1SHMCTL(2) Linux Programmer's Manual SHMCTL(2)
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6 shmctl - System V shared memory control
7
9 #include <sys/ipc.h>
10 #include <sys/shm.h>
11 int shmctl(int shmid, int cmd, struct shmid_ds *buf);
13
15 shmctl() performs the control operation specified by cmd on the System
16 V shared memory segment whose identifier is given in shmid.
17 The buf argument is a pointer to a shmid_ds structure, defined in
19 <sys/shm.h> as follows:
20 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 The ipc_perm structure is defined as follows (the highlighted fields
34 are settable using IPC_SET):
35 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 Valid values for cmd are:
48 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 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 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 The caller must ensure that a segment is eventually
73 destroyed; otherwise its pages that were faulted in will
74 remain in memory or swap.
75 See also the description of /proc/sys/kernel/shm_rmid_forced
77 in proc(5).
78 IPC_INFO (Linux-specific)
80 Returns information about system-wide shared memory limits
81 and parameters in the structure pointed to by buf. This
82 structure is of type shminfo (thus, a cast is required),
83 defined in <sys/shm.h> if the _GNU_SOURCE feature test macro
84 is defined:
85 struct shminfo {
87 unsigned long shmmax; /* Maximum segment size */
88 unsigned long shmmin; /* Minimum segment size;
89 always 1 */
90 unsigned long shmmni; /* Maximum number of segments */
91 unsigned long shmseg; /* Maximum number of segments
92 that a process can attach;
93 unused within kernel */
94 unsigned long shmall; /* Maximum number of pages of
95 shared memory, system-wide */
96 };
97 The shmmni, shmmax, and shmall settings can be changed via
99 /proc files of the same name; see proc(5) for details.
100 SHM_INFO (Linux-specific)
102 Returns a shm_info structure whose fields contain information
103 about system resources consumed by shared memory. This
104 structure is defined in <sys/shm.h> if the _GNU_SOURCE fea‐
105 ture test macro is defined:
106 struct shm_info {
108 int used_ids; /* # of currently existing
109 segments */
110 unsigned long shm_tot; /* Total number of shared
111 memory pages */
112 unsigned long shm_rss; /* # of resident shared
113 memory pages */
114 unsigned long shm_swp; /* # of swapped shared
115 memory pages */
116 unsigned long swap_attempts;
117 /* Unused since Linux 2.4 */
118 unsigned long swap_successes;
119 /* Unused since Linux 2.4 */
120 };
121 SHM_STAT (Linux-specific)
123 Returns a shmid_ds structure as for IPC_STAT. However, the
124 shmid argument is not a segment identifier, but instead an
125 index into the kernel's internal array that maintains infor‐
126 mation about all shared memory segments on the system.
127 The caller can prevent or allow swapping of a shared memory segment
129 with the following cmd values:
130 SHM_LOCK (Linux-specific)
132 Prevent swapping of the shared memory segment. The caller
133 must fault in any pages that are required to be present after
134 locking is enabled. If a segment has been locked, then the
135 (nonstandard) SHM_LOCKED flag of the shm_perm.mode field in
136 the associated data structure retrieved by IPC_STAT will be
137 set.
138 SHM_UNLOCK (Linux-specific)
140 Unlock the segment, allowing it to be swapped out.
141 In kernels before 2.6.10, only a privileged process could employ
143 SHM_LOCK and SHM_UNLOCK. Since kernel 2.6.10, an unprivileged process
144 can employ these operations if its effective UID matches the owner or
145 creator UID of the segment, and (for SHM_LOCK) the amount of memory to
146 be locked falls within the RLIMIT_MEMLOCK resource limit (see setr‐
147 limit(2)).
148
150 A successful IPC_INFO or SHM_INFO operation returns the index of the
151 highest used entry in the kernel's internal array recording information
152 about all shared memory segments. (This information can be used with
153 repeated SHM_STAT operations to obtain information about all shared
154 memory segments on the system.) A successful SHM_STAT operation
155 returns the identifier of the shared memory segment whose index was
156 given in shmid. Other operations return 0 on success.
157 On error, -1 is returned, and errno is set appropriately.
159
161 EACCES IPC_STAT or SHM_STAT is requested and shm_perm.mode does not
162 allow read access for shmid, and the calling process does not
163 have the CAP_IPC_OWNER capability.
164 EFAULT The argument cmd has value IPC_SET or IPC_STAT but the address
166 pointed to by buf isn't accessible.
167 EIDRM shmid points to a removed identifier.
169 EINVAL shmid is not a valid identifier, or cmd is not a valid command.
171 Or: for a SHM_STAT operation, the index value specified in shmid
172 referred to an array slot that is currently unused.
173 ENOMEM (In kernels since 2.6.9), SHM_LOCK was specified and the size of
175 the to-be-locked segment would mean that the total bytes in
176 locked shared memory segments would exceed the limit for the
177 real user ID of the calling process. This limit is defined by
178 the RLIMIT_MEMLOCK soft resource limit (see setrlimit(2)).
179 EOVERFLOW
181 IPC_STAT is attempted, and the GID or UID value is too large to
182 be stored in the structure pointed to by buf.
183 EPERM IPC_SET or IPC_RMID is attempted, and the effective user ID of
185 the calling process is not that of the creator (found in
186 shm_perm.cuid), or the owner (found in shm_perm.uid), and the
187 process was not privileged (Linux: did not have the
188 CAP_SYS_ADMIN capability).
189 Or (in kernels before 2.6.9), SHM_LOCK or SHM_UNLOCK was speci‐
191 fied, but the process was not privileged (Linux: did not have
192 the CAP_IPC_LOCK capability). (Since Linux 2.6.9, this error
193 can also occur if the RLIMIT_MEMLOCK is 0 and the caller is not
194 privileged.)
195
197 SVr4, POSIX.1-2001.
198
200 The inclusion of <sys/types.h> and <sys/ipc.h> isn't required on Linux
201 or by any version of POSIX. However, some old implementations required
202 the inclusion of these header files, and the SVID also documented their
203 inclusion. Applications intended to be portable to such old systems
204 may need to include these header files.
205 The IPC_INFO, SHM_STAT and SHM_INFO operations are used by the ipcs(1)
207 program to provide information on allocated resources. In the future
208 these may modified or moved to a /proc file system interface.
209 Linux permits a process to attach (shmat(2)) a shared memory segment
211 that has already been marked for deletion using shmctl(IPC_RMID). This
212 feature is not available on other UNIX implementations; portable appli‐
213 cations should avoid relying on it.
214 Various fields in a struct shmid_ds were typed as short under Linux 2.2
216 and have become long under Linux 2.4. To take advantage of this, a
217 recompilation under glibc-2.1.91 or later should suffice. (The kernel
218 distinguishes old and new calls by an IPC_64 flag in cmd.)
219
221 mlock(2), setrlimit(2), shmget(2), shmop(2), capabilities(7), svipc(7)
222
224 This page is part of release 3.53 of the Linux man-pages project. A
225 description of the project, and information about reporting bugs, can
226 be found at http://www.kernel.org/doc/man-pages/.
227Linux 2013-03-18 SHMCTL(2)