builtinalloca

builtinalloca refers to the compiler builtin for allocating memory on the current stack frame, most commonly accessed in C as __builtin_alloca. It is a non-standard extension provided by several compilers, notably GCC and Clang, and is distinct from the standard heap allocation facilities.

Semantics and lifetime: memory allocated by a builtin alloca resides on the program’s stack. The returned pointer is valid for the duration of the current function; the storage is automatically freed when the function returns or the stack frame is unwound. The mechanism is typically implemented by adjusting the stack pointer, potentially with appropriate alignment. Because the allocation happens within the stack frame, using it across function boundaries, after a long jump, or after a function has returned is undefined behavior on many targets.

Usage and implementation notes: code can obtain memory via __builtin_alloca(size) or via the alloca function where available. The size may be dynamic, enabling stack-based buffers whose size is known only at runtime, similar to a variable-length array, but implemented via the compiler builtin rather than a standard language feature. The operation can be more efficient than heap allocation, but it carries a risk of stack overflow if the requested size is large relative to the stack. Some environments provide alloca as a library function, while others expose it only as a compiler builtin.

Portability and cautions: builtinalloca is not part of the C standard, and behavior can vary across platforms and calling conventions. Code that relies on it tends to be less portable and harder to reason about in terms of stack usage. As an alternative, developers often prefer heap allocation (malloc/free) or fixed-size stack buffers, with careful bounds checking, to maintain portability and safety.