.\" $NetBSD: msgrcv.2,v 1.1 1995/10/16 23:49:20 jtc Exp $ .\" .\" Copyright (c) 1995 Frank van der Linden .\" All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" 3. All advertising materials mentioning features or use of this software .\" must display the following acknowledgement: .\" This product includes software developed for the NetBSD Project .\" by Frank van der Linden .\" 4. The name of the author may not be used to endorse or promote products .\" derived from this software without specific prior written permission .\" .\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR .\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES .\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. .\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, .\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT .\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, .\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY .\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT .\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF .\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. .\" .\" $FreeBSD: src/lib/libc/gen/msgrcv.3,v 1.8.2.7 2003/03/15 15:11:05 trhodes Exp $ .\" .Dd January 4, 2014 .Dt MSGRCV 2 .Os .Sh NAME .Nm msgrcv .Nd receive a message from a message queue .Sh LIBRARY .Lb libc .Sh SYNOPSIS .In sys/types.h .In sys/ipc.h .In sys/msg.h .Ft int .Fn msgrcv "int msqid" "void *msgp" "size_t msgsz" "long msgtyp" "int msgflg" .Sh DESCRIPTION The .Fn msgrcv system call receives a message from the message queue specified in .Fa msqid , and places it into the structure pointed to by .Fa msgp . This structure should consist of the following members: .Bd -literal long mtype; /* message type */ char mtext[1]; /* body of message */ .Ed .Pp .Va mtype is an integer greater than 0 that can be used for selecting messages, .Va mtext is an array of bytes, with a size up to that of the system limit .Pf ( Dv MSGMAX ) . .Pp The value of .Fa msgtyp has one of the following meanings: .Bl -bullet .It The .Fa msgtyp argument is greater than 0. The first message of type .Fa msgtyp will be received. .It The .Fa msgtyp argument is equal to 0. The first message on the queue will be received. .It The .Fa msgtyp argument is less than 0. The first message of the lowest message type that is less than or equal to the absolute value of .Fa msgtyp will be received. .El .Pp The .Fa msgsz argument specifies the maximum length of the requested message. If the received message has a length greater than .Fa msgsz it will be silently truncated if the .Dv MSG_NOERROR flag is set in .Fa msgflg , otherwise an error will be returned. .Pp If no matching message is present on the message queue specified by .Fa msqid , the behavior of .Fn msgrcv depends on whether the .Dv IPC_NOWAIT flag is set in .Fa msgflg or not. If .Dv IPC_NOWAIT is set, .Fn msgrcv will immediately return a value of -1, and set .Va errno to .Er EAGAIN . If .Dv IPC_NOWAIT is not set, the calling process will be blocked until: .Bl -bullet .It A message of the requested type becomes available on the message queue. .It The message queue is removed, in which case -1 will be returned, and .Va errno set to .Er EINVAL . .It A signal is received and caught. -1 is returned, and .Va errno set to .Er EINTR . .El .Pp If a message is successfully received, the data structure associated with .Fa msqid is updated as follows: .Bl -bullet .It .Va msg_cbytes is decremented by the size of the message. .It .Va msg_lrpid is set to the pid of the caller. .It .Va msg_lrtime is set to the current time. .It .Va msg_qnum is decremented by 1. .El .Sh RETURN VALUES Upon successful completion, .Fn msgrcv returns the number of bytes received into the .Va mtext field of the structure pointed to by .Fa msgp . Otherwise, -1 is returned, and .Va errno set to indicate the error. .Sh ENVIRONMENT The XSI Interprocess Communication family of functions is also available as an implementation in userspace. To use it, the .Xr sysvipcd 8 daemon has to be running. .Pp If the .Ev USR_SYSVIPC variable is set in a process' environment, the process and its children will use the userspace implementation. .Sh ERRORS The .Fn msgrcv system call will fail if: .Bl -tag -width Er .It Bq Er EINVAL The .Fa msqid argument is not a valid message queue identifier. .Pp The message queue was removed while .Fn msgrcv was waiting for a message of the requested type to become available on it. .Pp The .Fa msgsz argument is less than 0. .It Bq Er E2BIG A matching message was received, but its size was greater than .Fa msgsz and the .Dv MSG_NOERROR flag was not set in .Fa msgflg . .It Bq Er EACCES The calling process does not have read access to the message queue. .It Bq Er EFAULT The .Fa msgp argument points to an invalid address. .It Bq Er EINTR The system call was interrupted by the delivery of a signal. .It Bq Er EAGAIN There is no message of the requested type available on the message queue, and .Dv IPC_NOWAIT is set in .Fa msgflg . .El .Sh SEE ALSO .Xr msgctl 2 , .Xr msgget 2 , .Xr msgsnd 2 .Sh HISTORY Message queues appeared in the first release of .At V . .Sh AUTHORS .An -nosplit The .Dx specific userspace implementation (see .Sx ENVIRONMENT ) was written by .An Larisa Grigore . .Sh BUGS .Nx , .Dx , and .Fx do not define the .Er EIDRM error value, which should be used in the case of a removed message queue, nor the .Er ENOMSG value, which should be used when no suitable message is available and .Dv IPC_NOWAIT is set.