42 lines
2.0 KiB
Plaintext
42 lines
2.0 KiB
Plaintext
@c Documentation of gnulib module 'alloca-opt'.
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@c Copyright (C) 2004, 2007, 2009--2024 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 Texts. A
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@c copy of the license is at <https://www.gnu.org/licenses/fdl-1.3.en.html>.
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@mindex alloca-opt
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The @code{alloca-opt} 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
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@code{alloca-opt} module provides no replacement, just a preprocessor macro
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HAVE_ALLOCA.
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The user can @code{#include <alloca.h>} on all platforms, and use
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@code{alloca} on those platforms where the preprocessor macro HAVE_ALLOCA
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evaluates to true. If HAVE_ALLOCA is false, the code should use a heap-based
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memory allocation based on @code{malloc} or (in C++) @code{new}. Note that
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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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