| @c Documentation of gnulib module 'alloca'. | |
| @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 | |
| The @code{alloca} 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 alloca | |
| module provides a @code{malloc} based emulation. This emulation will not free a | |
| memory block immediately when the calling function returns, but rather will | |
| wait until the next @code{alloca} call from a function with the same or a | |
| shorter stack length. Thus, in some cases, a few memory blocks will be kept | |
| although they are not needed any more. | |
| The user can @code{#include <alloca.h>} and use @code{alloca} on all platforms. | |
| 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 | |
| @mindex alloca-opt | |
| An alternative to this module is the @samp{alloca-opt} module. |