MSGRCV(2) System Calls MSGRCV(2)


msgrcv - message receive operation


#include <sys/msg.h>

ssize_t msgrcv(int msqid, void *msgp, size_t msgsz,
long int msgtyp, int msgflg);


The msgrcv() function reads a message from the queue associated with the
message queue identifier specified by msqid and places it in the user-
defined buffer pointed to by msgp.

The msgp argument points to a user-defined buffer that must contain first
a field of type long int that will specify the type of the message, and
then a data portion that will hold the data bytes of the message. The
structure below is an example of what this user-defined buffer might look

struct mymsg {
long int mtype; /* message type */
char mtext[1]; /* message text */

The mtype member is the received message's type as specified by the
sending process.

The mtext member is the text of the message.

The msgsz argument specifies the size in bytes of mtext. The received
message is truncated to msgsz bytes if it is larger than msgsz and
(msgflg&MSG_NOERROR) is non-zero. The truncated part of the message is
lost and no indication of the truncation is given to the calling process.

The msgtyp argument specifies the type of message requested as follows:

o If msgtyp is 0, the first message on the queue is received.

o If msgtyp is greater than 0, the first message of type msgtyp
is received.

o If msgtyp is less than 0, the first message of the lowest type
that is less than or equal to the absolute value of msgtyp is

The msgflg argument specifies which of the following actions is to be
taken if a message of the desired type is not on the queue:

o If (msgflg&IPC_NOWAIT) is non-zero, the calling process will
return immediately with a return value of -1 and errno set to

o If (msgflg&IPC_NOWAIT) is 0, the calling process will suspend
execution until one of the following occurs:

o A message of the desired type is placed on the queue.

o The message queue identifier msqid is removed from the
system (see msgctl(2)); when this occurs, errno is set
equal to EIDRM and -1 is returned.

o The calling process receives a signal that is to be
caught; in this case a message is not received and the
calling process resumes execution in the manner prescribed
in sigaction(2).

Upon successful completion, the following actions are taken with respect
to the data structure associated with msqid (see Intro(2)):

o msg_qnum is decremented by 1.

o msg_lrpid is set equal to the process ID of the calling

o msg_rtime is set equal to the current time.


Upon successful completion, msgrcv() returns a value equal to the number
of bytes actually placed into the buffer mtext. Otherwise, -1 is
returned, no message is received, and errno is set to indicate the error.


The msgrcv() function will fail if:

The value of mtext is greater than msgsz and
(msgflg&MSG_NOERROR) is 0.

Operation permission is denied to the calling process. See

The message queue identifier msqid is removed from the system.

The msgrcv() function was interrupted by a signal.

The msqid argument is not a valid message queue identifier.

The queue does not contain a message of the desired type and
(msgflg&IPC_NOWAIT) is non-zero.

The msgrcv() function may fail if:

The msgp argument points to an illegal address.


The value passed as the msgp argument should be converted to type void *.


See attributes(7) for descriptions of the following attributes:

|Interface Stability | Standard |


Intro(2), msgctl(2), msgget(2), msgsnd(2), sigaction(2), attributes(7),

May 19, 1999 MSGRCV(2)