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man pages section 2: System Calls

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Updated: July 2017



msgsnap - message queue snapshot operation


#include <sys/msg.h>

msgsnap(int msqid, void *buf, size_t bufsz, long msgtyp);


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) boundary.

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:

  • If msgtyp is 0, all of the messages on the queue are read.

  • If msgtyp is greater than 0, all messages of type msgtyp are read.

  • 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.


The msgsnap() function will fail if:


Operation permission is denied to the calling process. See Intro(2).


The msqid argument is not a valid message queue identifier or the value of bufsz is less than sizeof(struct msgsnap_head).


The buf argument points to an illegal address.


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().


Example 1 msgsnap() example

This is sample C code indicating how to use the msgsnap function (see msgids(2)).

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) {
          if (bufsize >= buf->msgsnap_size)  /* we got them all */
          /* 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)));



See attributes(5) for descriptions of the following attributes:


See Also

ipcrm(1), ipcs(1), Intro(2), msgctl(2), msgget(2), msgids(2), msgrcv(2), msgsnd(2), attributes(5)