1SHMCTL(2)                  Linux Programmer's Manual                 SHMCTL(2)
2
3
4

NAME

6       shmctl - shared memory control
7

SYNOPSIS

9       #include <sys/ipc.h>
10       #include <sys/shm.h>
11
12       int shmctl(int shmid, int cmd, struct shmid_ds *buf);
13

DESCRIPTION

15       shmctl()  performs the control operation specified by cmd on the shared
16       memory segment whose identifier is given in shmid.
17
18       The buf argument is a pointer  to  a  shmid_ds  structure,  defined  in
19       <sys/shm.h> as follows:
20
21           struct shmid_ds {
22               struct ipc_perm shm_perm;    /* Ownership and permissions */
23               size_t          shm_segsz;   /* Size of segment (bytes) */
24               time_t          shm_atime;   /* Last attach time */
25               time_t          shm_dtime;   /* Last detach time */
26               time_t          shm_ctime;   /* Last change time */
27               pid_t           shm_cpid;    /* PID of creator */
28               pid_t           shm_lpid;    /* PID of last shmat(2)/shmdt(2) */
29               shmatt_t        shm_nattch;  /* No. of current attaches */
30               ...
31           };
32
33       The  ipc_perm structure is defined in <sys/ipc.h> as follows (the high‐
34       lighted fields are settable using IPC_SET):
35
36           struct ipc_perm {
37               key_t          __key;    /* Key supplied to shmget(2) */
38               uid_t          uid;      /* Effective UID of owner */
39               gid_t          gid;      /* Effective GID of owner */
40               uid_t          cuid;     /* Effective UID of creator */
41               gid_t          cgid;     /* Effective GID of creator */
42               unsigned short mode;     /* Permissions + SHM_DEST and
43                                           SHM_LOCKED flags */
44               unsigned short __seq;    /* Sequence number */
45           };
46
47       Valid values for cmd are:
48
49       IPC_STAT  Copy information from the kernel  data  structure  associated
50                 with  shmid  into  the  shmid_ds structure pointed to by buf.
51                 The caller must have read permission  on  the  shared  memory
52                 segment.
53
54       IPC_SET   Write  the  values  of some members of the shmid_ds structure
55                 pointed to by buf to the  kernel  data  structure  associated
56                 with  this shared memory segment, updating also its shm_ctime
57                 member.  The following fields can be  changed:  shm_perm.uid,
58                 shm_perm.gid,   and   (the   least  significant  9  bits  of)
59                 shm_perm.mode.  The effective UID of the calling process must
60                 match  the owner (shm_perm.uid) or creator (shm_perm.cuid) of
61                 the shared memory segment, or the caller must be privileged.
62
63       IPC_RMID  Mark the segment to be  destroyed.   The  segment  will  only
64                 actually  be  destroyed  after  the  last process detaches it
65                 (i.e., when the shm_nattch member of the associated structure
66                 shmid_ds  is zero).  The caller must be the owner or creator,
67                 or be privileged.  If a segment has been marked for  destruc‐
68                 tion,   then   the   (nonstandard)   SHM_DEST   flag  of  the
69                 shm_perm.mode  field  in  the   associated   data   structure
70                 retrieved by IPC_STAT will be set.
71
72       The  caller  must ensure that a segment is eventually destroyed; other‐
73       wise its pages that were faulted in will remain in memory or swap.
74
75       IPC_INFO (Linux-specific)
76                 Returns information about system-wide  shared  memory  limits
77                 and  parameters  in  the  structure  pointed to by buf.  This
78                 structure is of type shminfo  (thus,  a  cast  is  required),
79                 defined  in <sys/shm.h> if the _GNU_SOURCE feature test macro
80                 is defined:
81
82                     struct  shminfo {
83                         unsigned long shmmax; /* Maximum segment size */
84                         unsigned long shmmin; /* Minimum segment size;
85                                                  always 1 */
86                         unsigned long shmmni; /* Maximum number of segments */
87                         unsigned long shmseg; /* Maximum number of segments
88                                                  that a process can attach;
89                                                  unused within kernel */
90                         unsigned long shmall; /* Maximum number of pages of
91                                                  shared memory, system-wide */
92                     };
93
94                 The shmmni, shmmax, and shmall settings can  be  changed  via
95                 /proc files of the same name; see proc(5) for details.
96
97       SHM_INFO (Linux-specific)
98                 Returns a shm_info structure whose fields contain information
99                 about system  resources  consumed  by  shared  memory.   This
100                 structure  is  defined in <sys/shm.h> if the _GNU_SOURCE fea‐
101                 ture test macro is defined:
102
103                     struct shm_info {
104                         int           used_ids; /* # of currently existing
105                                                    segments */
106                         unsigned long shm_tot;  /* Total number of shared
107                                                    memory pages */
108                         unsigned long shm_rss;  /* # of resident shared
109                                                    memory pages */
110                         unsigned long shm_swp;  /* # of swapped shared
111                                                    memory pages */
112                         unsigned long swap_attempts;
113                                                 /* Unused since Linux 2.4 */
114                         unsigned long swap_successes;
115                                                 /* Unused since Linux 2.4 */
116                     };
117
118       SHM_STAT (Linux-specific)
119                 Returns a shmid_ds structure as for IPC_STAT.   However,  the
120                 shmid  argument  is  not a segment identifier, but instead an
121                 index into the kernel's internal array that maintains  infor‐
122                 mation about all shared memory segments on the system.
123
124       The  caller  can  prevent  or allow swapping of a shared memory segment
125       with the following cmd values:
126
127       SHM_LOCK (Linux-specific)
128                 Prevent swapping of the shared memory  segment.   The  caller
129                 must fault in any pages that are required to be present after
130                 locking is enabled.  If a segment has been locked,  then  the
131                 (nonstandard)  SHM_LOCKED  flag of the shm_perm.mode field in
132                 the associated data structure retrieved by IPC_STAT  will  be
133                 set.
134
135       SHM_UNLOCK (Linux-specific)
136                 Unlock the segment, allowing it to be swapped out.
137
138       In  kernels  before  2.6.10,  only  a  privileged  process could employ
139       SHM_LOCK and SHM_UNLOCK.  Since kernel 2.6.10, an unprivileged  process
140       can  employ  these operations if its effective UID matches the owner or
141       creator UID of the segment, and (for SHM_LOCK) the amount of memory  to
142       be  locked  falls  within  the RLIMIT_MEMLOCK resource limit (see setr‐
143       limit(2)).
144

RETURN VALUE

146       A successful IPC_INFO or SHM_INFO operation returns the  index  of  the
147       highest used entry in the kernel's internal array recording information
148       about all shared memory segments.  (This information can be  used  with
149       repeated  SHM_STAT  operations  to  obtain information about all shared
150       memory segments  on  the  system.)   A  successful  SHM_STAT  operation
151       returns  the  identifier  of  the shared memory segment whose index was
152       given in shmid.  Other operations return 0 on success.
153
154       On error, -1 is returned, and errno is set appropriately.
155

ERRORS

157       EACCES IPC_STAT or SHM_STAT is requested  and  shm_perm.mode  does  not
158              allow  read  access  for shmid, and the calling process does not
159              have the CAP_IPC_OWNER capability.
160
161       EFAULT The argument cmd has value IPC_SET or IPC_STAT but  the  address
162              pointed to by buf isn't accessible.
163
164       EIDRM  shmid points to a removed identifier.
165
166       EINVAL shmid  is not a valid identifier, or cmd is not a valid command.
167              Or: for a SHM_STAT operation, the index value specified in shmid
168              referred to an array slot that is currently unused.
169
170       ENOMEM (In kernels since 2.6.9), SHM_LOCK was specified and the size of
171              the to-be-locked segment would mean  that  the  total  bytes  in
172              locked  shared  memory  segments  would exceed the limit for the
173              real user ID of the calling process.  This limit is  defined  by
174              the RLIMIT_MEMLOCK soft resource limit (see setrlimit(2)).
175
176       EOVERFLOW
177              IPC_STAT  is attempted, and the GID or UID value is too large to
178              be stored in the structure pointed to by buf.
179
180       EPERM  IPC_SET or IPC_RMID is attempted, and the effective user  ID  of
181              the  calling  process  is  not  that  of  the  creator (found in
182              shm_perm.cuid), or the owner (found in  shm_perm.uid),  and  the
183              process   was   not   privileged   (Linux:   did  not  have  the
184              CAP_SYS_ADMIN capability).
185
186              Or (in kernels before 2.6.9), SHM_LOCK or SHM_UNLOCK was  speci‐
187              fied,  but  the  process was not privileged (Linux: did not have
188              the CAP_IPC_LOCK capability).  (Since Linux  2.6.9,  this  error
189              can  also occur if the RLIMIT_MEMLOCK is 0 and the caller is not
190              privileged.)
191

CONFORMING TO

193       SVr4, POSIX.1-2001.
194

NOTES

196       The IPC_INFO, SHM_STAT and SHM_INFO operations are used by the  ipcs(1)
197       program  to  provide information on allocated resources.  In the future
198       these may modified or moved to a /proc file system interface.
199
200       Linux permits a process to attach (shmat(2)) a  shared  memory  segment
201       that has already been marked for deletion using shmctl(IPC_RMID).  This
202       feature is not available on other Unix implementations; portable appli‐
203       cations should avoid relying on it.
204
205       Various fields in a struct shmid_ds were typed as short under Linux 2.2
206       and have become long under Linux 2.4.  To take  advantage  of  this,  a
207       recompilation  under glibc-2.1.91 or later should suffice.  (The kernel
208       distinguishes old and new calls by an IPC_64 flag in cmd.)
209

SEE ALSO

211       mlock(2), setrlimit(2), shmget(2), shmop(2), capabilities(7), svipc(7)
212

COLOPHON

214       This page is part of release 3.25 of the Linux  man-pages  project.   A
215       description  of  the project, and information about reporting bugs, can
216       be found at http://www.kernel.org/doc/man-pages/.
217
218
219
220Linux                             2008-08-07                         SHMCTL(2)
Impressum