doc/alloca-opt.texi

@c Documentation of gnulib module 'alloca-opt'.

@c Copyright (C) 2004, 2007, 2009--2026 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>.

@mindex alloca-opt
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