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SHMCTL(2)                DragonFly System Calls Manual               SHMCTL(2)


NAME

     shmctl - shared memory control


LIBRARY

     Standard C Library (libc, -lc)


SYNOPSIS

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

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


DESCRIPTION

     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.

     The shmid_ds struct is defined as follows:

     typedef unsigned int shmatt_t;

     struct shmid_ds {
         struct ipc_perm shm_perm;   /* operation permission structure */
         size_t          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 */
         shmatt_t        shm_nattch; /* number of current attaches */
         time_t          shm_atime;  /* time of last shmat() */
         time_t          shm_dtime;  /* time of last shmdt() */
         time_t          shm_ctime;  /* time of 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>.


RETURN VALUES

     The shmctl() function returns the value 0 if successful; otherwise the
     value -1 is returned and the global variable errno is set to indicate the
     error.


ENVIRONMENT

     The XSI Interprocess Communication family of functions is also available
     as an implementation in userspace.  To use it, the sysvipcd(8) daemon has
     to be running.

     If the USR_SYSVIPC variable is set in a process' environment, the process
     and its children will use the userspace implementation.


ERRORS

     Shmctl() will fail if:

     [EINVAL]           Invalid operation, or no shared memory segment was
                        found corresponding to shmid.

     [EPERM]            The calling process's effective uid does not match the
                        uid of the shared memory segment's owner or creator.

     [EACCES]           Permission denied due to mismatch between operation
                        and mode of shared memory segment.


SEE ALSO

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


AUTHORS

     The DragonFly specific userspace implementation (see ENVIRONMENT) was
     written by Larisa Grigore.

DragonFly 5.7-DEVELOPMENT     September 12, 2019     DragonFly 5.7-DEVELOPMENT

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