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Diffstat (limited to 'lib/malloca.c')
| -rw-r--r-- | lib/malloca.c | 137 |
1 files changed, 137 insertions, 0 deletions
diff --git a/lib/malloca.c b/lib/malloca.c new file mode 100644 index 00000000..7905e615 --- /dev/null +++ b/lib/malloca.c @@ -0,0 +1,137 @@ +/* Safe automatic memory allocation. + Copyright (C) 2003, 2006-2007 Free Software Foundation, Inc. + Written by Bruno Haible <bruno@clisp.org>, 2003. + + This program is free software; you can redistribute it and/or modify + it under the terms of the GNU Lesser General Public License as published by + the Free Software Foundation; either version 2, or (at your option) + any later version. + + This program is distributed in the hope that it will be useful, + but WITHOUT ANY WARRANTY; without even the implied warranty of + MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the + GNU Lesser General Public License for more details. + + You should have received a copy of the GNU Lesser General Public License + along with this program; if not, write to the Free Software Foundation, + Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301, USA. */ + +#include <config.h> + +/* Specification. */ +#include "malloca.h" + +/* The speed critical point in this file is freea() applied to an alloca() + result: it must be fast, to match the speed of alloca(). The speed of + mmalloca() and freea() in the other case are not critical, because they + are only invoked for big memory sizes. */ + +#if HAVE_ALLOCA + +/* Store the mmalloca() results in a hash table. This is needed to reliably + distinguish a mmalloca() result and an alloca() result. + + Although it is possible that the same pointer is returned by alloca() and + by mmalloca() at different times in the same application, it does not lead + to a bug in freea(), because: + - Before a pointer returned by alloca() can point into malloc()ed memory, + the function must return, and once this has happened the programmer must + not call freea() on it anyway. + - Before a pointer returned by mmalloca() can point into the stack, it + must be freed. The only function that can free it is freea(), and + when freea() frees it, it also removes it from the hash table. */ + +#define MAGIC_NUMBER 0x1415fb4a +#define MAGIC_SIZE sizeof (int) +/* This is how the header info would look like without any alignment + considerations. */ +struct preliminary_header { void *next; char room[MAGIC_SIZE]; }; +/* But the header's size must be a multiple of sa_alignment_max. */ +#define HEADER_SIZE \ + (((sizeof (struct preliminary_header) + sa_alignment_max - 1) / sa_alignment_max) * sa_alignment_max) +struct header { void *next; char room[HEADER_SIZE - sizeof (struct preliminary_header) + MAGIC_SIZE]; }; +/* Verify that HEADER_SIZE == sizeof (struct header). */ +typedef int verify1[2 * (HEADER_SIZE == sizeof (struct header)) - 1]; +/* We make the hash table quite big, so that during lookups the probability + of empty hash buckets is quite high. There is no need to make the hash + table resizable, because when the hash table gets filled so much that the + lookup becomes slow, it means that the application has memory leaks. */ +#define HASH_TABLE_SIZE 257 +static void * mmalloca_results[HASH_TABLE_SIZE]; + +#endif + +void * +mmalloca (size_t n) +{ +#if HAVE_ALLOCA + /* Allocate one more word, that serves as an indicator for malloc()ed + memory, so that freea() of an alloca() result is fast. */ + size_t nplus = n + HEADER_SIZE; + + if (nplus >= n) + { + char *p = (char *) malloc (nplus); + + if (p != NULL) + { + size_t slot; + + p += HEADER_SIZE; + + /* Put a magic number into the indicator word. */ + ((int *) p)[-1] = MAGIC_NUMBER; + + /* Enter p into the hash table. */ + slot = (unsigned long) p % HASH_TABLE_SIZE; + ((struct header *) (p - HEADER_SIZE))->next = mmalloca_results[slot]; + mmalloca_results[slot] = p; + + return p; + } + } + /* Out of memory. */ + return NULL; +#else +# if !MALLOC_0_IS_NONNULL + if (n == 0) + n = 1; +# endif + return malloc (n); +#endif +} + +#if HAVE_ALLOCA +void +freea (void *p) +{ + /* mmalloca() may have returned NULL. */ + if (p != NULL) + { + /* Attempt to quickly distinguish the mmalloca() result - which has + a magic indicator word - and the alloca() result - which has an + uninitialized indicator word. It is for this test that sa_increment + additional bytes are allocated in the alloca() case. */ + if (((int *) p)[-1] == MAGIC_NUMBER) + { + /* Looks like a mmalloca() result. To see whether it really is one, + perform a lookup in the hash table. */ + size_t slot = (unsigned long) p % HASH_TABLE_SIZE; + void **chain = &mmalloca_results[slot]; + for (; *chain != NULL;) + { + if (*chain == p) + { + /* Found it. Remove it from the hash table and free it. */ + char *p_begin = (char *) p - HEADER_SIZE; + *chain = ((struct header *) p_begin)->next; + free (p_begin); + return; + } + chain = &((struct header *) ((char *) *chain - HEADER_SIZE))->next; + } + } + /* At this point, we know it was not a mmalloca() result. */ + } +} +#endif |