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msgrcv(2) [netbsd man page]

MSGRCV(2)						      BSD System Calls Manual							 MSGRCV(2)

NAME
msgrcv -- receive a message from a message queue LIBRARY
Standard C Library (libc, -lc) SYNOPSIS
#include <sys/msg.h> ssize_t msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp, int msgflg); DESCRIPTION
The msgrcv() function receives a message from the message queue specified in msqid, and places it into the user-defined structure pointed to by msgp. This structure must contain a first field of type long that will indicate the user-defined type of the message. The remaining fields will contain the contents of the message. The following is an example of what this user-defined structure might look like: struct mymsg { long mtype; /* message type */ char mtext[1]; /* body of message */ }; mtype is an integer greater than 0 that can be used to select messages. mtext is an array of bytes, with size up to the system limit MSGMAX. The value of msgtyp has one of the following meanings: o msgtyp is greater than 0. The first message of type msgtyp will be received. o msgtyp is equal to 0. The first message on the queue will be received. o msgtyp is less than 0. The first message of the lowest message type that is less than or equal to the absolute value of msgtyp will be received. msgsz specifies the maximum length of the requested message. If the received message has a length greater than msgsz it will be silently truncated if the MSG_NOERROR flag is set in msgflg, otherwise an error will be returned. If no matching message is present on the message queue specified by msqid, the behaviour of msgrcv() depends on whether the IPC_NOWAIT flag is set in msgflg or not. If IPC_NOWAIT is set, then msgrcv() will immediately return a value of -1 and set errno to EAGAIN. If IPC_NOWAIT is not set, the calling process will block until: o A message of the requested type becomes available on the message queue. o The message queue is removed, in which case -1 will be returned and errno set to EIDRM. o A signal is received and caught. -1 is returned and errno is set to EINTR. If a message is successfully received, the data structure associated with msqid is updated as follows: o msg_lrpid is set to the pid of the caller. o msg_lrtime is set to the current time. o msg_qnum is decremented by 1. RETURN VALUES
Upon successful completion, msgrcv() returns the number of bytes received into the mtext field of the structure pointed to by msgp. Other- wise, -1 is returned, and errno set to indicate the error. ERRORS
msgrcv() will fail if: [E2BIG] A matching message was received, but its size was greater than msgsz and the MSG_NOERROR flag was not set in msgflg. [EACCES] The calling process does not have read access to the message queue. [EAGAIN] There is no message of the requested type available on the message queue, and IPC_NOWAIT is set in msgflg. [EFAULT] msgp points to an invalid address. [EIDRM] The message queue identifier msqid is removed from the system. [EINTR] The system call was interrupted by the delivery of a signal. [EINVAL] msqid is not a valid message queue identifier The message queue was removed while msgrcv() was waiting for a message of the requested type to become available in it. msgsz is greater than SSIZE_MAX. [ENOMSG] The queue does not contain a message of the desired type and IPC_NOWAIT is set. SEE ALSO
msgctl(2), msgget(2), msgsnd(2) STANDARDS
The msgrcv system call conforms to X/Open System Interfaces and Headers Issue 5 (``XSH5''). HISTORY
Message queues appeared in the first release of AT&T System V UNIX. BSD
January 26, 2009 BSD

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msgrcv(2)							   System Calls 							 msgrcv(2)

NAME
msgrcv - message receive operation SYNOPSIS
#include <sys/msg.h> ssize_t msgrcv(int msqid, void *msgp, size_t msgsz, long int msgtyp, int msgflg); DESCRIPTION
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 mes- sage, 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 like: 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 received. 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 ENOMSG. 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 process. o msg_rtime is set equal to the current time. RETURN VALUES
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. ERRORS
The msgrcv() function will fail if: E2BIG The value of mtext is greater than msgsz and (msgflg&MSG_NOERROR) is 0. EACCES Operation permission is denied to the calling process. See Intro(2). EIDRM The message queue identifier msqid is removed from the system. EINTR The msgrcv() function was interrupted by a signal. EINVAL The msqid argument is not a valid message queue identifier. ENOMSG The queue does not contain a message of the desired type and (msgflg&IPC_NOWAIT) is non-zero. The msgrcv() function may fail if: EFAULT The msgp argument points to an illegal address. USAGE
The value passed as the msgp argument should be converted to type void *. ATTRIBUTES
See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Interface Stability |Standard | +-----------------------------+-----------------------------+ SEE ALSO
Intro(2), msgctl(2), msgget(2), msgsnd(2), sigaction(2), attributes(5), standards(5) SunOS 5.11 19 May 1999 msgrcv(2)

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