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SHMCTL (2) | System calls | Unix Manual Pages | :man


shmctl - shared memory control


Return Values


.Lb libc


.In machine/param.h
.In sys/types.h
.In sys/ipc.h
.In sys/shm.h int shmctl "int shmid" "int cmd" "struct shmid_ds *buf"


Performs the action specified by cmd on the shared memory segment identified by shmid:
IPC_STAT Fetch the segment’s "struct shmid_ds", storing it in the memory pointed to by buf.
IPC_SET Changes the shm_perm.uid, shm_perm.gid, and shm_perm.mode members of the segment’s "struct shmid_ds" to match those of the struct pointed to by buf. The calling process’s effective uid must match either shm_perm.uid or shm_perm.cuid, or it must have superuser privileges.
IPC_RMID Removes the segment from the system. The removal will not take effect until all processes having attached the segment have exited; however, once the IPC_RMID operation has taken place, no further processes will be allowed to attach the segment. For the operation to succeed, the calling process’s effective uid must match shm_perm.uid or shm_perm.cuid, or the process must have superuser privileges.

.Vt shmid_ds structure is defined as follows:
struct shmid_ds {
struct ipc_perm shm_perm; /* operation permission structure */
int shm_segsz; /* size of segment in bytes */
pid_t shm_lpid; /* process ID of last shared memory op */
pid_t shm_cpid; /* process ID of creator */
short shm_nattch; /* number of current attaches */
time_tshm_atime; /* time of last shmat() */
time_tshm_dtime; /* time of last shmdt() */
time_tshm_ctime; /* time of last change by shmctl() */
void *shm_internal; /* sysv stupidity */


.Rv -std shmctl


The shmctl system call will fail if:
Invalid operation, or no shared memory segment was found corresponding to shmid.
The calling process’s effective uid does not match the uid of the shared memory segment’s owner or creator.
Permission denied due to mismatch between operation and mode of shared memory segment.


shmat(2), shmdt(2), shmget(2), ftok(3)

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