Copyright (c) 1995 Frank van der Linden
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$FreeBSD: releng/10.2/lib/libc/sys/msgrcv.2 195656 2009-07-13 12:53:43Z trasz $
/
.Dd July 9, 2009 .Dt MSGRCV 2 .Os .Sh NAME .Nm msgrcv .Nd receive a message from a message queue .Sh LIBRARY .Lb libc .Sh SYNOPSIS n sys/types.h n sys/ipc.h n sys/msg.h .Ft int .Fn msgrcv "int msqid" "void *msgp" "size_t msgsz" "long msgtyp" "int msgflg" .Sh DESCRIPTION The .Fn msgrcv function 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: d -literal long mtype; /* message type */ char mtext[1]; /* body of message */ .Ed
p .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
f ( Dv MSGMAX ) .
p The value of .Fa msgtyp has one of the following meanings: l -bullet t The .Fa msgtyp argument is greater than 0. The first message of type .Fa msgtyp will be received. t The .Fa msgtyp argument is equal to 0. The first message on the queue will be received. t 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
p 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.
p 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 ENOMSG . If .Dv IPC_NOWAIT is not set, the calling process will be blocked until: l -bullet t A message of the requested type becomes available on the message queue. t The message queue is removed, in which case -1 will be returned, and .Va errno set to .Er EINVAL . t A signal is received and caught. -1 is returned, and .Va errno set to .Er EINTR . .El
p If a message is successfully received, the data structure associated with .Fa msqid is updated as follows: l -bullet t .Va msg_cbytes is decremented by the size of the message. t .Va msg_lrpid is set to the pid of the caller. t .Va msg_lrtime is set to the current time. t .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 ERRORS The .Fn msgrcv function will fail if: l -tag -width Er t Bq Er EINVAL The .Fa msqid argument is not a valid message queue identifier.
p The message queue was removed while .Fn msgrcv was waiting for a message of the requested type to become available on it.
p The .Fa msgsz argument is less than 0. t 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 . t Bq Er EACCES The calling process does not have read access to the message queue. t Bq Er EFAULT The .Fa msgp argument points to an invalid address. t Bq Er EINTR The system call was interrupted by the delivery of a signal. t Bq Er ENOMSG 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 .