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/* Helper code for POSIX semaphore implementation.
Copyright (C) 2021-2025 Free Software Foundation, Inc.
This file is part of the GNU C Library.
The GNU C Library 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 2.1 of the License, or (at your option) any later version.
The GNU C Library 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 the GNU C Library; if not, see
<https://www.gnu.org/licenses/>. */
#include <search.h>
#include <semaphoreP.h>
#include <sys/mman.h>
#include <sem_routines.h>
/* Keeping track of currently used mappings. */
struct inuse_sem
{
dev_t dev;
ino64_t ino;
int refcnt;
sem_t *sem;
char name[];
};
struct search_sem
{
dev_t dev;
ino64_t ino;
int refcnt;
sem_t *sem;
char name[NAME_MAX + 1];
};
/* Comparison function for search of existing mapping. */
static int
sem_search (const void *a, const void *b)
{
const struct inuse_sem *as = (const struct inuse_sem *) a;
const struct inuse_sem *bs = (const struct inuse_sem *) b;
if (as->ino != bs->ino)
/* Cannot return the difference the type is larger than int. */
return as->ino < bs->ino ? -1 : (as->ino == bs->ino ? 0 : 1);
if (as->dev != bs->dev)
/* Cannot return the difference the type is larger than int. */
return as->dev < bs->dev ? -1 : (as->dev == bs->dev ? 0 : 1);
return strcmp (as->name, bs->name);
}
/* The search tree for existing mappings. */
static void *sem_mappings;
/* Lock to protect the search tree. */
static int sem_mappings_lock = LLL_LOCK_INITIALIZER;
/* Search for existing mapping and if possible add the one provided. */
sem_t *
__sem_check_add_mapping (const char *name, int fd, sem_t *existing)
{
size_t namelen = strlen (name);
if (namelen > NAME_MAX)
return SEM_FAILED;
namelen += 1;
sem_t *result = SEM_FAILED;
/* Get the information about the file. */
struct __stat64_t64 st;
if (__fstat64_time64 (fd, &st) == 0)
{
/* Get the lock. */
lll_lock (sem_mappings_lock, LLL_PRIVATE);
/* Search for an existing mapping given the information we have. */
struct search_sem fake;
memcpy (fake.name, name, namelen);
fake.dev = st.st_dev;
fake.ino = st.st_ino;
struct inuse_sem **foundp = __tfind (&fake, &sem_mappings, sem_search);
if (foundp != NULL)
{
/* There is already a mapping. Use it. */
result = (*foundp)->sem;
++(*foundp)->refcnt;
}
else
{
/* We haven't found a mapping. Install ione. */
struct inuse_sem *newp;
newp = (struct inuse_sem *) malloc (sizeof (*newp) + namelen);
if (newp != NULL)
{
/* If the caller hasn't provided any map it now. */
if (existing == SEM_FAILED)
existing = (sem_t *) __mmap (NULL, sizeof (sem_t),
PROT_READ | PROT_WRITE,
MAP_SHARED, fd, 0);
newp->dev = st.st_dev;
newp->ino = st.st_ino;
newp->refcnt = 1;
newp->sem = existing;
memcpy (newp->name, name, namelen);
/* Insert the new value. */
if (existing != MAP_FAILED
&& __tsearch (newp, &sem_mappings, sem_search) != NULL)
/* Successful. */
result = existing;
else
/* Something went wrong while inserting the new
value. We fail completely. */
free (newp);
}
}
/* Release the lock. */
lll_unlock (sem_mappings_lock, LLL_PRIVATE);
}
if (result != existing && existing != SEM_FAILED && existing != MAP_FAILED)
{
/* Do not disturb errno. */
int save = errno;
__munmap (existing, sizeof (sem_t));
errno = save;
}
return result;
}
struct walk_closure
{
sem_t *the_sem;
struct inuse_sem *rec;
};
static void
walker (const void *inodep, VISIT which, void *closure0)
{
struct walk_closure *closure = closure0;
struct inuse_sem *nodep = *(struct inuse_sem **) inodep;
if (nodep->sem == closure->the_sem)
closure->rec = nodep;
}
bool
__sem_remove_mapping (sem_t *sem)
{
bool ret = true;
/* Get the lock. */
lll_lock (sem_mappings_lock, LLL_PRIVATE);
/* Locate the entry for the mapping the caller provided. */
struct inuse_sem *rec;
{
struct walk_closure closure = { .the_sem = sem, .rec = NULL };
__twalk_r (sem_mappings, walker, &closure);
rec = closure.rec;
}
if (rec != NULL)
{
/* Check the reference counter. If it is going to be zero, free
all the resources. */
if (--rec->refcnt == 0)
{
/* Remove the record from the tree. */
__tdelete (rec, &sem_mappings, sem_search);
if (__munmap (rec->sem, sizeof (sem_t)) == -1)
ret = false;
free (rec);
}
}
else
ret = false;
/* Release the lock. */
lll_unlock (sem_mappings_lock, LLL_PRIVATE);
return ret;
}
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