msgrcv
MSGOP(2) Linux Programmer's Manual MSGOP(2)
NAME
msgop - message operations
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
int msgsnd(int msqid, struct msgbuf *msgp, size_t msgsz, int msgflg);
ssize_t msgrcv(int msqid, struct msgbuf *msgp, size_t msgsz, long msg-
typ, int msgflg);
DESCRIPTION
To send or receive a message, the calling process allocates a structure
of the following general form:
struct msgbuf {
long mtype; /* message type, must be > 0 */
char mtext[1]; /* message data */
};
The mtext field is an array (or other structure) whose size is speci-
fied by msgsz, a non-negative integer value. Messages of zero length
(i.e., no mtext field) are permitted. The mtype field must have a
strictly positive integer value that can be used by the receiving pro-
cess for message selection (see the section about msgrcv).
The calling process must have write permission to send and read permis-
sion to receive a message on the queue.
The msgsnd system call appends a copy of the message pointed to by msgp
to the message queue whose identifier is specified by msqid.
If sufficient space is available on the queue, msgsnd succeeds immedi-
ately. (The queue capacity is defined by the msg_bytes field in the
associated data structure for the message queue. During queue creation
this field is initialised to MSGMNB bytes, but this limit can be modi-
fied using msgctl.) If insufficient space is available on the queue,
then the default behaviour of msgsnd is to block until space becomes
available. If IPC_NOWAIT is asserted in msgflg then the call instead
fails with the error EAGAIN.
A blocked msgsnd call may also fail if the queue is removed (in which
case the system call fails with errno set to EIDRM), or a signal is
caught (in which case the system call fails with errno set to EINTR).
(msgsnd and msgrcv are never automatically restarted after being inter-
rupted by a signal handler, regardless of the setting of the
SA_RESTART flag when establishing a signal handler.)
Upon successful completion the message queue data structure is updated
as follows:
msg_lspid is set to the process ID of the calling process.
msg_qnum is incremented by 1.
msg_stime is set to the current time.
The system call msgrcv reads a message from the message queue specified
by msqid into the msgbuf pointed to by the msgp argument, removing the
read message from the queue.
The argument msgsz specifies the maximum size in bytes for the member
mtext of the structure pointed to by the msgp argument. If the message
text has length greater than msgsz, then if the msgflg argument asserts
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 errno set to E2BIG.
The argument msgtyp specifies the type of message requested as follows:
If msgtyp is 0, then the first message in the queue is read.
If msgtyp is greater than 0, then the first message on the queue
of type msgtyp is read, unless MSG_EXCEPT was asserted in
msgflg, in which case the first message on the queue of type not
equal to msgtyp will be read.
If msgtyp is less than 0, then the first message on the queue
with the lowest type less than or equal to the absolute value of
msgtyp will be read.
The msgflg argument asserts none, one or more (or-ing them) of the fol-
lowing flags:
IPC_NOWAIT For immediate return if no message of the requested
type is on the queue. The system call fails with errno set to
ENOMSG.
MSG_EXCEPT Used with msgtyp greater than 0 to read the first
message on the queue with message type that differs from msgtyp.
MSG_NOERROR To truncate the message text if longer than msgsz
bytes.
If no message of the requested type is available and IPC_NOWAIT isn't
asserted in msgflg, the calling process is blocked until one of the
following conditions occurs:
A message of the desired type is placed on the queue.
The message queue is removed from the system. In this case the
system call fails with errno set to EIDRM.
The calling process catches a signal. In this case the system
call fails with errno set to EINTR.
Upon successful completion the message queue data structure is updated
as follows:
msg_lrpid is set to the process ID of the calling process.
msg_qnum is decremented by 1.
msg_rtime is set to the current time.
RETURN VALUE
On a failure both functions return -1 with errno indicating the error,
otherwise msgsnd returns 0 and msgrvc returns the number of bytes actu-
ally copied into the mtext array.
ERRORS
When msgsnd fails, at return errno will be set to one among the follow-
ing values:
EAGAIN The message can't be sent due to the msg_qbytes limit for
the queue and IPC_NOWAIT was asserted in mgsflg.
EACCES The calling process has no write permission on the message
queue.
EFAULT The address pointed to by msgp isn't accessible.
EIDRM The message queue was removed.
EINTR Sleeping on a full message queue condition, the process
caught a signal.
EINVAL Invalid msqid value, or nonpositive mtype value, or invalid
msgsz value (less than 0 or greater than the system value
MSGMAX).
ENOMEM The system has not enough memory to make a copy of the sup-
plied msgbuf.
When msgrcv fails, at return errno will be set to one among the follow-
ing values:
E2BIG The message text length is greater than msgsz and MSG_NOER-
ROR isn't asserted in msgflg.
EACCES The calling process does not have read permission on the
message queue.
EFAULT The address pointed to by msgp isn't accessible.
EIDRM While the process was sleeping to receive a message, the
message queue was removed.
EINTR While the process was sleeping to receive a message, the
process received a signal that had to be caught.
EINVAL Illegal msgqid value, or msgsz less than 0.
ENOMSG IPC_NOWAIT was asserted in msgflg and no message of the
requested type existed on the message queue.
NOTES
The followings are system limits affecting a msgsnd system call:
MSGMAX Maximum size for a message text: the implementation set this
value to 8192 bytes.
MSGMNB Default maximum size in bytes of a message queue: 16384
bytes. The super-user can increase the size of a message
queue beyond MSGMNB by a msgctl system call.
The implementation has no intrinsic limits for the system wide maximum
number of message headers (MSGTQL) and for the system wide maximum size
in bytes of the message pool (MSGPOOL).
CONFORMING TO
SVr4, SVID.
NOTE
The pointer argument is declared as struct msgbuf * with libc4, libc5,
glibc 2.0, glibc 2.1. It is declared as void * (const void * for
msgsnd()) with glibc 2.2, following the SUSv2.
SEE ALSO
ipc(5), msgctl(2), msgget(2), msgrcv(2), msgsnd(2)
Linux 2.4.18 2002-06-01 MSGOP(2)
