NAME
msgrcv, msgsnd - message operations
SYNOPSIS
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>
int msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg);
ssize_t msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp,
int msgflg);
DESCRIPTION
The msgsnd() and msgrcv() system calls are used, respectively, to send
messages to, and receive messages from, a message queue. The calling
process must have write permission on the message queue in order to
send a message, and read permission to receive a message.
The msgp argument is a pointer to caller-defined 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
specified by msgsz, a nonnegative integer value. Messages of zero
length (i.e., no mtext field) are permitted. The mtype field must have
a strictly positive integer value. This value can be used by the
receiving process for message selection (see the description of
msgrcv() below).
msgsnd()
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 in the queue, msgsnd() succeeds
immediately. (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 initialized to MSGMNB bytes, but this limit can
be modified using msgctl(2).) If insufficient space is available in
the queue, then the default behavior of msgsnd() is to block until
space becomes available. If IPC_NOWAIT is specified 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;see signal(7). (msgsnd() is never automatically
restarted after being interrupted 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.
msgrcv()
The msgrcv() system call removes a message from the queue specified by
msqid and places it in the buffer pointed to by msgp.
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 the behavior depends on
whether MSG_NOERROR is specified in msgflg. If MSG_NOERROR is
specified, then the message text will be truncated (and the truncated
part will be lost); if MSG_NOERROR is not specified, then the message
isn’t removed from the queue and the system call fails returning -1
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 in the queue of
type msgtyp is read, unless MSG_EXCEPT was specified in msgflg, in
which case the first message in the queue of type not equal to msgtyp
will be read.
* If msgtyp is less than 0, then the first message in the queue with
the lowest type less than or equal to the absolute value of msgtyp
will be read.
The msgflg argument is a bit mask constructed by ORing together zero or
more of the following flags:
IPC_NOWAIT
Return immediately if no message of the requested type is in the
queue. The system call fails with errno set to ENOMSG.
MSG_EXCEPT
Used with msgtyp greater than 0 to read the first message in 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
specified in msgflg, the calling process is blocked until one of the
following conditions occurs:
* A message of the desired type is placed in 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. (msgrcv() is never automatically
restarted after being interrupted 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_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 failure both functions return -1 with errno indicating the error,
otherwise msgsnd() returns 0 and msgrcv() returns the number of bytes
actually copied into the mtext array.
ERRORS
When msgsnd() fails, errno will be set to one among the following
values:
EACCES The calling process does not have write permission on the
message queue, and does not have the CAP_IPC_OWNER capability.
EAGAIN The message can’t be sent due to the msg_qbytes limit for the
queue and IPC_NOWAIT was specified in msgflg.
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 does not have enough memory to make a copy of the
message pointed to by msgp.
When msgrcv() fails, errno will be set to one among the following
values:
E2BIG The message text length is greater than msgsz and MSG_NOERROR
isn’t specified in msgflg.
EACCES The calling process does not have read permission on the message
queue, and does not have the CAP_IPC_OWNER capability.
EAGAIN No message was available in the queue and IPC_NOWAIT was
specified in msgflg.
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
caught a signal; see signal(7).
EINVAL msgqid was invalid, or msgsz was less than 0.
ENOMSG IPC_NOWAIT was specified in msgflg and no message of the
requested type existed on the message queue.
CONFORMING TO
SVr4, POSIX.1-2001.
NOTES
The msgp argument is declared as struct msgbuf * with libc4, libc5,
glibc 2.0, glibc 2.1. It is declared as void * with glibc 2.2 and
later, as required by SUSv2 and SUSv3.
The following limits on message queue resources affect the msgsnd()
call:
MSGMAX Maximum size for a message text: 8192 bytes (on Linux, this
limit can be read and modified via /proc/sys/kernel/msgmax).
MSGMNB Default maximum size in bytes of a message queue: 16384 bytes
(on Linux, this limit can be read and modified via
/proc/sys/kernel/msgmnb). The superuser can increase the size
of a message queue beyond MSGMNB by a msgctl(2) 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).
SEE ALSO
msgctl(2), msgget(2), capabilities(7), mq_overview(7), svipc(7)
COLOPHON
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