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glibc/nptl/sem_open.c

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/* Copyright (C) 2002, 2003, 2006, 2007 Free Software Foundation, Inc.
This file is part of the GNU C Library.
Contributed by Ulrich Drepper <drepper@redhat.com>, 2002.
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, write to the Free
Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
02111-1307 USA. */
#include <errno.h>
#include <fcntl.h>
#include <mntent.h>
#include <paths.h>
#include <pthread.h>
#include <search.h>
#include <semaphore.h>
#include <stdarg.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <sys/statfs.h>
#include <linux_fsinfo.h>
#include "semaphoreP.h"
/* Information about the mount point. */
struct mountpoint_info mountpoint attribute_hidden;
/* This is the default mount point. */
static const char defaultmount[] = "/dev/shm";
/* This is the default directory. */
static const char defaultdir[] = "/dev/shm/sem.";
/* Protect the `mountpoint' variable above. */
pthread_once_t __namedsem_once attribute_hidden = PTHREAD_ONCE_INIT;
/* Determine where the shmfs is mounted (if at all). */
void
attribute_hidden
__where_is_shmfs (void)
{
char buf[512];
struct statfs f;
struct mntent resmem;
struct mntent *mp;
FILE *fp;
/* The canonical place is /dev/shm. This is at least what the
documentation tells everybody to do. */
if (__statfs (defaultmount, &f) == 0 && f.f_type == SHMFS_SUPER_MAGIC)
{
/* It is in the normal place. */
mountpoint.dir = (char *) defaultdir;
mountpoint.dirlen = sizeof (defaultdir) - 1;
return;
}
/* OK, do it the hard way. Look through the /proc/mounts file and if
this does not exist through /etc/fstab to find the mount point. */
fp = __setmntent ("/proc/mounts", "r");
if (__builtin_expect (fp == NULL, 0))
{
fp = __setmntent (_PATH_MNTTAB, "r");
if (__builtin_expect (fp == NULL, 0))
/* There is nothing we can do. Blind guesses are not helpful. */
return;
}
/* Now read the entries. */
while ((mp = __getmntent_r (fp, &resmem, buf, sizeof buf)) != NULL)
/* The original name is "shm" but this got changed in early Linux
2.4.x to "tmpfs". */
if (strcmp (mp->mnt_type, "tmpfs") == 0
|| strcmp (mp->mnt_type, "shm") == 0)
{
/* Found it. There might be more than one place where the
filesystem is mounted but one is enough for us. */
size_t namelen;
/* First make sure this really is the correct entry. At least
some versions of the kernel give wrong information because
of the implicit mount of the shmfs for SysV IPC. */
if (__statfs (mp->mnt_dir, &f) != 0 || f.f_type != SHMFS_SUPER_MAGIC)
continue;
namelen = strlen (mp->mnt_dir);
if (namelen == 0)
/* Hum, maybe some crippled entry. Keep on searching. */
continue;
mountpoint.dir = (char *) malloc (namelen + 4 + 2);
if (mountpoint.dir != NULL)
{
char *cp = __mempcpy (mountpoint.dir, mp->mnt_dir, namelen);
if (cp[-1] != '/')
*cp++ = '/';
cp = stpcpy (cp, "sem.");
mountpoint.dirlen = cp - mountpoint.dir;
}
break;
}
/* Close the stream. */
__endmntent (fp);
}
/* Comparison function for search of existing mapping. */
int
attribute_hidden
__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. */
void *__sem_mappings attribute_hidden;
/* Lock to protect the search tree. */
int __sem_mappings_lock attribute_hidden = LLL_LOCK_INITIALIZER;
/* Search for existing mapping and if possible add the one provided. */
static sem_t *
check_add_mapping (const char *name, size_t namelen, int fd, sem_t *existing)
{
sem_t *result = SEM_FAILED;
/* Get the information about the file. */
struct stat64 st;
if (__fxstat64 (_STAT_VER, fd, &st) == 0)
{
/* Get the lock. */
lll_lock (__sem_mappings_lock, LLL_PRIVATE);
/* Search for an existing mapping given the information we have. */
struct inuse_sem *fake;
fake = (struct inuse_sem *) alloca (sizeof (*fake) + namelen);
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. */
INTERNAL_SYSCALL_DECL (err);
INTERNAL_SYSCALL (munmap, err, 2, existing, sizeof (sem_t));
}
return result;
}
sem_t *
sem_open (const char *name, int oflag, ...)
{
char *finalname;
sem_t *result = SEM_FAILED;
int fd;
/* Determine where the shmfs is mounted. */
INTUSE(__pthread_once) (&__namedsem_once, __where_is_shmfs);
/* If we don't know the mount points there is nothing we can do. Ever. */
if (mountpoint.dir == NULL)
{
__set_errno (ENOSYS);
return SEM_FAILED;
}
/* Construct the filename. */
while (name[0] == '/')
++name;
if (name[0] == '\0')
{
/* The name "/" is not supported. */
__set_errno (EINVAL);
return SEM_FAILED;
}
size_t namelen = strlen (name) + 1;
/* Create the name of the final file. */
finalname = (char *) alloca (mountpoint.dirlen + namelen);
__mempcpy (__mempcpy (finalname, mountpoint.dir, mountpoint.dirlen),
name, namelen);
/* If the semaphore object has to exist simply open it. */
if ((oflag & O_CREAT) == 0 || (oflag & O_EXCL) == 0)
{
try_again:
fd = __libc_open (finalname,
(oflag & ~(O_CREAT|O_ACCMODE)) | O_NOFOLLOW | O_RDWR);
if (fd == -1)
{
/* If we are supposed to create the file try this next. */
if ((oflag & O_CREAT) != 0 && errno == ENOENT)
goto try_create;
/* Return. errno is already set. */
}
else
/* Check whether we already have this semaphore mapped and
create one if necessary. */
result = check_add_mapping (name, namelen, fd, SEM_FAILED);
}
else
{
/* We have to open a temporary file first since it must have the
correct form before we can start using it. */
char *tmpfname;
mode_t mode;
unsigned int value;
va_list ap;
try_create:
va_start (ap, oflag);
mode = va_arg (ap, mode_t);
value = va_arg (ap, unsigned int);
va_end (ap);
if (value > SEM_VALUE_MAX)
{
__set_errno (EINVAL);
return SEM_FAILED;
}
/* Create the initial file content. */
sem_t initsem;
struct new_sem *iinitsem = (struct new_sem *) &initsem;
iinitsem->value = value;
iinitsem->private = 0;
iinitsem->nwaiters = 0;
/* Initialize the remaining bytes as well. */
memset ((char *) &initsem + sizeof (struct new_sem), '\0',
sizeof (sem_t) - sizeof (struct new_sem));
tmpfname = (char *) alloca (mountpoint.dirlen + 6 + 1);
char *xxxxxx = __mempcpy (tmpfname, mountpoint.dir, mountpoint.dirlen);
int retries = 0;
#define NRETRIES 50
while (1)
{
/* Add the suffix for mktemp. */
strcpy (xxxxxx, "XXXXXX");
/* We really want to use mktemp here. We cannot use mkstemp
since the file must be opened with a specific mode. The
mode cannot later be set since then we cannot apply the
file create mask. */
if (mktemp (tmpfname) == NULL)
return SEM_FAILED;
/* Open the file. Make sure we do not overwrite anything. */
fd = __libc_open (tmpfname, O_RDWR | O_CREAT | O_EXCL, mode);
if (fd == -1)
{
if (errno == EEXIST)
{
if (++retries < NRETRIES)
continue;
__set_errno (EAGAIN);
}
return SEM_FAILED;
}
/* We got a file. */
break;
}
if (TEMP_FAILURE_RETRY (__libc_write (fd, &initsem, sizeof (sem_t)))
== sizeof (sem_t)
/* Map the sem_t structure from the file. */
&& (result = (sem_t *) mmap (NULL, sizeof (sem_t),
PROT_READ | PROT_WRITE, MAP_SHARED,
fd, 0)) != MAP_FAILED)
{
/* Create the file. Don't overwrite an existing file. */
if (link (tmpfname, finalname) != 0)
{
/* Undo the mapping. */
(void) munmap (result, sizeof (sem_t));
/* Reinitialize 'result'. */
result = SEM_FAILED;
/* This failed. If O_EXCL is not set and the problem was
that the file exists, try again. */
if ((oflag & O_EXCL) == 0 && errno == EEXIST)
{
/* Remove the file. */
(void) unlink (tmpfname);
/* Close the file. */
(void) __libc_close (fd);
goto try_again;
}
}
else
/* Insert the mapping into the search tree. This also
determines whether another thread sneaked by and already
added such a mapping despite the fact that we created it. */
result = check_add_mapping (name, namelen, fd, result);
}
/* Now remove the temporary name. This should never fail. If
it fails we leak a file name. Better fix the kernel. */
(void) unlink (tmpfname);
}
/* Map the mmap error to the error we need. */
if (MAP_FAILED != (void *) SEM_FAILED && result == MAP_FAILED)
result = SEM_FAILED;
/* We don't need the file descriptor anymore. */
if (fd != -1)
{
/* Do not disturb errno. */
INTERNAL_SYSCALL_DECL (err);
INTERNAL_SYSCALL (close, err, 1, fd);
}
return result;
}
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