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ALLOCA(3)                  Library Functions Manual                  ALLOCA(3)

NAME
     alloca -- memory allocator

LIBRARY
     Standard C Library (libc, -lc)

SYNOPSIS
     #include <stdlib.h>

     void *
     alloca(size_t size);

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.

SEE ALSO
     cc(1), brk(2), calloc(3), getpagesize(3), malloc(3), realloc(3),
     security(7)

CAVEATS
     Few limitations can be mentioned:

     o   The alloca() function is not part of any C standard and its use is
         not portable.

     o   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.

     o   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.

     o   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.

NetBSD 7.1.2                   October 24, 2012                   NetBSD 7.1.2