alloca(3) - NetBSD Manual Pages

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ALLOCA(3)               NetBSD 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:

     ·   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 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 7.0                     October 24, 2012                     NetBSD 7.0