Library Functions Manual
NAME alloca — memory allocator
LIBRARY Standard C Library (libc, -lc)
SYNOPSIS #include <stdlib.h>
DESCRIPTION The alloca() function allocates size bytes of space in the stack frame of the caller. This temporary space is automatically freed on return.
RETURN VALUES The alloca() function returns a pointer to the beginning of the allocated space. If the allocation failed, a NULL pointer is returned.
Few limitations can be mentioned:
The alloca() function is not part of any C standard and its use is not portable.
The alloca() function should be supplied by the compiler because the compiler is allowed to make assumptions about the stack and frame pointers. The libc alloca() implementation cannot account for those assumptions. While there is a machine dependent implementation of alloca() in libc, its use is discouraged and in most cases it will not work. Using this implementation will produce linker warnings.
The alloca() function is unsafe because it cannot ensure that the pointer returned points to a valid and usable block of memory. The allocation made may exceed the bounds of the stack, or even go further into other objects in memory, and alloca() cannot determine such an error. For that all alloca() allocations should be bounded and limited to a small size.
Since alloca() modifies the stack at runtime and the stack usage of each function frame cannot be predicted, it makes many compiler security features (such as cc(1) -fstack-protector) useless for the calling function. See security(7) for a discussion.