.\" Copyright 1993 Giorgio Ciucci .\" .\" Permission is granted to make and distribute verbatim copies of this .\" manual provided the copyright notice and this permission notice are .\" preserved on all copies. .\" .\" Permission is granted to copy and distribute modified versions of this .\" manual under the conditions for verbatim copying, provided that the .\" entire resulting derived work is distributed under the terms of a .\" permission notice identical to this one. .\" .\" Since the Linux kernel and libraries are constantly changing, this .\" manual page may be incorrect or out-of-date. The author(s) assume no .\" responsibility for errors or omissions, or for damages resulting from .\" the use of the information contained herein. The author(s) may not .\" have taken the same level of care in the production of this manual, .\" which is licensed free of charge, as they might when working .\" professionally. .\" .\" Formatted or processed versions of this manual, if unaccompanied by .\" the source, must acknowledge the copyright and authors of this work. .\" .\" Modified Tue Oct 22 16:40:11 1996 by Eric S. Raymond .\" Modified Mon Jul 10 21:09:59 2000 by aeb .\" Modified 1 Jun 2002, Michael Kerrisk .\" Language clean-ups. .\" Enhanced and corrected information on msg_qbytes, MSGMNB and MSGMAX .\" Added note on restart behaviour of msgsnd and msgrcv .\" Formatting clean-ups (argument and field names marked as .I .\" instead of .B) .\" Modified, 27 May 2004, Michael Kerrisk .\" Added notes on capability requirements .\" .TH MSGOP 2 2004-05-27 "Linux 2.6.6" "Linux Programmer's Manual" .SH NAME msgop \- message operations .SH SYNOPSIS .nf .B #include .br .B #include .br .B #include .fi .sp .BI "int msgsnd(int " msqid , .BI "struct msgbuf *" msgp , .BI "size_t " msgsz , .BI "int " msgflg ); .sp .BI "ssize_t msgrcv(int " msqid , .BI "struct msgbuf *" msgp , .BI "size_t " msgsz , .BI "long " msgtyp , .BI "int " msgflg ); .SH DESCRIPTION To send or receive a message, the calling process allocates a structure of the following general form: .sp .B struct msgbuf { .br .B long mtype; /* message type, must be > 0 */ .br .B char mtext[1]; /* message data */ .br .B }; .sp The .I mtext field is an array (or other structure) whose size is specified by .IR msgsz , a non-negative integer value. Messages of zero length (i.e., no .I mtext field) are permitted. The .I mtype field must have a strictly positive integer value that can be used by the receiving process for message selection (see the section about .BR msgrcv ). .PP The calling process must have write permission to send and read permission to receive a message on the queue. .PP The .B msgsnd system call appends a copy of the message pointed to by .I msgp to the message queue whose identifier is specified by .IR msqid . .PP If sufficient space is available on the queue, .B msgsnd succeeds immediately. (The queue capacity is defined by the .I msg_bytes field in the associated data structure for the message queue. During queue creation this field is initialised to .B MSGMNB bytes, but this limit can be modified using .BR msgctl .) If insufficient space is available on the queue, then the default behaviour of .B msgsnd is to block until space becomes available. If .B IPC_NOWAIT is asserted in .B msgflg then the call instead fails with the error .BR EAGAIN . A blocked .B msgsnd call may also fail if the queue is removed (in which case the system call fails with .I errno set to .BR EIDRM ), or a signal is caught (in which case the system call fails with .I errno set to .BR EINTR ). .RB ( msgsnd " and " msgrcv are never automatically restarted after being interrupted by a signal handler, regardless of the setting of the .B SA_RESTART flag when establishing a signal handler.) .PP Upon successful completion the message queue data structure is updated as follows: .IP .I msg_lspid is set to the process ID of the calling process. .IP .I msg_qnum is incremented by 1. .IP .I msg_stime is set to the current time. .PP The system call .B msgrcv reads a message from the message queue specified by .I msqid into the .I msgbuf pointed to by the .I msgp argument, removing the read message from the queue. .PP The argument .I msgsz specifies the maximum size in bytes for the member .I mtext of the structure pointed to by the .I msgp argument. If the message text has length greater than .IR msgsz , then if the .I msgflg argument asserts .BR MSG_NOERROR , the message text will be truncated (and the truncated part will be lost), otherwise the message isn't removed from the queue and the system call fails returning with .I errno set to .BR E2BIG . .PP The argument .I msgtyp specifies the type of message requested as follows: .IP If .I msgtyp is .BR 0 , then the first message in the queue is read. .IP If .I msgtyp is greater than .BR 0 , then the first message on the queue of type .I msgtyp is read, unless .B MSG_EXCEPT was asserted in .IR msgflg , in which case the first message on the queue of type not equal to .I msgtyp will be read. .IP If .I msgtyp is less than .BR 0 , then the first message on the queue with the lowest type less than or equal to the absolute value of .I msgtyp will be read. .PP The .I msgflg argument asserts none, one or more (or\-ing them) of the following flags: .IP .B IPC_NOWAIT For immediate return if no message of the requested type is on the queue. The system call fails with errno set to .BR ENOMSG . .IP .B MSG_EXCEPT Used with .I msgtyp greater than .B 0 to read the first message on the queue with message type that differs from .IR msgtyp . .IP .B MSG_NOERROR To truncate the message text if longer than .I msgsz bytes. .PP If no message of the requested type is available and .B IPC_NOWAIT isn't asserted in .IR msgflg , the calling process is blocked until one of the following conditions occurs: .IP A message of the desired type is placed on the queue. .IP The message queue is removed from the system. In this case the system call fails with .I errno set to .BR EIDRM . .IP The calling process catches a signal. In this case the system call fails with .I errno set to .BR EINTR . .PP Upon successful completion the message queue data structure is updated as follows: .IP .I msg_lrpid is set to the process ID of the calling process. .IP .I msg_qnum is decremented by 1. .IP .I msg_rtime is set to the current time. .SH "RETURN VALUE" On a failure both functions return .B \-1 with .I errno indicating the error, otherwise .B msgsnd returns .B 0 and .B msgrvc returns the number of bytes actually copied into the .I mtext array. .SH ERRORS When .B msgsnd fails, at return .I errno will be set to one among the following values: .TP .B EACCES The calling process does not have write permission on the message queue, and does not have the .BR CAP_IPC_OWNER capability. .TP 11 .B EAGAIN The message can't be sent due to the .I msg_qbytes limit for the queue and .B IPC_NOWAIT was asserted in .IR mgsflg . .TP .B EFAULT The address pointed to by .I msgp isn't accessible. .TP .B EIDRM The message queue was removed. .TP .B EINTR Sleeping on a full message queue condition, the process caught a signal. .TP .B EINVAL Invalid .I msqid value, or nonpositive .I mtype value, or invalid .I msgsz value (less than 0 or greater than the system value .BR MSGMAX ). .TP .B ENOMEM The system has not enough memory to make a copy of the supplied .IR msgbuf . .PP When .B msgrcv fails, at return .I errno will be set to one among the following values: .TP 11 .B E2BIG The message text length is greater than .I msgsz and .B MSG_NOERROR isn't asserted in .IR msgflg . .TP .B EACCES The calling process does not have read permission on the message queue, and does not have the .BR CAP_IPC_OWNER capability. .TP .B EFAULT The address pointed to by .I msgp isn't accessible. .TP .B EIDRM While the process was sleeping to receive a message, the message queue was removed. .TP .B EINTR While the process was sleeping to receive a message, the process received a signal that had to be caught. .TP .B EINVAL Illegal .I msgqid value, or .I msgsz less than .BR 0 . .TP .B ENOMSG .B IPC_NOWAIT was asserted in .I msgflg and no message of the requested type existed on the message queue. .SH NOTES The followings are system limits affecting a .B msgsnd system call: .TP 11 .B MSGMAX Maximum size for a message text: the implementation set this value to 8192 bytes. .TP .B MSGMNB Default maximum size in bytes of a message queue: 16384 bytes. The super\-user can increase the size of a message queue beyond .B MSGMNB by a .B msgctl system call. .PP The implementation has no intrinsic limits for the system wide maximum number of message headers .RB ( MSGTQL ) and for the system wide maximum size in bytes of the message pool .RB ( MSGPOOL ). .SH "CONFORMING TO" SVr4, SVID. .SH NOTE The pointer argument is declared as \fIstruct msgbuf *\fP with libc4, libc5, glibc 2.0, glibc 2.1. It is declared as \fIvoid *\fP (\fIconst void *\fP for \fImsgsnd()\fP) with glibc 2.2, following the SUSv2. .SH "SEE ALSO" .BR msgctl (2), .BR msgget (2), .BR msgrcv (2), .BR msgsnd (2), .BR ipc (5), .BR capabilities (7)