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GNU/Linux |
CentOS 4.8 |
i386 |
 |
shmctl(2) |
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shmctl − shared memory control
#include <sys/ipc.h>
#include <sys/shm.h>
int shmctl(int shmid, int cmd, struct shmid_ds *buf);
shmctl() allows the user to receive information on a shared memory segment, set the owner, group, and permissions of a shared memory segment, 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 */
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; /* 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:
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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. | ||
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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. | ||
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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 member 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, processes with appropriate privileges can prevent or allow swapping of a shared memory segment with the following cmds: (Linux only)
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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. | ||
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SHM_UNLOCK |
allows the shared memory segment to be swapped out. |
Processes are permitted to use SHM_LOCK and SHM_UNLOCK if they running with the CAP_IPC_LOCK capability (normally only true for root) or if their current RLIMIT_MEMLOCK resource limit is non-zero.
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.
0 is returned on success, −1 on error.
On error, errno will be set to one of the following:
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EACCES |
is returned if IPC_STAT is requested and shm_perm.modes does not allow read access for shmid. | ||
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EFAULT |
The argument cmd has value IPC_SET or IPC_STAT but the address pointed to by buf isn’t accessible. | ||
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EINVAL |
is returned if shmid is not a valid identifier, or cmd is not a valid command. | ||
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EIDRM |
is returned if shmid points to a removed identifier. | ||
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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. | ||
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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. |
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 recompilation under glibc-2.1.91 or later should suffice. (The kernel distinguishes old and new calls by a IPC_64 flag in cmd.)
SVr4, SVID. SVr4 documents additional error conditions EINVAL, ENOENT, ENOSPC, ENOMEM, EEXIST. Neither SVr4 nor SVID documents an EIDRM error condition.
shmget(2), shmop(2)
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shmctl(2) | |