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msgsnd(2) System Calls Manual msgsnd(2) NAME msgsnd - Sends a message to a message queue SYNOPSIS #include <sys/msg.h> int msgsnd( int msqid, const void *msgp, size_t msgsz, int msgflg); Application developers may want to specify #include statements for <sys/types.h> and <sys/ipc.h> before the one for <sys/msg.h> if pro- grams are being developed for multiple platforms. The additional #in- clude statements are not required on DIGITAL UNIX systems or by ISO or X/Open standards, but may be required on other vendors' systems that conform to these standards. STANDARDS Interfaces documented on this reference page conform to industry stan- dards as follows: msgsnd(): XPG4, XPG4-UNIX Refer to the standards(5) reference page for more information about in- dustry standards and associated tags. PARAMETERS Specifies the ID of the message queue on which to place the message. The ID is typically returned by a previous msgget() function. Speci- fies a pointer to the msgbuf structure that contains the message. Specifies the size of the data array in the msgbuf structure. Speci- fies the action to be taken by the kernel if it runs out of internal buffer space. DESCRIPTION The msgsnd() function sends a message to the queue associated with the msqid parameter. When the kernel sends a message, it allocates space for the message and copies the data from user space. The kernel then allocates a structure of type msg (message header), sets its fields, and inserts the structure at the end of the message queue associated with the message queue ID. The msg structure is defined as follows: struct msg { struct msg *msg_next; long msg_type; long msg_ts; caddr_t msg_addr; }; The msg_next field is a pointer to the next message in the queue. The msg_type field is the message type supplied in the user-specified msg- buf structure. The msg_ts field is the size of the message text. The msg_addr field is the address of the message text. You pass the message that the kernel adds to the message queue in the msgp parameter. This parameter points to a msgbuf structure that you define as follows: struct msgbuf { long int mtype; char mtext[]; } The mytpe field is a positive integer that indicates the type of the message. A receiving process can use this message type to select only those messages it wants to receive from the queue. For more informa- tion about the values you can specify in this field, see msgrcv(2). The mtext field contains the text of the message. You specify the size in bytes of the message in the msgsz parameter. You can specify a value between 0 (zero) and, by default, 8192 bytes. Note that the max- imum number of bytes allowed in a message string is configured at sys- tem configuration time using the msgmax configuration file keyword, so the limit might be different for your system. The msgflg parameter specifies the action that the kernel should take if either or both of the following are true: The current number of bytes in the message queue is equal to msg_qbytes (in the msqid_ds structure). The total number of messages on all message queues is equal to the limit defined by the msgtql configuration file keyword. By default the limit is 40, but it might have been changed for your system at system configuration time. One of two kernel actions can be specified, as follows: If IPC_NOWAIT is set, the kernel does not send the message and returns to the calling process immediately. If IPC_NOWAIT is not set, the kernel suspends the calling process. The process remains suspended until one of the fol- lowing occurs: The blocking condition is removed. In this case, the kernel sends the message. The specified message queue ID is removed from the system. In this case, the kernel sets errno to [EIDRM] and returns -1 to the calling process. The calling process catches a sig- nal. In this case, the message is not sent and the process resumes ex- ecution as directed by the signal() function. If the msgsnd() function completes successfully, the kernel updates the msqid_ds structure associated with the msgid parameter. Specifically, it: Increments msg_qnum by 1. Increments msg_cbytes by the message text size. Sets msg_lspid equal to the process ID of the calling process. Sets msg_stime equal to the current time. RETURN VALUES Upon successful completion, the msgrcv() function returns a value of 0 (zero). Otherwise, the function returns a value of -1 and sets errno to indicate the error. ERRORS The msgsnd() function sets errno to the specified values for the fol- lowing conditions: The calling process does not have permission for the operation. The IPC_NOWAIT flag is set, and either the maximum number of message headers has been allocated or the message queue is full. The message queue identified by the msqid parameter has been removed from the system. The operation was interrupted by a signal. The msqid parameter is not a valid message queue ID, mtype is less than 1, or ms- gsz is less than 0 (zero) or greater than the limit defined by the msg- max configuration file keyword. By default the limit is 8192 bytes, but it might have been changed for your system at system configuration time. RELATED INFORMATION Functions: msgctl(2), msgget(2), msgrcv(2), sigaction(2) Data Structures: msqid_ds(4) Standards: standards(5) delim off msgsnd(2)