#include <sys/types.h> #include <sys/ipc.h> #include <sys/msg.h>
int msgctl(int msqid, int cmd, struct msqid_ds *buf);
struct msqid_ds {
struct ipc_perm msg_perm; /* Ownership and permissions
time_t msg_stime; /* Time of last msgsnd() */
time_t msg_rtime; /* Time of last msgrcv() */
time_t msg_ctime; /* Time of last change */
unsigned long __msg_cbytes; /* Current number of bytes in
queue (non-standard) */
msgqnum_t msg_qnum; /* Current number of messages
in queue */
msglen_t msg_qbytes; /* Maximum number of bytes
allowed in queue */
pid_t msg_lspid; /* PID of last msgsnd() */
pid_t msg_lrpid; /* PID of last msgrcv() */
};
The ipc_perm structure is defined in <sys/ipc.h> as follows (the highlighted fields are settable using IPC_SET):
struct ipc_perm {
key_t key; /* Key supplied to msgget() */
uid_t uid; /* Effective UID of owner */
gid_t gid; /* Effective GID of owner */
uid_t cuid; /* Effective UID of creator */
gid_t cgid; /* Effective GID of creator */
unsigned short mode; /* Permissions */
unsigned short seq; /* Sequence number */
};
Valid values for cmd are:
| 標(biāo)簽 | 描述 | |
|---|---|---|
| IPC_STAT | ||
| Copy information from the kernel data structure associated withmsqid into the msqid_ds 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 kernel data structure associated with this message queue, updating also its msg_ctime member. The following members of the structure are updated: msg_qbytes,msg_perm.uid, msg_perm.gid, and (the least significant 9 bits of)msg_perm.mode. The effective UID of the calling process must match the owner (msg_perm.uid) or creator (msg_perm.cuid) of the message queue, or the caller must be privileged. Appropriate privilege (Linux: the CAP_IPC_RESOURCEcapability) is required to raise the msg_qbytes value beyond the system parameter MSGMNB. | ||
| IPC_RMID | ||
| Immediately remove the message queue, awakening all waiting reader and writer processes (with an error return and errno set to EIDRM). The calling process must have appropriate privileges or its effective user ID must be either that of the creator or owner of the message queue. | ||
| IPC_INFO (Linux specific) | ||
Returns information about system-wide message queue limits and parameters in the structure pointed to by buf. This structure is of type msginfo (thus, a cast is required), defined in<sys/msg.h> if the _GNU_SOURCE feature test macro is defined:
|
||
| MSG_INFO (Linux specific) | ||
| Returns a msginfo structure containing the same information as for IPC_INFO, except that the following fields are returned with information about system resources consumed by message queues: the msgpool field returns the number of message queues that currently exist on the system; the msgmap field returns the total number of messages in all queues on the system; and the msgtql field returns the total number of bytes in all messages in all queues on the system. | ||
| MSG_STAT (Linux specific) | ||
| Returns a msqid_ds structure as for IPC_STAT. However, themsqid argument is not a queue identifier, but instead an index into the kernel’s internal array that maintains information about all message queues on the system. | ||
On error, -1 is returned with errno indicating the error.
| 標(biāo)簽 | 描述 |
|---|---|
| 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_OWNERcapability. |
| 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. Or: for a MSG_STAT operation, the index value specified in msqid referred to an array slot that is currently unused. |
| 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 inmsg_perm.uid) of the message queue, and the process is not privileged (Linux: it does not have the CAP_SYS_ADMINcapability). |
Various fields in the 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 an IPC_64 flag incmd.)
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