'\" t .\" Copyright (c) 1993 Luigi P. Bai (lpb@softint.com) July 28, 1993 .\" and Copyright 1993 Giorgio Ciucci .\" and Copyright 2004, 2005 Michael Kerrisk .\" .\" SPDX-License-Identifier: Linux-man-pages-copyleft .\" .\" Modified 1993-07-28, Rik Faith .\" Modified 1993-11-28, Giorgio Ciucci .\" Modified 1997-01-31, Eric S. Raymond .\" Modified 2001-02-18, Andries Brouwer .\" Modified 2002-01-05, 2004-05-27, 2004-06-17, .\" Michael Kerrisk .\" Modified 2004-10-11, aeb .\" Modified, Nov 2004, Michael Kerrisk .\" Language and formatting clean-ups .\" Updated shmid_ds structure definitions .\" Added information on SHM_DEST and SHM_LOCKED flags .\" Noted that CAP_IPC_LOCK is not required for SHM_UNLOCK .\" since Linux 2.6.9 .\" Modified, 2004-11-25, mtk, notes on 2.6.9 RLIMIT_MEMLOCK changes .\" 2005-04-25, mtk -- noted aberrant Linux behavior w.r.t. new .\" attaches to a segment that has already been marked for deletion. .\" 2005-08-02, mtk: Added IPC_INFO, SHM_INFO, SHM_STAT descriptions. .\" 2018-03-20, dbueso: Added SHM_STAT_ANY description. .\" .TH shmctl 2 2023-03-30 "Linux man-pages 6.04" .SH NAME shmctl \- System V shared memory control .SH LIBRARY Standard C library .RI ( libc ", " \-lc ) .SH SYNOPSIS .nf .B #include .PP .BI "int shmctl(int " shmid ", int " cmd ", struct shmid_ds *" buf ); .fi .SH DESCRIPTION .BR shmctl () performs the control operation specified by .I cmd on the System\ V shared memory segment whose identifier is given in .IR shmid . .PP The .I buf argument is a pointer to a \fIshmid_ds\fP structure, defined in \fI\fP as follows: .PP .in +4n .EX struct shmid_ds { struct ipc_perm shm_perm; /* Ownership and permissions */ size_t shm_segsz; /* Size of segment (bytes) */ time_t shm_atime; /* Last attach time */ time_t shm_dtime; /* Last detach time */ time_t shm_ctime; /* Creation time/time of last modification via shmctl() */ pid_t shm_cpid; /* PID of creator */ pid_t shm_lpid; /* PID of last shmat(2)/shmdt(2) */ shmatt_t shm_nattch; /* No. of current attaches */ ... }; .EE .in .PP The fields of the .I shmid_ds structure are as follows: .TP 12 .I shm_perm This is an .I ipc_perm structure (see below) that specifies the access permissions on the shared memory segment. .TP .I shm_segsz Size in bytes of the shared memory segment. .TP .I shm_atime Time of the last .BR shmat (2) system call that attached this segment. .TP .I shm_dtime Time of the last .BR shmdt (2) system call that detached tgis segment. .TP .I shm_ctime Time of creation of segment or time of the last .BR shmctl () .B IPC_SET operation. .TP .I shm_cpid ID of the process that created the shared memory segment. .TP .I shm_lpid ID of the last process that executed a .BR shmat (2) or .BR shmdt (2) system call on this segment. .TP .I shm_nattch Number of processes that have this segment attached. .PP The .I ipc_perm structure is defined as follows (the highlighted fields are settable using .BR IPC_SET ): .PP .in +4n .EX struct ipc_perm { key_t __key; /* Key supplied to shmget(2) */ uid_t \fBuid\fP; /* Effective UID of owner */ gid_t \fBgid\fP; /* Effective GID of owner */ uid_t cuid; /* Effective UID of creator */ gid_t cgid; /* Effective GID of creator */ unsigned short \fBmode\fP; /* \fBPermissions\fP + SHM_DEST and SHM_LOCKED flags */ unsigned short __seq; /* Sequence number */ }; .EE .in .PP The least significant 9 bits of the .I mode field of the .I ipc_perm structure define the access permissions for the shared memory segment. The permission bits are as follows: .TS l l. 0400 Read by user 0200 Write by user 0040 Read by group 0020 Write by group 0004 Read by others 0002 Write by others .TE .PP Bits 0100, 0010, and 0001 (the execute bits) are unused by the system. (It is not necessary to have execute permission on a segment in order to perform a .BR shmat (2) call with the .B SHM_EXEC flag.) .PP Valid values for .I cmd are: .TP .B IPC_STAT Copy information from the kernel data structure associated with .I shmid into the .I shmid_ds structure pointed to by \fIbuf\fP. The caller must have read permission on the shared memory segment. .TP .B IPC_SET Write the values of some members of the .I shmid_ds structure pointed to by .I buf to the kernel data structure associated with this shared memory segment, updating also its .I shm_ctime member. .IP The following fields are updated: \fIshm_perm.uid\fP, \fIshm_perm.gid\fP, and (the least significant 9 bits of) \fIshm_perm.mode\fP. .IP The effective UID of the calling process must match the owner .RI ( shm_perm.uid ) or creator .RI ( shm_perm.cuid ) of the shared memory segment, or the caller must be privileged. .TP .B IPC_RMID Mark the segment to be destroyed. The segment will actually be destroyed only after the last process detaches it (i.e., when the .I shm_nattch member of the associated structure .I shmid_ds is zero). The caller must be the owner or creator of the segment, or be privileged. The .I buf argument is ignored. .IP If a segment has been marked for destruction, then the (nonstandard) .B SHM_DEST flag of the .I shm_perm.mode field in the associated data structure retrieved by .B IPC_STAT will be set. .IP The caller \fImust\fP ensure that a segment is eventually destroyed; otherwise its pages that were faulted in will remain in memory or swap. .IP See also the description of .I /proc/sys/kernel/shm_rmid_forced in .BR proc (5). .TP .BR IPC_INFO " (Linux-specific)" Return information about system-wide shared memory limits and parameters in the structure pointed to by .IR buf . This structure is of type .I shminfo (thus, a cast is required), defined in .I if the .B _GNU_SOURCE feature test macro is defined: .IP .in +4n .EX struct shminfo { unsigned long shmmax; /* Maximum segment size */ unsigned long shmmin; /* Minimum segment size; always 1 */ unsigned long shmmni; /* Maximum number of segments */ unsigned long shmseg; /* Maximum number of segments that a process can attach; unused within kernel */ unsigned long shmall; /* Maximum number of pages of shared memory, system\-wide */ }; .EE .in .IP The .IR shmmni , .IR shmmax , and .I shmall settings can be changed via .I /proc files of the same name; see .BR proc (5) for details. .TP .BR SHM_INFO " (Linux-specific)" Return a .I shm_info structure whose fields contain information about system resources consumed by shared memory. This structure is defined in .I if the .B _GNU_SOURCE feature test macro is defined: .IP .in +4n .EX struct shm_info { int used_ids; /* # of currently existing segments */ unsigned long shm_tot; /* Total number of shared memory pages */ unsigned long shm_rss; /* # of resident shared memory pages */ unsigned long shm_swp; /* # of swapped shared memory pages */ unsigned long swap_attempts; /* Unused since Linux 2.4 */ unsigned long swap_successes; /* Unused since Linux 2.4 */ }; .EE .in .TP .BR SHM_STAT " (Linux-specific)" Return a .I shmid_ds structure as for .BR IPC_STAT . However, the .I shmid argument is not a segment identifier, but instead an index into the kernel's internal array that maintains information about all shared memory segments on the system. .TP .BR SHM_STAT_ANY " (Linux-specific, since Linux 4.17)" Return a .I shmid_ds structure as for .BR SHM_STAT . However, .I shm_perm.mode is not checked for read access for .IR shmid , meaning that any user can employ this operation (just as any user may read .I /proc/sysvipc/shm to obtain the same information). .PP The caller can prevent or allow swapping of a shared memory segment with the following \fIcmd\fP values: .TP .BR SHM_LOCK " (Linux-specific)" Prevent swapping of the shared memory segment. The caller must fault in any pages that are required to be present after locking is enabled. If a segment has been locked, then the (nonstandard) .B SHM_LOCKED flag of the .I shm_perm.mode field in the associated data structure retrieved by .B IPC_STAT will be set. .TP .BR SHM_UNLOCK " (Linux-specific)" Unlock the segment, allowing it to be swapped out. .PP Before Linux 2.6.10, only a privileged process could employ .B SHM_LOCK and .BR SHM_UNLOCK . Since Linux 2.6.10, an unprivileged process can employ these operations if its effective UID matches the owner or creator UID of the segment, and (for .BR SHM_LOCK ) the amount of memory to be locked falls within the .B RLIMIT_MEMLOCK resource limit (see .BR setrlimit (2)). .\" There was some weirdness in Linux 2.6.9: SHM_LOCK and SHM_UNLOCK could .\" be applied to a segment, regardless of ownership of the segment. .\" This was a botch-up in the move to RLIMIT_MEMLOCK, and was fixed .\" in Linux 2.6.10. MTK, May 2005 .SH RETURN VALUE A successful .B IPC_INFO or .B SHM_INFO operation returns the index of the highest used entry in the kernel's internal array recording information about all shared memory segments. (This information can be used with repeated .B SHM_STAT or .B SHM_STAT_ANY operations to obtain information about all shared memory segments on the system.) A successful .B SHM_STAT operation returns the identifier of the shared memory segment whose index was given in .IR shmid . Other operations return 0 on success. .PP On error, \-1 is returned, and .I errno is set to indicate the error. .SH ERRORS .TP .B EACCES \fBIPC_STAT\fP or \fBSHM_STAT\fP is requested and \fIshm_perm.mode\fP does not allow read access for .IR shmid , and the calling process does not have the .B CAP_IPC_OWNER capability in the user namespace that governs its IPC namespace. .TP .B EFAULT The argument .I cmd has value .B IPC_SET or .B IPC_STAT but the address pointed to by .I buf isn't accessible. .TP .B EIDRM \fIshmid\fP points to a removed identifier. .TP .B EINVAL \fIshmid\fP is not a valid identifier, or \fIcmd\fP is not a valid command. Or: for a .B SHM_STAT or .B SHM_STAT_ANY operation, the index value specified in .I shmid referred to an array slot that is currently unused. .TP .B ENOMEM (Since Linux 2.6.9), .B SHM_LOCK was specified and the size of the to-be-locked segment would mean that the total bytes in locked shared memory segments would exceed the limit for the real user ID of the calling process. This limit is defined by the .B RLIMIT_MEMLOCK soft resource limit (see .BR setrlimit (2)). .TP .B EOVERFLOW \fBIPC_STAT\fP is attempted, and the GID or UID value is too large to be stored in the structure pointed to by .IR buf . .TP .B EPERM \fBIPC_SET\fP or \fBIPC_RMID\fP is attempted, and the effective user ID of the calling process is not that of the creator (found in .IR shm_perm.cuid ), or the owner (found in .IR shm_perm.uid ), and the process was not privileged (Linux: did not have the .B CAP_SYS_ADMIN capability). .IP Or (before Linux 2.6.9), .B SHM_LOCK or .B SHM_UNLOCK was specified, but the process was not privileged (Linux: did not have the .B CAP_IPC_LOCK capability). (Since Linux 2.6.9, this error can also occur if the .B RLIMIT_MEMLOCK is 0 and the caller is not privileged.) .SH VERSIONS Linux permits a process to attach .RB ( shmat (2)) a shared memory segment that has already been marked for deletion using .IR shmctl(IPC_RMID) . This feature is not available on other UNIX implementations; portable applications should avoid relying on it. .SH STANDARDS POSIX.1-2008. .SH HISTORY POSIX.1-2001, SVr4. .\" SVr4 documents additional error conditions EINVAL, .\" ENOENT, ENOSPC, ENOMEM, EEXIST. Neither SVr4 nor SVID documents .\" an EIDRM error condition. .PP Various fields in a \fIstruct shmid_ds\fP were typed as .I short under Linux 2.2 and have become .I long under Linux 2.4. To take advantage of this, a recompilation under glibc-2.1.91 or later should suffice. (The kernel distinguishes old and new calls by an .B IPC_64 flag in .IR cmd .) .SH NOTES The .BR IPC_INFO , .BR SHM_STAT , and .B SHM_INFO operations are used by the .BR ipcs (1) program to provide information on allocated resources. In the future, these may modified or moved to a .I /proc filesystem interface. .SH SEE ALSO .BR mlock (2), .BR setrlimit (2), .BR shmget (2), .BR shmop (2), .BR capabilities (7), .BR sysvipc (7)