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+/* alloca.c -- allocate automatically reclaimed memory
+ This file is in the public domain. */
+
+/* (Mostly) portable implementation -- D A Gwyn
+
+ This implementation of the PWB library alloca function,
+ which is used to allocate space off the run-time stack so
+ that it is automatically reclaimed upon procedure exit,
+ was inspired by discussions with J. Q. Johnson of Cornell.
+ J.Otto Tennant <jot@cray.com> contributed the Cray support.
+
+ There are some preprocessor constants that can
+ be defined when compiling for your specific system, for
+ improved efficiency; however, the defaults should be okay.
+
+ The general concept of this implementation is to keep
+ track of all alloca-allocated blocks, and reclaim any
+ that are found to be deeper in the stack than the current
+ invocation. This heuristic does not reclaim storage as
+ soon as it becomes invalid, but it will do so eventually.
+
+ As a special case, alloca(0) reclaims storage without
+ allocating any. It is a good idea to use alloca(0) in
+ your main control loop, etc. to force garbage collection. */
+
+#include <config.h>
+
+#include <alloca.h>
+
+#include <string.h>
+#include <stdlib.h>
+
+#ifdef emacs
+# include "lisp.h"
+# include "blockinput.h"
+# ifdef EMACS_FREE
+# undef free
+# define free EMACS_FREE
+# endif
+#else
+# define memory_full() abort ()
+#endif
+
+/* If compiling with GCC or clang, this file is not needed. */
+#if !(defined __GNUC__ || defined __clang__)
+
+/* If someone has defined alloca as a macro,
+ there must be some other way alloca is supposed to work. */
+# ifndef alloca
+
+# ifdef emacs
+# ifdef static
+/* actually, only want this if static is defined as ""
+ -- this is for usg, in which emacs must undefine static
+ in order to make unexec workable
+ */
+# ifndef STACK_DIRECTION
+you
+lose
+-- must know STACK_DIRECTION at compile-time
+/* Using #error here is not wise since this file should work for
+ old and obscure compilers. */
+# endif /* STACK_DIRECTION undefined */
+# endif /* static */
+# endif /* emacs */
+
+/* Define STACK_DIRECTION if you know the direction of stack
+ growth for your system; otherwise it will be automatically
+ deduced at run-time.
+
+ STACK_DIRECTION > 0 => grows toward higher addresses
+ STACK_DIRECTION < 0 => grows toward lower addresses
+ STACK_DIRECTION = 0 => direction of growth unknown */
+
+# ifndef STACK_DIRECTION
+# define STACK_DIRECTION 0 /* Direction unknown. */
+# endif
+
+# if STACK_DIRECTION != 0
+
+# define STACK_DIR STACK_DIRECTION /* Known at compile-time. */
+
+# else /* STACK_DIRECTION == 0; need run-time code. */
+
+static int stack_dir; /* 1 or -1 once known. */
+# define STACK_DIR stack_dir
+
+static int
+find_stack_direction (int *addr, int depth)
+{
+ int dir, dummy = 0;
+ if (! addr)
+ addr = &dummy;
+ *addr = addr < &dummy ? 1 : addr == &dummy ? 0 : -1;
+ dir = depth ? find_stack_direction (addr, depth - 1) : 0;
+ return dir + dummy;
+}
+
+# endif /* STACK_DIRECTION == 0 */
+
+/* An "alloca header" is used to:
+ (a) chain together all alloca'ed blocks;
+ (b) keep track of stack depth.
+
+ It is very important that sizeof(header) agree with malloc
+ alignment chunk size. The following default should work okay. */
+
+# ifndef ALIGN_SIZE
+# define ALIGN_SIZE sizeof(double)
+# endif
+
+typedef union hdr
+{
+ char align[ALIGN_SIZE]; /* To force sizeof(header). */
+ struct
+ {
+ union hdr *next; /* For chaining headers. */
+ char *deep; /* For stack depth measure. */
+ } h;
+} header;
+
+static header *last_alloca_header = NULL; /* -> last alloca header. */
+
+/* Return a pointer to at least SIZE bytes of storage,
+ which will be automatically reclaimed upon exit from
+ the procedure that called alloca. Originally, this space
+ was supposed to be taken from the current stack frame of the
+ caller, but that method cannot be made to work for some
+ implementations of C, for example under Gould's UTX/32. */
+
+void *
+alloca (size_t size)
+{
+ auto char probe; /* Probes stack depth: */
+ register char *depth = &probe;
+
+# if STACK_DIRECTION == 0
+ if (STACK_DIR == 0) /* Unknown growth direction. */
+ STACK_DIR = find_stack_direction (NULL, (size & 1) + 20);
+# endif
+
+ /* Reclaim garbage, defined as all alloca'd storage that
+ was allocated from deeper in the stack than currently. */
+
+ {
+ register header *hp; /* Traverses linked list. */
+
+# ifdef emacs
+ BLOCK_INPUT;
+# endif
+
+ for (hp = last_alloca_header; hp != NULL;)
+ if ((STACK_DIR > 0 && hp->h.deep > depth)
+ || (STACK_DIR < 0 && hp->h.deep < depth))
+ {
+ register header *np = hp->h.next;
+
+ free (hp); /* Collect garbage. */
+
+ hp = np; /* -> next header. */
+ }
+ else
+ break; /* Rest are not deeper. */
+
+ last_alloca_header = hp; /* -> last valid storage. */
+
+# ifdef emacs
+ UNBLOCK_INPUT;
+# endif
+ }
+
+ if (size == 0)
+ return NULL; /* No allocation required. */
+
+ /* Allocate combined header + user data storage. */
+
+ {
+ /* Address of header. */
+ register header *new;
+
+ size_t combined_size = sizeof (header) + size;
+ if (combined_size < sizeof (header))
+ memory_full ();
+
+ new = malloc (combined_size);
+
+ if (! new)
+ memory_full ();
+
+ new->h.next = last_alloca_header;
+ new->h.deep = depth;
+
+ last_alloca_header = new;
+
+ /* User storage begins just after header. */
+
+ return (void *) (new + 1);
+ }
+}
+
+# endif /* no alloca */
+#endif /* not GCC || clang */