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ALLOCA(3) NetBSD Library Functions Manual ALLOCA(3)
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alloca -- memory allocator
Standard C Library (libc, -lc)
The alloca() function allocates size bytes of space in the stack frame of
the caller. This temporary space is automatically freed on return.
The alloca() function returns a pointer to the beginning of the allocated
space. If the allocation failed, a NULL pointer is returned.
cc(1), brk(2), calloc(3), getpagesize(3), malloc(3), realloc(3),
Few limitations can be mentioned:
· The alloca() function is not part of any C standard and its use is
· 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 fur-
ther 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 secu-
rity features (such as cc(1) -fstack-protector) useless for the call-
ing function. See security(7) for a discussion.
NetBSD 9.0 October 24, 2012 NetBSD 9.0