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44 lines
2.1 KiB
Plaintext
44 lines
2.1 KiB
Plaintext
@c Documentation of gnulib module 'alloca'.
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@c Copyright (C) 2004, 2007, 2009-2017 Free Software Foundation, Inc.
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@c Permission is granted to copy, distribute and/or modify this document
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@c under the terms of the GNU Free Documentation License, Version 1.3 or
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@c any later version published by the Free Software Foundation; with no
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@c Invariant Sections, no Front-Cover Texts, and no Back-Cover
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@c Texts. A copy of the license is included in the ``GNU Free
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@c Documentation License'' file as part of this distribution.
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The alloca module provides for a function @code{alloca} which allocates
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memory on the stack, where the system allows it. A memory block allocated with
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@code{alloca} exists only until the function that calls @code{alloca} returns
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or exits abruptly.
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There are a few systems where this is not possible: HP-UX systems, and some
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other platforms when the C++ compiler is used. On these platforms the alloca
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module provides a @code{malloc} based emulation. This emulation will not free a
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memory block immediately when the calling function returns, but rather will
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wait until the next @code{alloca} call from a function with the same or a
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shorter stack length. Thus, in some cases, a few memory blocks will be kept
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although they are not needed any more.
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The user can @code{#include <alloca.h>} and use @code{alloca} on all platforms.
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Note that the @code{#include <alloca.h>} must be the first one after the
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autoconf-generated @file{config.h}, for AIX 3 compatibility. Thanks to IBM for
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this nice restriction!
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Note that GCC 3.1 and 3.2 can @emph{inline} functions that call @code{alloca}.
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When this happens, the memory blocks allocated with @code{alloca} will not be
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freed until @emph{the end of the calling function}. If this calling function
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runs a loop calling the function that uses @code{alloca}, the program easily
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gets a stack overflow and crashes. To protect against this compiler behaviour,
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you can mark the function that uses @code{alloca} with the following attribute:
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@smallexample
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#ifdef __GNUC__
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__attribute__ ((__noinline__))
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#endif
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@end smallexample
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An alternative to this module is the @samp{alloca-opt} module.
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