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diff --git a/man2/msgop.2 b/man2/msgop.2 new file mode 100644 index 0000000..381875e --- /dev/null +++ b/man2/msgop.2 @@ -0,0 +1,684 @@ +.\" Copyright 1993 Giorgio Ciucci <giorgio@crcc.it> +.\" and Copyright 2015 Bill Pemberton <wfp5p@worldbroken.com> +.\" +.\" SPDX-License-Identifier: Linux-man-pages-copyleft +.\" +.\" Modified Tue Oct 22 16:40:11 1996 by Eric S. Raymond <esr@thyrsus.com> +.\" Modified Mon Jul 10 21:09:59 2000 by aeb +.\" Modified 1 Jun 2002, Michael Kerrisk <mtk.manpages@gmail.com> +.\" Language clean-ups. +.\" Enhanced and corrected information on msg_qbytes, MSGMNB and MSGMAX +.\" Added note on restart behavior of msgsnd() and msgrcv() +.\" Formatting clean-ups (argument and field names marked as .I +.\" instead of .B) +.\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com> +.\" Added notes on capability requirements +.\" Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com> +.\" Language and formatting clean-ups +.\" Added notes on /proc files +.\" +.TH MSGOP 2 2023-05-03 "Linux man-pages 6.05.01" +.SH NAME +msgrcv, msgsnd \- System V message queue operations +.SH LIBRARY +Standard C library +.RI ( libc ", " \-lc ) +.SH SYNOPSIS +.nf +.B #include <sys/msg.h> +.PP +.BI "int msgsnd(int " msqid ", const void " msgp [. msgsz "], size_t " msgsz , +.BI " int " msgflg ); +.PP +.BI "ssize_t msgrcv(int " msqid ", void " msgp [. msgsz "], size_t " msgsz \ +", long " msgtyp , +.BI " int " msgflg ); +.fi +.SH DESCRIPTION +The +.BR msgsnd () +and +.BR msgrcv () +system calls are used to send messages to, +and receive messages from, a System\ V message queue. +The calling process must have write permission on the message queue +in order to send a message, and read permission to receive a message. +.PP +The +.I msgp +argument is a pointer to a caller-defined structure +of the following general form: +.PP +.in +4n +.EX +struct msgbuf { + long mtype; /* message type, must be > 0 */ + char mtext[1]; /* message data */ +}; +.EE +.in +.PP +The +.I mtext +field is an array (or other structure) whose size is specified by +.IR msgsz , +a nonnegative integer value. +Messages of zero length (i.e., no +.I mtext +field) are permitted. +The +.I mtype +field must have a strictly positive integer value. +This value can be +used by the receiving process for message selection +(see the description of +.BR msgrcv () +below). +.SS msgsnd() +The +.BR msgsnd () +system call appends a copy of the message pointed to by +.I msgp +to the message queue whose identifier is specified +by +.IR msqid . +.PP +If sufficient space is available in the queue, +.BR msgsnd () +succeeds immediately. +The queue capacity is governed by the +.I msg_qbytes +field in the associated data structure for the message queue. +During queue creation this field is initialized to +.B MSGMNB +bytes, but this limit can be modified using +.BR msgctl (2). +A message queue is considered to be full if either of the following +conditions is true: +.IP \[bu] 3 +Adding a new message to the queue would cause the total number of bytes +in the queue to exceed the queue's maximum size (the +.I msg_qbytes +field). +.IP \[bu] +Adding another message to the queue would cause the total number of messages +in the queue to exceed the queue's maximum size (the +.I msg_qbytes +field). +This check is necessary to prevent an unlimited number of zero-length +messages being placed on the queue. +Although such messages contain no data, +they nevertheless consume (locked) kernel memory. +.PP +If insufficient space is available in the queue, then the default +behavior of +.BR msgsnd () +is to block until space becomes available. +If +.B IPC_NOWAIT +is specified in +.IR msgflg , +then the call instead fails with the error +.BR EAGAIN . +.PP +A blocked +.BR msgsnd () +call may also fail if: +.IP \[bu] 3 +the queue is removed, +in which case the system call fails with +.I errno +set to +.BR EIDRM ; +or +.IP \[bu] +a signal is caught, in which case the system call fails +with +.I errno +set to +.BR EINTR ; see +.BR signal (7). +.RB ( msgsnd () +is never automatically restarted after being interrupted by a +signal handler, regardless of the setting of the +.B SA_RESTART +flag when establishing a signal handler.) +.PP +Upon successful completion the message queue data structure is updated +as follows: +.IP \[bu] 3 +.I msg_lspid +is set to the process ID of the calling process. +.IP \[bu] +.I msg_qnum +is incremented by 1. +.IP \[bu] +.I msg_stime +is set to the current time. +.SS msgrcv() +The +.BR msgrcv () +system call removes a message from the queue specified by +.I msqid +and places it in the buffer +pointed to by +.IR msgp . +.PP +The argument +.I msgsz +specifies the maximum size in bytes for the member +.I mtext +of the structure pointed to by the +.I msgp +argument. +If the message text has length greater than +.IR msgsz , +then the behavior depends on whether +.B MSG_NOERROR +is specified in +.IR msgflg . +If +.B MSG_NOERROR +is specified, then +the message text will be truncated (and the truncated part will be +lost); if +.B MSG_NOERROR +is not specified, then +the message isn't removed from the queue and +the system call fails returning \-1 with +.I errno +set to +.BR E2BIG . +.PP +Unless +.B MSG_COPY +is specified in +.I msgflg +(see below), +the +.I msgtyp +argument specifies the type of message requested, as follows: +.IP \[bu] 3 +If +.I msgtyp +is 0, +then the first message in the queue is read. +.IP \[bu] +If +.I msgtyp +is greater than 0, +then the first message in the queue of type +.I msgtyp +is read, unless +.B MSG_EXCEPT +was specified in +.IR msgflg , +in which case +the first message in the queue of type not equal to +.I msgtyp +will be read. +.IP \[bu] +If +.I msgtyp +is less than 0, +then the first message in the queue with the lowest type less than or +equal to the absolute value of +.I msgtyp +will be read. +.PP +The +.I msgflg +argument is a bit mask constructed by ORing together zero or more +of the following flags: +.TP +.B IPC_NOWAIT +Return immediately if no message of the requested type is in the queue. +The system call fails with +.I errno +set to +.BR ENOMSG . +.TP +.BR MSG_COPY " (since Linux 3.8)" +.\" commit 4a674f34ba04a002244edaf891b5da7fc1473ae8 +Nondestructively fetch a copy of the message at the ordinal position +in the queue specified by +.I msgtyp +(messages are considered to be numbered starting at 0). +.IP +This flag must be specified in conjunction with +.BR IPC_NOWAIT , +with the result that, if there is no message available at the given position, +the call fails immediately with the error +.BR ENOMSG . +Because they alter the meaning of +.I msgtyp +in orthogonal ways, +.B MSG_COPY +and +.B MSG_EXCEPT +may not both be specified in +.IR msgflg . +.IP +The +.B MSG_COPY +flag was added for the implementation of +the kernel checkpoint-restore facility and +is available only if the kernel was built with the +.B CONFIG_CHECKPOINT_RESTORE +option. +.TP +.B MSG_EXCEPT +Used with +.I msgtyp +greater than 0 +to read the first message in the queue with message type that differs +from +.IR msgtyp . +.TP +.B MSG_NOERROR +To truncate the message text if longer than +.I msgsz +bytes. +.PP +If no message of the requested type is available and +.B IPC_NOWAIT +isn't specified in +.IR msgflg , +the calling process is blocked until one of the following conditions occurs: +.IP \[bu] 3 +A message of the desired type is placed in the queue. +.IP \[bu] +The message queue is removed from the system. +In this case, the system call fails with +.I errno +set to +.BR EIDRM . +.IP \[bu] +The calling process catches a signal. +In this case, the system call fails with +.I errno +set to +.BR EINTR . +.RB ( msgrcv () +is never automatically restarted after being interrupted by a +signal handler, regardless of the setting of the +.B SA_RESTART +flag when establishing a signal handler.) +.PP +Upon successful completion the message queue data structure is updated +as follows: +.IP +.I msg_lrpid +is set to the process ID of the calling process. +.IP +.I msg_qnum +is decremented by 1. +.IP +.I msg_rtime +is set to the current time. +.SH RETURN VALUE +On success, +.BR msgsnd () +returns 0 +and +.BR msgrcv () +returns the number of bytes actually copied into the +.I mtext +array. +On failure, both functions return \-1, and set +.I errno +to indicate the error. +.SH ERRORS +.BR msgsnd () +can fail with the following errors: +.TP +.B EACCES +The calling process does not have write permission on the message queue, +and does not have the +.B CAP_IPC_OWNER +capability in the user namespace that governs its IPC namespace. +.TP +.B EAGAIN +The message can't be sent due to the +.I msg_qbytes +limit for the queue and +.B IPC_NOWAIT +was specified in +.IR msgflg . +.TP +.B EFAULT +The address pointed to by +.I msgp +isn't accessible. +.TP +.B EIDRM +The message queue was removed. +.TP +.B EINTR +Sleeping on a full message queue condition, the process caught a signal. +.TP +.B EINVAL +Invalid +.I msqid +value, or nonpositive +.I mtype +value, or +invalid +.I msgsz +value (less than 0 or greater than the system value +.BR MSGMAX ). +.TP +.B ENOMEM +The system does not have enough memory to make a copy of the +message pointed to by +.IR msgp . +.PP +.BR msgrcv () +can fail with the following errors: +.TP +.B E2BIG +The message text length is greater than +.I msgsz +and +.B MSG_NOERROR +isn't specified in +.IR msgflg . +.TP +.B EACCES +The calling process does not have read permission on the message queue, +and does not have the +.B CAP_IPC_OWNER +capability in the user namespace that governs its IPC namespace. +.TP +.B EFAULT +The address pointed to by +.I msgp +isn't accessible. +.TP +.B EIDRM +While the process was sleeping to receive a message, +the message queue was removed. +.TP +.B EINTR +While the process was sleeping to receive a message, +the process caught a signal; see +.BR signal (7). +.TP +.B EINVAL +.I msqid +was invalid, or +.I msgsz +was less than 0. +.TP +.BR EINVAL " (since Linux 3.14)" +.I msgflg +specified +.BR MSG_COPY , +but not +.BR IPC_NOWAIT . +.TP +.BR EINVAL " (since Linux 3.14)" +.I msgflg +specified both +.B MSG_COPY +and +.BR MSG_EXCEPT . +.TP +.B ENOMSG +.B IPC_NOWAIT +was specified in +.I msgflg +and no message of the requested type existed on the message queue. +.TP +.B ENOMSG +.B IPC_NOWAIT +and +.B MSG_COPY +were specified in +.I msgflg +and the queue contains less than +.I msgtyp +messages. +.TP +.BR ENOSYS " (since Linux 3.8)" +Both +.B MSG_COPY +and +.B IPC_NOWAIT +were specified in +.IR msgflg , +and this kernel was configured without +.BR CONFIG_CHECKPOINT_RESTORE . +.SH STANDARDS +POSIX.1-2008. +.PP +The +.B MSG_EXCEPT +and +.B MSG_COPY +flags are Linux-specific; +their definitions can be obtained by defining the +.B _GNU_SOURCE +.\" MSG_COPY since glibc 2.18 +feature test macro. +.SH HISTORY +POSIX.1-2001, SVr4. +.PP +The +.I msgp +argument is declared as \fIstruct msgbuf\ *\fP in +glibc 2.0 and 2.1. +It is declared as \fIvoid\ *\fP +in glibc 2.2 and later, as required by SUSv2 and SUSv3. +.SH NOTES +The following limits on message queue resources affect the +.BR msgsnd () +call: +.TP +.B MSGMAX +Maximum size of a message text, in bytes (default value: 8192 bytes). +On Linux, this limit can be read and modified via +.IR /proc/sys/kernel/msgmax . +.TP +.B MSGMNB +Maximum number of bytes that can be held in a message queue +(default value: 16384 bytes). +On Linux, this limit can be read and modified via +.IR /proc/sys/kernel/msgmnb . +A privileged process +(Linux: a process with the +.B CAP_SYS_RESOURCE +capability) +can increase the size of a message queue beyond +.B MSGMNB +using the +.BR msgctl (2) +.B IPC_SET +operation. +.PP +The implementation has no intrinsic system-wide limits on the +number of message headers +.RB ( MSGTQL ) +and the number of bytes in the message pool +.RB ( MSGPOOL ). +.SH BUGS +In Linux 3.13 and earlier, +if +.BR msgrcv () +was called with the +.B MSG_COPY +flag, but without +.BR IPC_NOWAIT , +and the message queue contained less than +.I msgtyp +messages, then the call would block until the next message is written +to the queue. +.\" http://marc.info/?l=linux-kernel&m=139048542803605&w=2 +At that point, the call would return a copy of the message, +.I regardless +of whether that message was at the ordinal position +.IR msgtyp . +This bug is fixed +.\" commit 4f87dac386cc43d5525da7a939d4b4e7edbea22c +in Linux 3.14. +.PP +Specifying both +.B MSG_COPY +and +.B MSC_EXCEPT +in +.I msgflg +is a logical error (since these flags impose different interpretations on +.IR msgtyp ). +In Linux 3.13 and earlier, +.\" http://marc.info/?l=linux-kernel&m=139048542803605&w=2 +this error was not diagnosed by +.BR msgrcv (). +This bug is fixed +.\" commit 4f87dac386cc43d5525da7a939d4b4e7edbea22c +in Linux 3.14. +.SH EXAMPLES +The program below demonstrates the use of +.BR msgsnd () +and +.BR msgrcv (). +.PP +The example program is first run with the \fB\-s\fP option to send a +message and then run again with the \fB\-r\fP option to receive a +message. +.PP +The following shell session shows a sample run of the program: +.PP +.in +4n +.EX +.RB "$" " ./a.out \-s" +sent: a message at Wed Mar 4 16:25:45 2015 +.PP +.RB "$" " ./a.out \-r" +message received: a message at Wed Mar 4 16:25:45 2015 +.EE +.in +.SS Program source +\& +.\" SRC BEGIN (msgop.c) +.EX +#include <errno.h> +#include <stdio.h> +#include <stdlib.h> +#include <sys/ipc.h> +#include <sys/msg.h> +#include <time.h> +#include <unistd.h> +\& +struct msgbuf { + long mtype; + char mtext[80]; +}; +\& +static void +usage(char *prog_name, char *msg) +{ + if (msg != NULL) + fputs(msg, stderr); +\& + fprintf(stderr, "Usage: %s [options]\en", prog_name); + fprintf(stderr, "Options are:\en"); + fprintf(stderr, "\-s send message using msgsnd()\en"); + fprintf(stderr, "\-r read message using msgrcv()\en"); + fprintf(stderr, "\-t message type (default is 1)\en"); + fprintf(stderr, "\-k message queue key (default is 1234)\en"); + exit(EXIT_FAILURE); +} +\& +static void +send_msg(int qid, int msgtype) +{ + time_t t; + struct msgbuf msg; +\& + msg.mtype = msgtype; +\& + time(&t); + snprintf(msg.mtext, sizeof(msg.mtext), "a message at %s", + ctime(&t)); +\& + if (msgsnd(qid, &msg, sizeof(msg.mtext), + IPC_NOWAIT) == \-1) + { + perror("msgsnd error"); + exit(EXIT_FAILURE); + } + printf("sent: %s\en", msg.mtext); +} +\& +static void +get_msg(int qid, int msgtype) +{ + struct msgbuf msg; +\& + if (msgrcv(qid, &msg, sizeof(msg.mtext), msgtype, + MSG_NOERROR | IPC_NOWAIT) == \-1) { + if (errno != ENOMSG) { + perror("msgrcv"); + exit(EXIT_FAILURE); + } + printf("No message available for msgrcv()\en"); + } else { + printf("message received: %s\en", msg.mtext); + } +} +\& +int +main(int argc, char *argv[]) +{ + int qid, opt; + int mode = 0; /* 1 = send, 2 = receive */ + int msgtype = 1; + int msgkey = 1234; +\& + while ((opt = getopt(argc, argv, "srt:k:")) != \-1) { + switch (opt) { + case \[aq]s\[aq]: + mode = 1; + break; + case \[aq]r\[aq]: + mode = 2; + break; + case \[aq]t\[aq]: + msgtype = atoi(optarg); + if (msgtype <= 0) + usage(argv[0], "\-t option must be greater than 0\en"); + break; + case \[aq]k\[aq]: + msgkey = atoi(optarg); + break; + default: + usage(argv[0], "Unrecognized option\en"); + } + } +\& + if (mode == 0) + usage(argv[0], "must use either \-s or \-r option\en"); +\& + qid = msgget(msgkey, IPC_CREAT | 0666); +\& + if (qid == \-1) { + perror("msgget"); + exit(EXIT_FAILURE); + } +\& + if (mode == 2) + get_msg(qid, msgtype); + else + send_msg(qid, msgtype); +\& + exit(EXIT_SUCCESS); +} +.EE +.\" SRC END +.SH SEE ALSO +.BR msgctl (2), +.BR msgget (2), +.BR capabilities (7), +.BR mq_overview (7), +.BR sysvipc (7) |