SHMCTL(2) BSD System Calls Manual SHMCTL(2)NAME
shmctl -- shared memory control operations
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
int
shmctl(int shmid, int cmd, struct shmid_ds *buf);
DESCRIPTION
The shmctl() system call performs some control operations on the shared memory area specified by shmid. Each shared memory segment has a
data structure associated with it, parts of which may be altered by shmctl() and parts of which determine the actions of shmctl(). This
structure is defined as follows in <sys/shm.h>:
struct shmid_ds {
struct ipc_perm shm_perm; /* operation permissions */
int shm_segsz; /* size of segment in bytes */
pid_t shm_lpid; /* pid of last shm op */
pid_t shm_cpid; /* pid of creator */
short shm_nattch; /* # of current attaches */
time_t shm_atime; /* last shmat() time*/
time_t shm_dtime; /* last shmdt() time */
time_t shm_ctime; /* last change by shmctl() */
void *shm_internal; /* sysv stupidity */
};
The ipc_perm structure used inside the shmid_ds structure is defined in <sys/ipc.h> and looks like this:
struct ipc_perm {
ushort cuid; /* creator user id */
ushort cgid; /* creator group id */
ushort uid; /* user id */
ushort gid; /* group id */
ushort mode; /* r/w permission (see chmod(2)) */
ushort seq; /* sequence # (to generate unique msg/sem/shm id) */
key_t key; /* user specified msg/sem/shm key */
};
The operation to be performed by shmctl() is specified in cmd and is one of:
IPC_STAT Gather information about the shared memory segment and place it in the structure pointed to by buf.
IPC_SET Set the value of the shm_perm.uid, shm_perm.gid and shm_perm.mode fields in the structure associated with shmid. The values are
taken from the corresponding fields in the structure pointed to by buf. This operation can only be executed by the super-user, or
a process that has an effective user id equal to either shm_perm.cuid or shm_perm.uid in the data structure associated with the
shared memory segment.
IPC_RMID Remove the shared memory segment specified by shmid and destroy the data associated with it. Only the super-user or a process with
an effective uid equal to the shm_perm.cuid or shm_perm.uid values in the data structure associated with the queue can do this.
The read and write permissions on a shared memory identifier are determined by the shm_perm.mode field in the same way as is done with files
(see chmod(2) ), but the effective uid can match either the shm_perm.cuid field or the shm_perm.uid field, and the effective gid can match
either shm_perm.cgid or shm_perm.gid.
RETURN VALUES
Upon successful completion, a value of 0 is returned. Otherwise, -1 is returned and the global variable errno is set to indicate the error.
ERRORS
shmctl() will fail if:
[EPERM] cmd is equal to IPC_SET or IPC_RMID and the caller is not the super-user, nor does the effective uid match either the
shm_perm.uid or shm_perm.cuid fields of the data structure associated with the shared memory segment. An attempt is made
to increase the value of shm_qbytes through IPC_SET but the caller is not the super-user.
[EACCES] The command is IPC_STAT and the caller has no read permission for this shared memory segment.
[EINVAL] shmid is not a valid shared memory segment identifier. cmd is not a valid command.
[EFAULT] buf specifies an invalid address.
SEE ALSO shmat(2), shmdt(2), shmget(2)BSD August 17, 1995 BSD
Check Out this Related Man Page
SHMCTL(2) Linux Programmer's Manual SHMCTL(2)NAME
shmctl - shared memory control
SYNOPSIS
#include <sys/ipc.h>
#include <sys/shm.h>
int shmctl(int shmid, int cmd, struct shmid_ds *buf);
DESCRIPTION
shmctl() allows the user to receive information on a shared memory segment, set the owner, group, and permissions of a shared memory seg-
ment, or destroy a segment. The information about the segment identified by shmid is returned in a shmid_ds structure:
struct shmid_ds {
struct ipc_perm shm_perm; /* operation perms */
int shm_segsz; /* size of segment (bytes) */
time_t shm_atime; /* last attach time */
time_t shm_dtime; /* last detach time */
time_t shm_ctime; /* last change time */
unsigned short shm_cpid; /* pid of creator */
unsigned short shm_lpid; /* pid of last operator */
short shm_nattch; /* no. of current attaches */
...
};
The highlighted fields in the member shm_perm can be set:
struct ipc_perm {
key_t key;
ushort uid; /* owner euid and egid */
ushort gid;
ushort cuid; /* creator euid and egid */
ushort cgid;
ushort mode; /* lower 9 bits of access modes */
ushort seq; /* sequence number */
};
The following cmds are available:
IPC_STAT is used to copy the information about the shared memory segment into the buffer buf. The user must have read access to the
shared memory segment.
IPC_SET is used to apply the changes the user has made to the uid, gid, or mode members of the shm_perms field. Only the lowest 9 bits
of mode are used. The shm_ctime member is also updated. The user must be the owner, creator, or the super-user.
IPC_RMID is used to mark the segment as destroyed. It will actually be destroyed after the last detach. (I.e., when the shm_nattch mem-
ber of the associated structure shmid_ds is zero.) The user must be the owner, creator, or the super-user.
The user must ensure that a segment is eventually destroyed; otherwise its pages that were faulted in will remain in memory or swap.
In addition, the super-user can prevent or allow swapping of a shared memory segment with the following cmds: (Linux only)
SHM_LOCK prevents swapping of a shared memory segment. The user must fault in any pages that are required to be present after locking is
enabled.
SHM_UNLOCK allows the shared memory segment to be swapped out.
The IPC_INFO, SHM_STAT and SHM_INFO control calls are used by the ipcs(8) program to provide information on allocated resources. In the
future, these may be modified as needed or moved to a proc file system interface.
RETURN VALUE
0 is returned on success, -1 on error.
ERRORS
On error, errno will be set to one of the following:
EACCES is returned if IPC_STAT is requested and shm_perm.modes does not allow read access for shmid.
EFAULT The argument cmd has value IPC_SET or IPC_STAT but the address pointed to by buf isn't accessible.
EINVAL is returned if shmid is not a valid identifier, or cmd is not a valid command.
EIDRM is returned if shmid points to a removed identifier.
EPERM is returned if IPC_SET or IPC_RMID is attempted, and the effective user ID of the calling process is not the creator (as found
in shm_perm.cuid), the owner (as found in shm_perm.uid), or the super-user.
EOVERFLOW is returned if IPC_STAT is attempted, and the gid or uid value is too large to be stored in the structure pointed to by buf.
NOTE
Various fields in a struct shmid_ds were shorts under Linux 2.2 and have become longs under Linux 2.4. To take advantage of this, a recom-
pilation under glibc-2.1.91 or later should suffice. (The kernel distinguishes old and new calls by a IPC_64 flag in cmd.)
CONFORMING TO
SVr4, SVID. SVr4 documents additional error conditions EINVAL, ENOENT, ENOSPC, ENOMEM, EEXIST. Neither SVr4 nor SVID documents an EIDRM
error condition.
SEE ALSO shmget(2), shmop(2)Linux 2.4.1 2002-01-05 SHMCTL(2)