man3/mq_getattr.3

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  23 .TH MQ_GETATTR 3 2006-02-25 "Linux 2.6.16" "Linux Programmer's Manual"
  24 .SH NAME
  25 mq_getattr, mq_setattr \- get/set message queue attributes
  26 .SH SYNOPSIS
  27 .nf
  28 .B #include <mqueue.h>
  29 .sp
  30 .BI "mqd_t mq_getattr(mqd_t " mqdes ", struct mq_attr *" attr );
  31 .BI "mqd_t mq_setattr(mqd_t " mqdes ", struct mq_attr *" newattr ","
  32 .BI "                 struct mq_attr *" oldattr );
  33 .fi
  34 .SH DESCRIPTION
  35 .BR mq_getattr ()
  36 and
  37 .BR mq_setattr ()
  38 respectively retrieve and modify attributes of the message queue
  39 referred to by the descriptor
  40 .IR mqdes .
  41
  42 .BR mq_getattr ()
  43 returns an
  44 .I mq_attr
  45 structure in the buffer pointed by
  46 .IR attr .
  47 This structure is defined as:
  48 .in +0.25i
  49 .nf
  50
  51 struct mq_attr {
  52     long mq_flags;       /* Flags: 0 or O_NONBLOCK */
  53     long mq_maxmsg;      /* Max. # of messages on queue */
  54     long mq_msgsize;     /* Max. message size (bytes) */
  55     long mq_curmsgs;     /* # of messages currently in queue */
  56 };
  57 .fi
  58 .in -0.25i
  59 .PP
  60 The
  61 .I mq_flags
  62 field contains flags associated with the open message queue description.
  63 This field is initialised when the queue is created by
  64 .BR mq_open ().
  65 The only flag that can appear in this field is
  66 .BR O_NONBLOCK .
  67
  68 The
  69 .I mq_maxmsg
  70 and
  71 .I mq_msgsize
  72 fields are set when the message queue is created by
  73 .BR mq_open ().
  74 The
  75 .I mq_maxmsg
  76 field is an upper limit on the number of messages
  77 that may be placed on the queue using
  78 .BR mq_send ().
  79 The
  80 .I mq_msgsize
  81 field is an upper limit on the size of messages
  82 that may be placed on the queue.
  83 Both of these fields must have a value greater than zero.
  84 Two
  85 .I /proc
  86 files that place ceilings on the values for these fields are described in
  87 .BR mq_open (3).
  88
  89 The
  90 .I mq_curmsgs
  91 field returns the number of messages currently held in the queue.
  92
  93 .BR mq_setattr ()
  94 sets message queue attributes using information supplied in the
  95 .I mq_attr
  96 structure pointed to by
  97 .IR newattr .
  98 The only attribute that can be modified is the setting of the
  99 .B O_NONBLOCK
 100 flag in
 101 .IR mq_flags .
 102 The other fields in
 103 .I newattr
 104 are ignored.
 105 If the
 106 .I oldattr
 107 field is not NULL,
 108 then the buffer that it points to is used to return an
 109 .I mq_attr
 110 structure that contains the same information that is returned by
 111 .BR mq_getattr ().
 112 .SH RETURN VALUE
 113 On success
 114 .BR mq_getattr ()
 115 and
 116 .BR mq_setattr ()
 117 return 0; on error, \-1 is returned, with
 118 .I errno
 119 set to indicate the error.
 120 .SH ERRORS
 121 .TP
 122 .B EBADF
 123 The descriptor specified in
 124 .I mqdes
 125 is invalid.
 126 .TP
 127 .B EINVAL
 128 .I newattr->mq_flags
 129 contained set bits other than
 130 .BR O_NONBLOCK .
 131 .SH CONFORMING TO
 132 POSIX.1-2001.
 133 .SH "SEE ALSO"
 134 .BR mq_close (3),
 135 .BR mq_notify (3),
 136 .BR mq_open (3),
 137 .BR mq_receive (3),
 138 .BR mq_send (3),
 139 .BR mq_unlink (3),
 140 .BR mq_overview (7)