summaryrefslogtreecommitdiff
path: root/sysdeps/unix/sysv/linux/shm-directory.c
blob: 43f044d77c31b0f40d16b09a8701d922b228cc1a (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
/* Determine directory for shm/sem files.  Linux version.
   Copyright (C) 2000-2020 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 "shm-directory.h"

#include <errno.h>
#include <mntent.h>
#include <paths.h>
#include <stdio.h>
#include <string.h>
#include <sys/statfs.h>
#include <libc-lock.h>
#include "linux_fsinfo.h"


/* Mount point of the shared memory filesystem.  */
static struct
{
  char *dir;
  size_t dirlen;
} mountpoint;

/* This is the default directory.  */
static const char defaultdir[] = "/dev/shm/";

/* Protect the `mountpoint' variable above.  */
__libc_once_define (static, once);


/* Determine where the shmfs is mounted (if at all).  */
static void
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 (defaultdir, &f) == 0 && (f.f_type == SHMFS_SUPER_MAGIC
                                         || f.f_type == RAMFS_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 (__glibc_unlikely (fp == NULL))
    {
      fp = __setmntent (_PATH_MNTTAB, "r");
      if (__glibc_unlikely (fp == NULL))
        /* 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
                                                && f.f_type != RAMFS_MAGIC))
          continue;

        namelen = strlen (mp->mnt_dir);

        if (namelen == 0)
          /* Hum, maybe some crippled entry.  Keep on searching.  */
          continue;

        mountpoint.dir = (char *) malloc (namelen + 2);
        if (mountpoint.dir != NULL)
          {
            char *cp = __mempcpy (mountpoint.dir, mp->mnt_dir, namelen);
            if (cp[-1] != '/')
              *cp++ = '/';
            *cp = '\0';
            mountpoint.dirlen = cp - mountpoint.dir;
          }

        break;
      }

  /* Close the stream.  */
  __endmntent (fp);
}


const char *
__shm_directory (size_t *len)
{
  /* Determine where the shmfs is mounted.  */
  __libc_once (once, where_is_shmfs);

  /* If we don't know the mount points there is nothing we can do.  Ever.  */
  if (__glibc_unlikely (mountpoint.dir == NULL))
    {
      __set_errno (ENOSYS);
      return NULL;
    }

  *len = mountpoint.dirlen;
  return mountpoint.dir;
}
#if IS_IN (libpthread)
hidden_def (__shm_directory)

/* Make sure the table is freed if we want to free everything before
   exiting.  */
void
__shm_directory_freeres (void)
{
  if (mountpoint.dir != defaultdir)
    free (mountpoint.dir);
}
#endif