Controlling shared memory

This section describes how to use the shmctl system call. The accompanying program illustrates its use.

Using shmctl

The synopsis found on the shmctl(2) manual page is as follows:

   #include <sys/types.h>
   #include <sys/ipc.h>
   #include <sys/shm.h>
   

   int shmctl (shmid, cmd, buf)
   int shmid, cmd;
   struct shmid_ds *buf;

The shmctl system call requires three arguments to be passed to it. It returns an integer value which will be zero for successful completion or -1 otherwise.

The shmid variable must be a valid, non-negative, integer value. In other words, it must have already been created by using the shmget system call.

The cmd argument can be replaced by one of following values:

IPC_STAT

return the status information contained in the associated data structure for the specified shmid and place it in the data structure pointed to by the buf pointer in the user memory area

IPC_SET

for the specified shmid, set the effective user and group identification, and operation permissions

IPC_RMID

remove the specified shmid with its associated shared memory segment data structure

SHM_LOCK

lock the specified shared memory segment in memory; must have P_SYSOPS privilege to perform this operation

SHM_UNLOCK

unlock the shared memory segment from memory; must have P_SYSOPS privilege to perform this operation

To perform an IPC_SET or IPC_RMID control command, a process must have:

• an effective user id of OWNER/CREATOR, or

• an effective user id of root (if the system is running with the SUM privilege module), or

• the P_OWNER privilege.

A process must have read permission to perform the IPC_STAT control command.

The details of this system call are discussed in the example program. If you need more information on the logic manipulations in this program, read ``Using shmget''. It goes into more detail than would be practical for every system call.