How does msgrcv work?
How does msgrcv work?
The msgrcv() function reads a message from the queue associated with the message queue identifier specified by msqid and places it in the user-defined buffer pointed to by msgp. The received message is truncated to msgsz bytes if it is larger than msgsz and (msgflg & MSG_NOERROR) is non-zero.
Does msgrcv remove the message from the queue?
The msgrcv() system call removes a message from the queue specified by msqid and places it in the buffer pointed to by msgp.
What does msgrcv return in c?
Returned value If successful, msgrcv() returns a value equal to the number of bytes actually placed into the mtext field of the user-defined buffer pointed to by msgp. A value of zero indicates that only the mtype field was received from the message queue.
Is Msgrcv blocked?
The msgsnd() and msgrcv() functions can be performed as either blocking or non-blocking operations. Non-blocking operations allow for asynchronous message transfer — the process is not suspended as a result of sending or receiving a message. The process receives a signal. The queue is removed.
What is FTOK?
The ftok() function returns a key based on path and id that is usable in subsequent calls to msgget() , semget() , and shmget() . The path argument must be the path name of an existing file that the process is able to stat() . If a different id value is given, or a different file is given, a different key is returned.
What is Posix message queue?
POSIX message queues allow processes to exchange data in the form of messages. Each message queue is identified by a name of the form /somename; that is, a null-terminated string of up to NAME_MAX (i.e., 255) characters consisting of an initial slash, followed by one or more characters, none of which are slashes.
What is the difference between message queue and shared memory?
Shared memory can be deemed as faster (low overhead, high volume of data passing) then queues. But queues on the other hand, requires high overhead (the set up for making a queue to be permanent etc) with low volume of data.
Which command is used to know the status of a message queue in IPC?
# ipcs -a : It provides details about message queue, semaphore and shared memory. All the IPC facility has unique key and identifier, which is used to identify an IPC facility.
Should I use FTOK?
You cannot just use a “simple number” as you don’t know whether the token might be for example an index to an system-internal table or something. In other words, you don’t know how that token is used internally so you do need to use ftok .
What is the use of FTOK?
The ftok() function shall return a key based on path and id that is usable in subsequent calls to msgget(), semget(), and shmget(). The application shall ensure that the path argument is the pathname of an existing file that the process is able to stat().
Is shared memory faster than message passing?
Shared memory allows maximum speed and convenience of communication, as it can be done at memory speeds when within a computer. Shared memory is faster than message passing, as message-passing systems are typically implemented using system calls and thus require the more time-consuming task of kernel intervention.
Is the errno of msgctl set to eidrm?
The official msgctl doc says that errno is set to EIDRM (43) for the reader (the client) and I would like to display a custom error when it happens. But when I try to read a message from a closed server, the function msgrcv (msqid, &message, 64, 0, IPC_NOWAIT) returns the EINVAL error. I assume that msqid is guilty
How does eidrm work in msgget message queue?
The server is started and puts one message on a queue. Then waits. The client is started and reads the first message, then blocks waiting for the second message that never arrives. The server removes the queue while the client is waiting for the second message and an EIDRM return code is seen.
How are msgsnd and msgrcv used in System V?
The msgsnd () and msgrcv () system calls are used to send messages to, and receive messages from, a System V message queue. The calling process must have write permission on the message queue in order to send a message, and read permission to receive a message.
How does the msgrcv function in msqid work?
The msgrcv () function shall read a message from the queue associated with the message queue identifier specified by msqid and place it in the user-defined buffer pointed to by msgp .