View Source: msgctl(2) - Waikato Linux Users Group

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!!NAME
msgctl - message control operations
!!SYNOPSIS
 __#include <sys/types.h>__
 __#include <sys/ipc.h>__
 __#include <sys/msg.h>__

 __int msgctl ( int__ ''msqid''__, int__ ''cmd''__, struct msqid_ds *__''buf'' __)__

!!DESCRIPTION
The function performs the control operation specified by ''cmd'' on the message queue with identifier ''msqid''. Legal values for ''cmd'' are:
;__IPC_STAT__: Copy info from the message queue data structure into the structure pointed to by ''buf''. The user must have read access privileges on the message queue.
;__IPC_SET__: Write the values of some members of the __msqid_ds__ structure pointed to by ''buf'' to the message queue data structure, updating also its __msg_ctime__ member. Considered members from the user supplied __struct msqid_ds__ pointed to by ''buf'' are
;:__msg_perm.uid__
;:__msg_perm.gid__
;:__msg_perm.mode__  /* only lowest 9-bits */
;:__msg_qbytes__
;:The calling process effective user-ID must be one among super-user, creator or owner of the message queue. Only the super-user can raise the __msg_qbytes__ value beyond the system parameter __MSGMNB__.

;__IPC_RMID__: Remove immediately the message queue and its data structures awakening all waiting reader and writer processes (with an error return and __errno__ set to [EIDRM]). The calling process effective user-ID must be one among super-user, creator or owner of the message queue.

!!RETURN VALUE
If successful, the return value will be __0__, otherwise __-1__ with __errno__ indicating the error.

!!ERRORS
For a failing return, __errno__ will be set to one among the following values:
;[EACCES]: The argument ''cmd'' is equal to __IPC_STAT__ but the calling process has no read access permissions on the message queue ''msqid''.
;[EFAULT]: The argument ''cmd'' has value __IPC_SET__ or __IPC_STAT__ but the address pointed to by ''buf'' isn't accessible.
;[EIDRM]: The message queue was removed.
;[EINVAL]: Invalid value for ''cmd'' or ''msqid''.
;[EPERM]: The argument ''cmd'' has value __IPC_SET__ or __IPC_RMID__ but the calling process effective user-ID has insufficient privileges to execute the command. Note
this is also the case of a non super-user process trying to increase the __msg_qbytes__ value beyond the value specified by the system parameter __MSGMNB__.

!!NOTES
The __IPC_INFO__, __MSG_STAT__ and __MSG_INFO__ control calls are used by the ipcs(8) program to provide information on allocated resources. In the future these can be modified as needed or moved to a proc file system interface.

Various fields in a ''struct msqid_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''.)

!!CONFORMING TO
SVr4, SVID. SVID does not document the [EIDRM] error condition.

!!SEE ALSO
ipc(5), msgget(2), msgsnd(2), msgrcv(2)

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Last edited on Saturday, March 15, 2003 1:11:11 am by PerryLorier

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