msgctl: operace ovládání zpráv

Originální popis anglicky: msgctl - message control operations

Návod, kniha: Linux Programmer's Manual

STRUČNĚ

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

int msgctl(int msqid, int cmd, struct msqid_ds *buf);

POPIS / INSTRUKCE

This 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 associated with msqid into the structure pointed to by buf. The caller must have read permission 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. The following members of the structure can be updated:

 
	msg_perm.uid
	msg_perm.gid
	msg_perm.mode	/* only lowest 9-bits */
	msg_qbytes
    

 

The calling process must have appropriate (probably, root) privileges or its effective user-ID must be that of the creator (msg_perm.cuid) or owner (msg_perm.uid) of the message queue. Appropriate privilege (Linux: the CAP_IPC_RESOURCE capability) is required to raise the msg_qbytes value beyond the system parameter MSGMNB.

IPC_RMID

Immediately remove the message queue and its associated data structure, awakening all waiting reader and writer processes (with an error return and errno set to EIDRM). The calling process must have appropriate (probably, root) privileges or its effective user-ID must be either that of the creator or owner of the message queue.

NÁVRATOVÁ HODNOTA

On success, the return value will be 0, otherwise -1 with errno indicating the error.

CHYBY / ERRORY

On failure, errno is set to one of the following:

EACCES

The argument cmd is equal to IPC_STAT or MSG_STAT, but the calling process does not have read permission on the message queue msqid, and does not have the CAP_IPC_OWNER capability.

EFAULT

The argument cmd has the 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 the value IPC_SET or IPC_RMID, but the effective user ID of the calling process is not the creator (as found in msg_perm.cuid) or the owner (as found in msg_perm.uid) of the message queue, and the process is not privileged (Linux: it does not have the CAP_SYS_ADMIN capability).

NOTES

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.)

ODPOVÍDAJÍCÍ

SVr4, SVID. SVID does not document the EIDRM error condition.

SOUVISEJÍCÍ

msgget(2), msgrcv(2), msgsnd(2), ipc(5), capabilities(7)