17.2 alloca-opt

The alloca-opt module provides for a function alloca which allocates memory on the stack, where the system allows it. A memory block allocated with alloca exists only until the function that calls 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-opt module provides no replacement, just a preprocessor macro HAVE_ALLOCA.

The user can #include <alloca.h> on all platforms, and use 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 malloc or (in C++) new. Note that the #include <alloca.h> must be the first one after the autoconf-generated config.h, for AIX 3 compatibility. Thanks to IBM for this nice restriction!

Note that GCC 3.1 and 3.2 can inline functions that call alloca. When this happens, the memory blocks allocated with alloca will not be freed until the end of the calling function. If this calling function runs a loop calling the function that uses alloca, the program easily gets a stack overflow and crashes. To protect against this compiler behaviour, you can mark the function that uses alloca with the following attribute:

#ifdef __GNUC__
__attribute__ ((__noinline__))
#endif