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man page for msgsnap

msgsnap(2)							   System Calls 							msgsnap(2)

msgsnap - message queue snapshot operation SYNOPSIS
#include <sys/msg.h> msgsnap(int msqid, void *buf, size_t bufsz, long msgtyp); DESCRIPTION
The msgsnap() function reads all of the messages of type msgtyp from the queue associated with the message queue identifier specified by msqid and places them in the user-defined buffer pointed to by buf. The buf argument points to a user-defined buffer that on return will contain first a buffer header structure: struct msgsnap_head { size_t msgsnap_size; /* bytes used/required in the buffer */ size_t msgsnap_nmsg; /* number of messages in the buffer */ }; followed by msgsnap_nmsg messages, each of which starts with a message header: struct msgsnap_mhead { size_t msgsnap_mlen; /* number of bytes in the message */ long msgsnap_mtype; /* message type */ }; and followed by msgsnap_mlen bytes containing the message contents. Each subsequent message header is located at the first byte following the previous message contents, rounded up to a sizeof(size_t) bound- ary. The bufsz argument specifies the size of buf in bytes. If bufsz is less than sizeof(msgsnap_head), msgsnap() fails with EINVAL. If bufsz is insufficient to contain all of the requested messages, msgsnap() succeeds but returns with msgsnap_nmsg set to 0 and with msgsnap_size set to the required size of the buffer in bytes. The msgtyp argument specifies the types of messages requested as follows: o If msgtyp is 0, all of the messages on the queue are read. o If msgtyp is greater than 0, all messages of type msgtyp are read. o If msgtyp is less than 0, all messages with type less than or equal to the absolute value of msgtyp are read. The msgsnap() function is a non-destructive operation. Upon completion, no changes are made to the data structures associated with msqid. RETURN VALUES
Upon successful completion, msgsnap() returns 0. Otherwise, -1 is returned and errno is set to indicate the error. ERRORS
The msgsnap() function will fail if: EACCES Operation permission is denied to the calling process. See intro(2). EINVAL The msqid argument is not a valid message queue identifier or the value of bufsz is less than sizeof(struct msgsnap_head). EFAULT The buf argument points to an illegal address. USAGE
The msgsnap() function returns a snapshot of messages on a message queue at one point in time. The queue contents can change immediately following return from msgsnap(). EXAMPLES
Example 1: msgsnap() example This is sample C code indicating how to use the msgsnap function (see msgids(2)). void process_msgid(int msqid) { size_t bufsize; struct msgsnap_head *buf; struct msgsnap_mhead *mhead; int i; /* allocate a minimum-size buffer */ buf = malloc(bufsize = sizeof(struct msgsnap_head)); /* read all of the messages from the queue */ for (;;) { if (msgsnap(msqid, buf, bufsize, 0) != 0) { perror("msgsnap"); free(buf); return; } if (bufsize >= buf->msgsnap_size) /* we got them all */ break; /* we need a bigger buffer */ buf = realloc(buf, bufsize = buf->msgsnap_size); } /* process each message in the queue (there may be none) */ mhead = (struct msgsnap_mhead *)(buf + 1); /* first message */ for (i = 0; i < buf->msgsnap_nmsg; i++) { size_t mlen = mhead->msgsnap_mlen; /* process the message contents */ process_message(mhead->msgsnap_mtype, (char *)(mhead+1), mlen); /* advance to the next message header */ mhead = (struct msgsnap_mhead *) ((char *)mhead + sizeof(struct msgsnap_mhead) + ((mlen + sizeof(size_t) - 1) & ~(sizeof(size_t) - 1))); } free(buf); } ATTRIBUTES
See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ | ATTRIBUTE TYPE | ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |MT-Level |Async-Signal-Safe | +-----------------------------+-----------------------------+ SEE ALSO
ipcrm(1), ipcs(1), intro(2), msgctl(2), msgget(2), msgids(2), msgrcv(2), msgsnd(2), attributes(5) SunOS 5.10 8 Mar 2000 msgsnap(2)

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