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MQ_GETATTR(3) Linux Programmer's Manual MQ_GETATTR(3)

NAME

     mq_getattr, mq_setattr - get/set message queue attributes

SYNOPSIS

     #include <mqueue.h>

     int mq_getattr(mqd_t mqdes, struct mq_attr *attr);

     int mq_setattr(mqd_t mqdes, const struct mq_attr *newattr,
                      struct mq_attr *oldattr);

     Link with -lrt.

DESCRIPTION

     mq_getattr()   and   mq_setattr()   respectively  retrieve  and  modify
     attributes of the message  queue  referred  to  by  the  message  queue
     descriptor mqdes.

     mq_getattr()  returns  an  mq_attr  structure  in the buffer pointed by
     attr.  This structure is defined as:

         struct mq_attr {
             long mq_flags;       /* Flags: 0 or O_NONBLOCK */
             long mq_maxmsg;      /* Max. # of messages on queue */
             long mq_msgsize;     /* Max. message size (bytes) */
             long mq_curmsgs;     /* # of messages currently in queue */ };

     The mq_flags field contains flags  associated  with  the  open  message
     queue description.  This field is initialized when the queue is created
     by mq_open(3).  The only flag that can appear in this field  is  O_NON-
     BLOCK.

     The  mq_maxmsg  and mq_msgsize fields are set when the message queue is
     created by mq_open(3).  The mq_maxmsg field is an upper  limit  on  the
     number  of  messages  that may be placed on the queue using mq_send(3).
     The mq_msgsize field is an upper limit on the size of messages that may
     be placed on the queue.  Both of these fields must have a value greater
     than zero.  Two /proc files that place ceilings on the values for these
     fields are described in mq_overview(7).

     The  mq_curmsgs  field returns the number of messages currently held in
     the queue.

     mq_setattr() sets message queue attributes using  information  supplied
     in  the  mq_attr  structure  pointed to by newattr.  The only attribute
     that can be modified is the setting of the O_NONBLOCK flag in mq_flags.
     The  other  fields in newattr are ignored.  If the oldattr field is not
     NULL, then the buffer that it points to is used to  return  an  mq_attr
     structure  that  contains  the  same  information  that  is returned by
     mq_getattr().

RETURN VALUE

     On success mq_getattr() and mq_setattr() return  0;  on  error,  -1  is
     returned, with errno set to indicate the error.

ERRORS

     EBADF  The message queue descriptor specified in mqdes is invalid.

     EINVAL newattr->mq_flags contained set bits other than O_NONBLOCK.

ATTRIBUTES

     For   an   explanation   of   the  terms  used  in  this  section,  see
     attributes(7).

     allbox; lbw26 lb lb l l l.  Interface Attribute Value T{  mq_getattr(),
     mq_setattr() T}   Thread safety  MT-Safe

CONFORMING TO

     POSIX.1-2001, POSIX.1-2008.

NOTES

     On  Linux,  mq_getattr() and mq_setattr() are library functions layered
     on top of the mq_getsetattr(2) system call.

EXAMPLE

     The program below can be used to show the default mq_maxmsg and mq_msg-
     size values that are assigned to a message queue that is created with a
     call to mq_open(3) in which the attr argument  is  NULL.   Here  is  an
     example run of the program:

         $  ./a.out /testq Maximum # of messages on queue:   10 Maximum mes-
         sage size:             8192

     Since Linux 3.5, the  following  /proc  files  (described  in  mq_over-
     view(7)) can be used to control the defaults:

         $  uname -sr Linux 3.8.0 $ cat /proc/sys/fs/mqueue/msg_default 10 $
         cat /proc/sys/fs/mqueue/msgsize_default 8192

 Program source
      #include <mqueue.h> #include <sys/stat.h> #include <fcntl.h>  #include
     <stdio.h> #include <stdlib.h> #include <unistd.h>

     #define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \
                             } while (0)

     int main(int argc, char *argv[]) {
         mqd_t mqd;
         struct mq_attr attr;

         if (argc != 2) {
             fprintf(stderr, "Usage: %s mq-name\n", argv[0]);
             exit(EXIT_FAILURE);
         }

         mqd = mq_open(argv[1], O_CREAT | O_EXCL, S_IRUSR | S_IWUSR, NULL);
         if (mqd == (mqd_t) -1)
             errExit("mq_open");

         if (mq_getattr(mqd, &attr) == -1)
             errExit("mq_getattr");

         printf("Maximum # of messages on queue:   %ld\n", attr.mq_maxmsg);
         printf("Maximum message size:             %ld\n", attr.mq_msgsize);

         if (mq_unlink(argv[1]) == -1)
             errExit("mq_unlink");

         exit(EXIT_SUCCESS); }