specific contexts: defined by a labeled-statement, as the target of a goto
statement, or, for gcc, as the operand of unary &&
.