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 Sys‐
16 tem 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 actually
64 be destroyed only after the last process detaches it (i.e.,
65 when the shm_nattch member of the associated structure
66 shmid_ds is zero). The caller must be the owner or creator
67 of the segment, or be privileged. The buf argument is
68 ignored.
69 If a segment has been marked for destruction, then the (non‐
71 standard) SHM_DEST flag of the shm_perm.mode field in the
72 associated data structure retrieved by IPC_STAT will be set.
73 The caller must ensure that a segment is eventually
75 destroyed; otherwise its pages that were faulted in will
76 remain in memory or swap.
77 See also the description of /proc/sys/kernel/shm_rmid_forced
79 in proc(5).
80 IPC_INFO (Linux-specific)
82 Return information about system-wide shared memory limits and
83 parameters in the structure pointed to by buf. This struc‐
84 ture is of type shminfo (thus, a cast is required), defined
85 in <sys/shm.h> if the _GNU_SOURCE feature test macro is
86 defined:
87 struct shminfo {
89 unsigned long shmmax; /* Maximum segment size */
90 unsigned long shmmin; /* Minimum segment size;
91 always 1 */
92 unsigned long shmmni; /* Maximum number of segments */
93 unsigned long shmseg; /* Maximum number of segments
94 that a process can attach;
95 unused within kernel */
96 unsigned long shmall; /* Maximum number of pages of
97 shared memory, system-wide */
98 };
99 The shmmni, shmmax, and shmall settings can be changed via
101 /proc files of the same name; see proc(5) for details.
102 SHM_INFO (Linux-specific)
104 Return a shm_info structure whose fields contain information
105 about system resources consumed by shared memory. This
106 structure is defined in <sys/shm.h> if the _GNU_SOURCE fea‐
107 ture test macro is defined:
108 struct shm_info {
110 int used_ids; /* # of currently existing
111 segments */
112 unsigned long shm_tot; /* Total number of shared
113 memory pages */
114 unsigned long shm_rss; /* # of resident shared
115 memory pages */
116 unsigned long shm_swp; /* # of swapped shared
117 memory pages */
118 unsigned long swap_attempts;
119 /* Unused since Linux 2.4 */
120 unsigned long swap_successes;
121 /* Unused since Linux 2.4 */
122 };
123 SHM_STAT (Linux-specific)
125 Return a shmid_ds structure as for IPC_STAT. However, the
126 shmid argument is not a segment identifier, but instead an
127 index into the kernel's internal array that maintains infor‐
128 mation about all shared memory segments on the system.
129 The caller can prevent or allow swapping of a shared memory segment
131 with the following cmd values:
132 SHM_LOCK (Linux-specific)
134 Prevent swapping of the shared memory segment. The caller
135 must fault in any pages that are required to be present after
136 locking is enabled. If a segment has been locked, then the
137 (nonstandard) SHM_LOCKED flag of the shm_perm.mode field in
138 the associated data structure retrieved by IPC_STAT will be
139 set.
140 SHM_UNLOCK (Linux-specific)
142 Unlock the segment, allowing it to be swapped out.
143 In kernels before 2.6.10, only a privileged process could employ
145 SHM_LOCK and SHM_UNLOCK. Since kernel 2.6.10, an unprivileged process
146 can employ these operations if its effective UID matches the owner or
147 creator UID of the segment, and (for SHM_LOCK) the amount of memory to
148 be locked falls within the RLIMIT_MEMLOCK resource limit (see setr‐
149 limit(2)).
150
152 A successful IPC_INFO or SHM_INFO operation returns the index of the
153 highest used entry in the kernel's internal array recording information
154 about all shared memory segments. (This information can be used with
155 repeated SHM_STAT operations to obtain information about all shared
156 memory segments on the system.) A successful SHM_STAT operation
157 returns the identifier of the shared memory segment whose index was
158 given in shmid. Other operations return 0 on success.
159 On error, -1 is returned, and errno is set appropriately.
161
163 EACCES IPC_STAT or SHM_STAT is requested and shm_perm.mode does not
164 allow read access for shmid, and the calling process does not
165 have the CAP_IPC_OWNER capability in the user namespace that
166 governs its IPC namespace.
167 EFAULT The argument cmd has value IPC_SET or IPC_STAT but the address
169 pointed to by buf isn't accessible.
170 EIDRM shmid points to a removed identifier.
172 EINVAL shmid is not a valid identifier, or cmd is not a valid command.
174 Or: for a SHM_STAT operation, the index value specified in shmid
175 referred to an array slot that is currently unused.
176 ENOMEM (In kernels since 2.6.9), SHM_LOCK was specified and the size of
178 the to-be-locked segment would mean that the total bytes in
179 locked shared memory segments would exceed the limit for the
180 real user ID of the calling process. This limit is defined by
181 the RLIMIT_MEMLOCK soft resource limit (see setrlimit(2)).
182 EOVERFLOW
184 IPC_STAT is attempted, and the GID or UID value is too large to
185 be stored in the structure pointed to by buf.
186 EPERM IPC_SET or IPC_RMID is attempted, and the effective user ID of
188 the calling process is not that of the creator (found in
189 shm_perm.cuid), or the owner (found in shm_perm.uid), and the
190 process was not privileged (Linux: did not have the
191 CAP_SYS_ADMIN capability).
192 Or (in kernels before 2.6.9), SHM_LOCK or SHM_UNLOCK was speci‐
194 fied, but the process was not privileged (Linux: did not have
195 the CAP_IPC_LOCK capability). (Since Linux 2.6.9, this error
196 can also occur if the RLIMIT_MEMLOCK is 0 and the caller is not
197 privileged.)
198
200 POSIX.1-2001, POSIX.1-2008, SVr4.
201
203 The inclusion of <sys/types.h> and <sys/ipc.h> isn't required on Linux
204 or by any version of POSIX. However, some old implementations required
205 the inclusion of these header files, and the SVID also documented their
206 inclusion. Applications intended to be portable to such old systems
207 may need to include these header files.
208 The IPC_INFO, SHM_STAT and SHM_INFO operations are used by the ipcs(1)
210 program to provide information on allocated resources. In the future,
211 these may modified or moved to a /proc filesystem interface.
212 Linux permits a process to attach (shmat(2)) a shared memory segment
214 that has already been marked for deletion using shmctl(IPC_RMID). This
215 feature is not available on other UNIX implementations; portable appli‐
216 cations should avoid relying on it.
217 Various fields in a struct shmid_ds were typed as short under Linux 2.2
219 and have become long under Linux 2.4. To take advantage of this, a
220 recompilation under glibc-2.1.91 or later should suffice. (The kernel
221 distinguishes old and new calls by an IPC_64 flag in cmd.)
222
224 mlock(2), setrlimit(2), shmget(2), shmop(2), capabilities(7), svipc(7)
225
227 This page is part of release 4.16 of the Linux man-pages project. A
228 description of the project, information about reporting bugs, and the
229 latest version of this page, can be found at
230 https://www.kernel.org/doc/man-pages/.
231Linux 2017-09-15 SHMCTL(2)