MSGRCV(2) BSD Programmer's Manual MSGRCV(2)
msgrcv - receive a message from a message queue
#include <sys/types.h> #include <sys/ipc.h> #include <sys/msg.h> int msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp, int msgflg);
The msgrcv() function receives a message from the message queue specified in msqid, and places it into the structure pointed to by msgp. This structure should consist of the following members: long mtype; /* message type */ char mtext; /* body of message */ mtype is an integer greater than 0 that can be used for selecting mes- sages, mtext is an array of bytes, with a size up to that of the system limit (MSGMAX). The value of msgtyp has one of the following meanings: • msgtyp is greater than 0. The first message of type msgtyp will be received. • msgtyp is equal to 0. The first message on the queue will be re- ceived. • 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 re- ceived message has a length greater than msgsz it will be silently trun- cated 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 behavior of msgrcv() depends on whether the IPC_NOWAIT flag is set in msgflg or not. If IPC_NOWAIT is set, msgrcv() will immediately re- turn a value of -1, and set errno to EAGAIN. If IPC_NOWAIT is not set, the calling process will be blocked until: • A message of the requested type becomes available on the message queue. • The message queue is removed, in which case -1 will be returned, and errno set to EINVAL. • A signal is received and caught. -1 is returned, and errno set to EINTR. If a message is successfully received, the data structure associated with msqid is updated as follows: • msg_cbytes is decremented by the size of the message. • msg_lrpid is set to the pid of the caller. • msg_lrtime is set to the current time. • msg_qnum is decremented by 1.
Upon successful completion, msgrcv() returns the number of bytes received into the mtext field of the structure pointed to by msgp. Otherwise, -1 is returned, and errno set to indicate the error.
msgrcv() will fail if: [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 on it. msgsz is less than 0. [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 mes- sage queue. [EFAULT] msgp points to an invalid address. [EINTR] The system call was interrupted by the delivery of a sig- nal. [EAGAIN] There is no message of the requested type available on the message queue, and IPC_NOWAIT is set in msgflg.
msgctl(2), msgget(2), msgsnd(2)
Message queues appeared in the first release of AT&T Unix System V.
OpenBSD does not define the EIDRM error value, which should be used in the case of a removed message queue, nor the ENOMSG value, which should be used when no suitable message is available and IPC_NOWAIT is set. MirOS BSD #10-current August 17, 1995 1
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