alloca

alloca is a function that allocates memory on the calling thread’s stack frame rather than on the heap. The allocated block is automatically reclaimed when the function that called alloca returns, so the programmer does not need to invoke a corresponding free operation. In most C and C++ environments alloca is declared in <alloca.h> or, on some systems, as a built‑in compiler intrinsic, for example __builtin_alloca in GCC and Clang.

Because the memory comes from the stack, allocation and deallocation are fast: the operation typically adjusts the stack pointer. However, the size of the allocation is limited by the available stack space, which can be much smaller than the heap, and excessive use of alloca may cause stack overflow. The lifetime of the memory is tied to the lexical scope of the function; returning a pointer to the allocated block or storing it in a static or global variable results in undefined behavior.

alloca is not part of the ISO C standard and its availability varies across platforms. It is defined by POSIX only as an optional extension, and many embedded or kernel environments omit it. Consequently, portable code usually prefers heap allocation with malloc/free or uses variable‑length arrays, which are standard in C99 and later.

Security considerations include the risk of stack smashing attacks when large or unchecked allocations are made. Some compilers provide warnings for alloca usage, and static analysis tools can flag potential misuse. When used judiciously—typically for small, short‑lived buffers—alloca can improve performance and simplify memory management, but developers should weigh its non‑portable nature and stack‑size limitations against alternative allocation strategies.