msgrcv.3p (7292B)
- '\" et
- .TH MSGRCV "3P" 2017 "IEEE/The Open Group" "POSIX Programmer's Manual"
- .\"
- .SH PROLOG
- This manual page is part of the POSIX Programmer's Manual.
- The Linux implementation of this interface may differ (consult
- the corresponding Linux manual page for details of Linux behavior),
- or the interface may not be implemented on Linux.
- .\"
- .SH NAME
- msgrcv
- \(em XSI message receive operation
- .SH SYNOPSIS
- .LP
- .nf
- #include <sys/msg.h>
- .P
- ssize_t msgrcv(int \fImsqid\fP, void *\fImsgp\fP, size_t \fImsgsz\fP, long \fImsgtyp\fP,
- int \fImsgflg\fP);
- .fi
- .SH DESCRIPTION
- The
- \fImsgrcv\fR()
- function operates on XSI message queues (see the Base Definitions volume of POSIX.1\(hy2017,
- .IR "Section 3.226" ", " "Message Queue").
- It is unspecified whether this function interoperates with the
- realtime interprocess communication facilities defined in
- .IR "Section 2.8" ", " "Realtime".
- .P
- The
- \fImsgrcv\fR()
- function shall read a message from the queue associated with the message
- queue identifier specified by
- .IR msqid
- and place it in the user-defined buffer pointed to by
- .IR msgp .
- .P
- The application shall ensure that the argument
- .IR msgp
- points to a user-defined buffer that contains first a field of type
- .BR long
- specifying the type of the message, and then a data portion that holds
- the data bytes of the message. The structure below is an example of
- what this user-defined buffer might look like:
- .sp
- .RS 4
- .nf
- struct mymsg {
- long mtype; /* Message type. */
- char mtext[1]; /* Message text. */
- }
- .fi
- .P
- .RE
- .P
- The structure member
- .IR mtype
- is the received message's type as specified by the sending process.
- .P
- The structure member
- .IR mtext
- is the text of the message.
- .P
- The argument
- .IR msgsz
- specifies the size in bytes of
- .IR mtext .
- The received message shall be truncated to
- .IR msgsz
- bytes if it is larger than
- .IR msgsz
- and (\fImsgflg\fP & MSG_NOERROR) is non-zero.
- The truncated part of the message shall be lost and no indication of
- the truncation shall be given to the calling process.
- .P
- If the value of
- .IR msgsz
- is greater than
- {SSIZE_MAX},
- the result is implementation-defined.
- .P
- The argument
- .IR msgtyp
- specifies the type of message requested as follows:
- .IP " *" 4
- If
- .IR msgtyp
- is 0, the first message on the queue shall be received.
- .IP " *" 4
- If
- .IR msgtyp
- is greater than 0, the first message of type
- .IR msgtyp
- shall be received.
- .IP " *" 4
- If
- .IR msgtyp
- is less than 0, the first message of the lowest type that is less than
- or equal to the absolute value of
- .IR msgtyp
- shall be received.
- .P
- The argument
- .IR msgflg
- specifies the action to be taken if a message of the desired type is
- not on the queue. These are as follows:
- .IP " *" 4
- If (\fImsgflg\fP & IPC_NOWAIT)
- is non-zero, the calling thread shall return immediately with a return
- value of \-1 and
- .IR errno
- set to
- .BR [ENOMSG] .
- .IP " *" 4
- If (\fImsgflg\fP & IPC_NOWAIT) is 0, the calling thread shall suspend
- execution until one of the following occurs:
- .RS 4
- .IP -- 4
- A message of the desired type is placed on the queue.
- .IP -- 4
- The message queue identifier
- .IR msqid
- is removed from the system; when this occurs,
- .IR errno
- shall be set to
- .BR [EIDRM]
- and \-1 shall be returned.
- .IP -- 4
- The calling thread receives a signal that is to be caught; in this case
- a message is not received and the calling thread resumes execution in
- the manner prescribed in
- .IR "\fIsigaction\fR\^(\|)".
- .RE
- .P
- Upon successful completion, the following actions are taken with
- respect to the data structure associated with
- .IR msqid :
- .IP " *" 4
- .BR msg_qnum
- shall be decremented by 1.
- .IP " *" 4
- .BR msg_lrpid
- shall be set to the process ID of the calling process.
- .IP " *" 4
- .BR msg_rtime
- shall be set to the current time, as described in
- .IR "Section 2.7.1" ", " "IPC General Description".
- .SH "RETURN VALUE"
- Upon successful completion,
- \fImsgrcv\fR()
- shall return a value equal to the number of bytes actually placed
- into the buffer
- .IR mtext .
- Otherwise, no message shall be received,
- \fImsgrcv\fR()
- shall return \-1, and
- .IR errno
- shall be set to indicate the error.
- .SH ERRORS
- The
- \fImsgrcv\fR()
- function shall fail if:
- .TP
- .BR E2BIG
- The value of
- .IR mtext
- is greater than
- .IR msgsz
- and (\fImsgflg\fP & MSG_NOERROR) is 0.
- .TP
- .BR EACCES
- Operation permission is denied to the calling process; see
- .IR "Section 2.7" ", " "XSI Interprocess Communication".
- .TP
- .BR EIDRM
- The message queue identifier
- .IR msqid
- is removed from the system.
- .TP
- .BR EINTR
- The
- \fImsgrcv\fR()
- function was interrupted by a signal.
- .TP
- .BR EINVAL
- .IR msqid
- is not a valid message queue identifier.
- .TP
- .BR ENOMSG
- The queue does not contain a message of the desired type and
- (\fImsgflg\fP & IPC_NOWAIT) is non-zero.
- .LP
- .IR "The following sections are informative."
- .SH EXAMPLES
- .SS "Receiving a Message"
- .P
- The following example receives the first message on the queue (based on
- the value of the
- .IR msgtyp
- argument, 0). The queue is identified by the
- .IR msqid
- argument (assuming that the value has previously been set). This call
- specifies that an error should be reported if no message is available,
- but not if the message is too large. The message size is calculated
- directly using the
- .IR sizeof
- operator.
- .sp
- .RS 4
- .nf
- #include <sys/msg.h>
- \&...
- int result;
- int msqid;
- struct message {
- long type;
- char text[20];
- } msg;
- long msgtyp = 0;
- \&...
- result = msgrcv(msqid, (void *) &msg, sizeof(msg.text),
- msgtyp, MSG_NOERROR | IPC_NOWAIT);
- .fi
- .P
- .RE
- .SH "APPLICATION USAGE"
- The POSIX Realtime Extension defines alternative interfaces for interprocess communication
- (IPC). Application developers who need to use IPC should design their
- applications so that modules using the IPC routines described in
- .IR "Section 2.7" ", " "XSI Interprocess Communication"
- can be easily modified to use the alternative interfaces.
- .SH RATIONALE
- None.
- .SH "FUTURE DIRECTIONS"
- None.
- .SH "SEE ALSO"
- .IR "Section 2.7" ", " "XSI Interprocess Communication",
- .IR "Section 2.8" ", " "Realtime",
- .IR "\fImq_close\fR\^(\|)",
- .IR "\fImq_getattr\fR\^(\|)",
- .IR "\fImq_notify\fR\^(\|)",
- .IR "\fImq_open\fR\^(\|)",
- .IR "\fImq_receive\fR\^(\|)",
- .IR "\fImq_send\fR\^(\|)",
- .IR "\fImq_setattr\fR\^(\|)",
- .IR "\fImq_unlink\fR\^(\|)",
- .IR "\fImsgctl\fR\^(\|)",
- .IR "\fImsgget\fR\^(\|)",
- .IR "\fImsgsnd\fR\^(\|)",
- .IR "\fIsigaction\fR\^(\|)"
- .P
- The Base Definitions volume of POSIX.1\(hy2017,
- .IR "Section 3.226" ", " "Message Queue",
- .IR "\fB<sys_msg.h>\fP"
- .\"
- .SH COPYRIGHT
- Portions of this text are reprinted and reproduced in electronic form
- from IEEE Std 1003.1-2017, Standard for Information Technology
- -- Portable Operating System Interface (POSIX), The Open Group Base
- Specifications Issue 7, 2018 Edition,
- Copyright (C) 2018 by the Institute of
- Electrical and Electronics Engineers, Inc and The Open Group.
- In the event of any discrepancy between this version and the original IEEE and
- The Open Group Standard, the original IEEE and The Open Group Standard
- is the referee document. The original Standard can be obtained online at
- http://www.opengroup.org/unix/online.html .
- .PP
- Any typographical or formatting errors that appear
- in this page are most likely
- to have been introduced during the conversion of the source files to
- man page format. To report such errors, see
- https://www.kernel.org/doc/man-pages/reporting_bugs.html .