| @c Documentation of gnulib module 'alloca-opt'. |
| |
| @c Copyright (C) 2004, 2007, 2009--2020 Free Software Foundation, Inc. |
| |
| @c Permission is granted to copy, distribute and/or modify this document |
| @c under the terms of the GNU Free Documentation License, Version 1.3 or |
| @c any later version published by the Free Software Foundation; with no |
| @c Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A |
| @c copy of the license is at <https://www.gnu.org/licenses/fdl-1.3.en.html>. |
| |
| The @code{alloca-opt} module provides for a function @code{alloca} which allocates |
| memory on the stack, where the system allows it. A memory block allocated with |
| @code{alloca} exists only until the function that calls @code{alloca} returns |
| or exits abruptly. |
| |
| There are a few systems where this is not possible: HP-UX systems, and some |
| other platforms when the C++ compiler is used. On these platforms the |
| @code{alloca-opt} module provides no replacement, just a preprocessor macro |
| HAVE_ALLOCA. |
| |
| The user can @code{#include <alloca.h>} on all platforms, and use |
| @code{alloca} on those platforms where the preprocessor macro HAVE_ALLOCA |
| evaluates to true. If HAVE_ALLOCA is false, the code should use a heap-based |
| memory allocation based on @code{malloc} or (in C++) @code{new}. Note that |
| the @code{#include <alloca.h>} must be the first one after the |
| autoconf-generated @file{config.h}, for AIX 3 compatibility. Thanks to IBM for |
| this nice restriction! |
| |
| Note that GCC 3.1 and 3.2 can @emph{inline} functions that call @code{alloca}. |
| When this happens, the memory blocks allocated with @code{alloca} will not be |
| freed until @emph{the end of the calling function}. If this calling function |
| runs a loop calling the function that uses @code{alloca}, the program easily |
| gets a stack overflow and crashes. To protect against this compiler behaviour, |
| you can mark the function that uses @code{alloca} with the following attribute: |
| |
| @smallexample |
| #ifdef __GNUC__ |
| __attribute__ ((__noinline__)) |
| #endif |
| @end smallexample |