1shmctl(2) System Calls Manual shmctl(2)
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6 shmctl - System V shared memory control
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9 Standard C library (libc, -lc)
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12 #include <sys/shm.h>
13 int shmctl(int shmid, int cmd, struct shmid_ds *buf);
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17 shmctl() performs the control operation specified by cmd on the Sys‐
18 tem V shared memory segment whose identifier is given in shmid.
19 The buf argument is a pointer to a shmid_ds structure, defined in
21 <sys/shm.h> as follows:
22 struct shmid_ds {
24 struct ipc_perm shm_perm; /* Ownership and permissions */
25 size_t shm_segsz; /* Size of segment (bytes) */
26 time_t shm_atime; /* Last attach time */
27 time_t shm_dtime; /* Last detach time */
28 time_t shm_ctime; /* Creation time/time of last
29 modification via shmctl() */
30 pid_t shm_cpid; /* PID of creator */
31 pid_t shm_lpid; /* PID of last shmat(2)/shmdt(2) */
32 shmatt_t shm_nattch; /* No. of current attaches */
33 ...
34 };
35 The fields of the shmid_ds structure are as follows:
37 shm_perm This is an ipc_perm structure (see below) that specifies
39 the access permissions on the shared memory segment.
40 shm_segsz Size in bytes of the shared memory segment.
42 shm_atime Time of the last shmat(2) system call that attached this
44 segment.
45 shm_dtime Time of the last shmdt(2) system call that detached tgis
47 segment.
48 shm_ctime Time of creation of segment or time of the last shmctl()
50 IPC_SET operation.
51 shm_cpid ID of the process that created the shared memory segment.
53 shm_lpid ID of the last process that executed a shmat(2) or shmdt(2)
55 system call on this segment.
56 shm_nattch Number of processes that have this segment attached.
58 The ipc_perm structure is defined as follows (the highlighted fields
60 are settable using IPC_SET):
61 struct ipc_perm {
63 key_t __key; /* Key supplied to shmget(2) */
64 uid_t uid; /* Effective UID of owner */
65 gid_t gid; /* Effective GID of owner */
66 uid_t cuid; /* Effective UID of creator */
67 gid_t cgid; /* Effective GID of creator */
68 unsigned short mode; /* Permissions + SHM_DEST and
69 SHM_LOCKED flags */
70 unsigned short __seq; /* Sequence number */
71 };
72 The least significant 9 bits of the mode field of the ipc_perm struc‐
74 ture define the access permissions for the shared memory segment. The
75 permission bits are as follows:
76 0400 Read by user
78 0200 Write by user
79 0040 Read by group
80 0020 Write by group
81 0004 Read by others
82 0002 Write by others
83 Bits 0100, 0010, and 0001 (the execute bits) are unused by the system.
85 (It is not necessary to have execute permission on a segment in order
86 to perform a shmat(2) call with the SHM_EXEC flag.)
87 Valid values for cmd are:
89 IPC_STAT
91 Copy information from the kernel data structure associated with
92 shmid into the shmid_ds structure pointed to by buf. The caller
93 must have read permission on the shared memory segment.
94 IPC_SET
96 Write the values of some members of the shmid_ds structure
97 pointed to by buf to the kernel data structure associated with
98 this shared memory segment, updating also its shm_ctime member.
99 The following fields are updated: shm_perm.uid, shm_perm.gid,
101 and (the least significant 9 bits of) shm_perm.mode.
102 The effective UID of the calling process must match the owner
104 (shm_perm.uid) or creator (shm_perm.cuid) of the shared memory
105 segment, or the caller must be privileged.
106 IPC_RMID
108 Mark the segment to be destroyed. The segment will actually be
109 destroyed only after the last process detaches it (i.e., when
110 the shm_nattch member of the associated structure shmid_ds is
111 zero). The caller must be the owner or creator of the segment,
112 or be privileged. The buf argument is ignored.
113 If a segment has been marked for destruction, then the (nonstan‐
115 dard) SHM_DEST flag of the shm_perm.mode field in the associated
116 data structure retrieved by IPC_STAT will be set.
117 The caller must ensure that a segment is eventually destroyed;
119 otherwise its pages that were faulted in will remain in memory
120 or swap.
121 See also the description of /proc/sys/kernel/shm_rmid_forced in
123 proc(5).
124 IPC_INFO (Linux-specific)
126 Return information about system-wide shared memory limits and
127 parameters in the structure pointed to by buf. This structure
128 is of type shminfo (thus, a cast is required), defined in
129 <sys/shm.h> if the _GNU_SOURCE feature test macro is defined:
130 struct shminfo {
132 unsigned long shmmax; /* Maximum segment size */
133 unsigned long shmmin; /* Minimum segment size;
134 always 1 */
135 unsigned long shmmni; /* Maximum number of segments */
136 unsigned long shmseg; /* Maximum number of segments
137 that a process can attach;
138 unused within kernel */
139 unsigned long shmall; /* Maximum number of pages of
140 shared memory, system-wide */
141 };
142 The shmmni, shmmax, and shmall settings can be changed via /proc
144 files of the same name; see proc(5) for details.
145 SHM_INFO (Linux-specific)
147 Return a shm_info structure whose fields contain information
148 about system resources consumed by shared memory. This struc‐
149 ture is defined in <sys/shm.h> if the _GNU_SOURCE feature test
150 macro is defined:
151 struct shm_info {
153 int used_ids; /* # of currently existing
154 segments */
155 unsigned long shm_tot; /* Total number of shared
156 memory pages */
157 unsigned long shm_rss; /* # of resident shared
158 memory pages */
159 unsigned long shm_swp; /* # of swapped shared
160 memory pages */
161 unsigned long swap_attempts;
162 /* Unused since Linux 2.4 */
163 unsigned long swap_successes;
164 /* Unused since Linux 2.4 */
165 };
166 SHM_STAT (Linux-specific)
168 Return a shmid_ds structure as for IPC_STAT. However, the shmid
169 argument is not a segment identifier, but instead an index into
170 the kernel's internal array that maintains information about all
171 shared memory segments on the system.
172 SHM_STAT_ANY (Linux-specific, since Linux 4.17)
174 Return a shmid_ds structure as for SHM_STAT. However,
175 shm_perm.mode is not checked for read access for shmid, meaning
176 that any user can employ this operation (just as any user may
177 read /proc/sysvipc/shm to obtain the same information).
178 The caller can prevent or allow swapping of a shared memory segment
180 with the following cmd values:
181 SHM_LOCK (Linux-specific)
183 Prevent swapping of the shared memory segment. The caller must
184 fault in any pages that are required to be present after locking
185 is enabled. If a segment has been locked, then the (nonstan‐
186 dard) SHM_LOCKED flag of the shm_perm.mode field in the associ‐
187 ated data structure retrieved by IPC_STAT will be set.
188 SHM_UNLOCK (Linux-specific)
190 Unlock the segment, allowing it to be swapped out.
191 Before Linux 2.6.10, only a privileged process could employ SHM_LOCK
193 and SHM_UNLOCK. Since Linux 2.6.10, an unprivileged process can employ
194 these operations if its effective UID matches the owner or creator UID
195 of the segment, and (for SHM_LOCK) the amount of memory to be locked
196 falls within the RLIMIT_MEMLOCK resource limit (see setrlimit(2)).
197
199 A successful IPC_INFO or SHM_INFO operation returns the index of the
200 highest used entry in the kernel's internal array recording information
201 about all shared memory segments. (This information can be used with
202 repeated SHM_STAT or SHM_STAT_ANY operations to obtain information
203 about all shared memory segments on the system.) A successful SHM_STAT
204 operation returns the identifier of the shared memory segment whose in‐
205 dex was given in shmid. Other operations return 0 on success.
206 On error, -1 is returned, and errno is set to indicate the error.
208
210 EACCES IPC_STAT or SHM_STAT is requested and shm_perm.mode does not al‐
211 low read access for shmid, and the calling process does not have
212 the CAP_IPC_OWNER capability in the user namespace that governs
213 its IPC namespace.
214 EFAULT The argument cmd has value IPC_SET or IPC_STAT but the address
216 pointed to by buf isn't accessible.
217 EIDRM shmid points to a removed identifier.
219 EINVAL shmid is not a valid identifier, or cmd is not a valid command.
221 Or: for a SHM_STAT or SHM_STAT_ANY operation, the index value
222 specified in shmid referred to an array slot that is currently
223 unused.
224 ENOMEM (Since Linux 2.6.9), SHM_LOCK was specified and the size of the
226 to-be-locked segment would mean that the total bytes in locked
227 shared memory segments would exceed the limit for the real user
228 ID of the calling process. This limit is defined by the
229 RLIMIT_MEMLOCK soft resource limit (see setrlimit(2)).
230 EOVERFLOW
232 IPC_STAT is attempted, and the GID or UID value is too large to
233 be stored in the structure pointed to by buf.
234 EPERM IPC_SET or IPC_RMID is attempted, and the effective user ID of
236 the calling process is not that of the creator (found in
237 shm_perm.cuid), or the owner (found in shm_perm.uid), and the
238 process was not privileged (Linux: did not have the CAP_SYS_AD‐
239 MIN capability).
240 Or (before Linux 2.6.9), SHM_LOCK or SHM_UNLOCK was specified,
242 but the process was not privileged (Linux: did not have the
243 CAP_IPC_LOCK capability). (Since Linux 2.6.9, this error can
244 also occur if the RLIMIT_MEMLOCK is 0 and the caller is not
245 privileged.)
246
248 Linux permits a process to attach (shmat(2)) a shared memory segment
249 that has already been marked for deletion using shmctl(IPC_RMID). This
250 feature is not available on other UNIX implementations; portable appli‐
251 cations should avoid relying on it.
252
254 POSIX.1-2008.
255
257 POSIX.1-2001, SVr4.
258 Various fields in a struct shmid_ds were typed as short under Linux 2.2
260 and have become long under Linux 2.4. To take advantage of this, a re‐
261 compilation under glibc-2.1.91 or later should suffice. (The kernel
262 distinguishes old and new calls by an IPC_64 flag in cmd.)
263
265 The IPC_INFO, SHM_STAT, and SHM_INFO operations are used by the ipcs(1)
266 program to provide information on allocated resources. In the future,
267 these may modified or moved to a /proc filesystem interface.
268
270 mlock(2), setrlimit(2), shmget(2), shmop(2), capabilities(7),
271 sysvipc(7)
272Linux man-pages 6.04 2023-03-30 shmctl(2)