man2/msgctl.2

   1 .\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
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  23 .\" Modified Tue Oct 22 08:11:14 EDT 1996 by Eric S. Raymond <esr@thyrsus.com>
  24 .\" Modified Sun Feb 18 01:59:29 2001 by Andries E. Brouwer <aeb@cwi.nl>
  25 .\" Modified, 27 May 2004, Michael Kerrisk <mtk-manpages@gmx.net>
  26 .\"     Added notes on CAP_IPC_OWNER requirement
  27 .\" Modified, 17 Jun 2004, Michael Kerrisk <mtk-manpages@gmx.net>
  28 .\"     Added notes on CAP_SYS_ADMIN requirement for IPC_SET and IPC_RMID
  29 .\" Modified, 11 Nov 2004, Michael Kerrisk <mtk-manpages@gmx.net>
  30 .\"     Language and formatting clean-ups
  31 .\"     Added msqid_ds and ipc_perm structure definitions
  32 .\"
  33 .TH MSGCTL 2 2004-11-10 "Linux 2.6.9" "Linux Programmer's Manual"
  34 .SH NAME
  35 msgctl \- message control operations
  36 .SH SYNOPSIS
  37 .nf
  38 .B
  39 #include <sys/types.h>
  40 .B
  41 #include <sys/ipc.h>
  42 .B
  43 #include <sys/msg.h>
  44 .fi
  45 .sp
  46 .BI "int msgctl(int " msqid ,
  47 .BI "int " cmd ,
  48 .BI "struct msqid_ds *" buf );
  49 .SH DESCRIPTION
  50 .B msgctl
  51 performs the control operation specified by
  52 .I cmd
  53 on the message queue with identifier
  54 .IR msqid .
  55 .PP
  56 The
  57 .I msqid_ds
  58 data structure is defined in <sys/msg.h> as follows:
  59 .nf
  60 .in +4n
  61
  62 struct msqid_ds {
  63     struct ipc_perm msg_perm;     /* Ownership and permissions
  64     time_t          msg_stime;    /* Time of last msgsnd() */
  65     time_t          msg_rtime;    /* Time of last msgrcv() */
  66     time_t          msg_ctime;    /* Time of last change */
  67     unsigned long   __msg_cbytes; /* Current number of bytes in
  68                                      queue (non-standard) */
  69     msgqnum_t       msg_qnum;     /* Current number of messages
  70                                      in queue */
  71     msglen_t        msg_qbytes;   /* Maximum number of bytes
  72                                      allowed in queue */
  73     pid_t           msg_lspid;    /* PID of last msgsnd() */
  74     pid_t           msg_lrpid;    /* PID of last msgrcv() */
  75
  76 };
  77 .in -4n
  78 .fi
  79 .PP
  80 The
  81 .I ipc_perm
  82 structure is defined in <sys/ipc.h> as follows
  83 (the highlighted fields are settable using
  84 .BR IPC_SET ):
  85 .PP
  86 .nf
  87 .in +4n
  88 struct ipc_perm {
  89     key_t key;            /* Key supplied to msgget() */
  90     uid_t \fBuid\fP;            /* Effective UID of owner */
  91     gid_t \fBgid\fP;            /* Effective GID of owner */
  92     uid_t cuid;           /* Effective UID of creator */
  93     gid_t cgid;           /* Effective GID of creator */
  94     unsigned short \fBmode\fP;  /* Permissions */
  95     unsigned short seq;   /* Sequence number */
  96 };
  97 .in -4n
  98 .fi
  99 .PP
 100 Valid values for
 101 .I cmd
 102 are:
 103 .TP
 104 .B IPC_STAT
 105 Copy information from the kernel data structure associated with
 106 .I msqid
 107 into the
 108 .I msqid_ds
 109 structure pointed to by
 110 .IR buf .
 111 The caller must have read permission on the message queue.
 112 .TP
 113 .B IPC_SET
 114 Write the values of some members of the
 115 .I msqid_ds
 116 structure pointed to by
 117 .I buf
 118 to the kernel data structure associated with this message queue,
 119 updating also its
 120 .I msg_ctime
 121 member.
 122 The following members of the structure are updated:
 123 .IR msg_qbytes ,
 124 .IR msg_perm.uid ,
 125 .IR msg_perm.gid ,
 126 and (the least significant 9 bits of)
 127 .IR msg_perm.mode .
 128 The effective UID of the calling process must match the owner
 129 .RI ( msg_perm.uid )
 130 or creator
 131 .RI ( msg_perm.cuid )
 132 of the message queue, or the caller must be privileged.
 133 Appropriate privilege (Linux: the
 134 .B CAP_IPC_RESOURCE
 135 capability) is required to raise the
 136 .I msg_qbytes
 137 value beyond the system parameter
 138 .BR MSGMNB .
 139 .TP
 140 .B IPC_RMID
 141 Immediately remove the message queue,
 142 awakening all waiting reader and writer processes (with an error
 143 return and
 144 .I errno
 145 set to
 146 .BR EIDRM ).
 147 The calling process must have appropriate privileges
 148 or its effective user\-ID must be either that of the creator or owner
 149 of the message queue.
 150 .SH "RETURN VALUE"
 151 On success, the return value will be 0
 152 otherwise \-1
 153 with
 154 .I errno
 155 indicating the error.
 156 .SH ERRORS
 157 On failure,
 158 .I errno
 159 is set to one of the following:
 160 .TP 11
 161 .B EACCES
 162 The argument
 163 .I cmd
 164 is equal to
 165 .B IPC_STAT
 166 or
 167 .BR MSG_STAT ,
 168 but the calling process does not have read permission on the message queue
 169 .IR msqid ,
 170 and does not have the
 171 .B CAP_IPC_OWNER
 172 capability.
 173 .TP
 174 .B EFAULT
 175 The argument
 176 .I cmd
 177 has the value
 178 .B IPC_SET
 179 or
 180 .BR IPC_STAT ,
 181 but the address pointed to by
 182 .I buf
 183 isn't accessible.
 184 .TP
 185 .B EIDRM
 186 The message queue was removed.
 187 .TP
 188 .B EINVAL
 189 Invalid value for
 190 .I cmd
 191 or
 192 .IR msqid .
 193 .TP
 194 .B EPERM
 195 The argument
 196 .I cmd
 197 has the value
 198 .B IPC_SET
 199 or
 200 .BR IPC_RMID ,
 201 but the effective user ID of the calling process is not the creator
 202 (as found in
 203 .IR msg_perm.cuid )
 204 or the owner
 205 (as found in
 206 .IR msg_perm.uid )
 207 of the message queue,
 208 and the process is not privileged (Linux: it does not have the
 209 .B CAP_SYS_ADMIN
 210 capability).
 211 .SH NOTES
 212 Various fields in the \fIstruct msqid_ds\fP were shorts under Linux 2.2
 213 and have become longs under Linux 2.4. To take advantage of this,
 214 a recompilation under glibc-2.1.91 or later should suffice.
 215 (The kernel distinguishes old and new calls by an IPC_64 flag in
 216 .IR cmd .)
 217 .SH "CONFORMING TO"
 218 SVr4, SVID.  SVID does not document the EIDRM error condition.
 219 .SH "SEE ALSO"
 220 .BR msgget (2),
 221 .BR msgrcv (2),
 222 .BR msgsnd (2),
 223 .BR ipc (5),
 224 .BR capabilities (7)