const-qualifying a variable doesn’t make it a compile-time constant (see C99 6.6 §6 for the defintion of an integer constant expression), and before the introduction of variable-length arrays with C99, array sizes needed to be compile-time constants. It’s rather obvious that const-qualify a variable doesn’t make it a compile-time constant, in particular in case of … Read more
Variable length arrays were added to C99. It’s described in the C99 rationale: 6.7.5.2 Array declarators C99 adds a new array type called a variable length array type. The inability to declare arrays whose size is known only at execution time was often cited as a primary deterrent to using C as a numerical computing … Read more
See http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37428 and also http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42121 Yes, it’s weird.
Diagnosis In the code in the question, the variable n is uninitialized when it is used in the definition of vla. Indeed, with GCC set fussy, the code shown produces a compilation error (it’ll give a warning if you are careless enough to omit -Werror from your compilation options — don’t do that!): $ gcc … Read more
There is no requirement that VLAs be allocated from the stack (the language standard doesn’t even mention stacks or heaps). The only requirement is as follows: 6.2.4 Storage durations of objects … 7 For such an object that does have a variable length array type, its lifetime extends from the declaration of the object until … Read more
A variable length array (i.e. an array sized with a runtime value) can’t be a global variable, because the expression you are using for the size must obviously be computed at compile time. It can only live on the stack. Presumably what you are getting is a static array with a size that depends on … Read more
Yes, this causes undefined behaviour. In N1570 6.5.3.4/2 we have: The sizeof operator yields the size (in bytes) of its operand, which may be an expression or the parenthesized name of a type. The size is determined from the type of the operand. The result is an integer. If the type of the operand is … Read more
VLA does have some overhead (compared to “ordinary” named compile-time-sized array). Firstly, it has run-time length and yet the language provides you with means to obtain the actual size of the array at run-time (using sizeof). This immediately means that the actual size of the array has to be stored somewhere. This results in some … Read more
In C99 and later, you can have a (one-dimensional) flexible array member (FAM) at the end of a structure: §6.7.2.1 Structure and union specifiers ¶18 As a special case, the last element of a structure with more than one named member may have an incomplete array type; this is called a flexible array member. In … Read more
Yes your reasoning is correct, that is how these different forms of array declarations and definitions are viewed by C and C++. As others already stated, VLA with a veritable variable length (non-const) in global scope is difficult to make sense. What would the evaluation order be, e.g if the the length expression would refer … Read more