Joshua Joshua - 1 year ago 73
C Question

What does a && operator do when there is no left side in C?

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?

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.

Answer Source

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 type void *. 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.

Recommended from our users: Dynamic Network Monitoring from WhatsUp Gold from IPSwitch. Free Download