| MSGOP(2) | Linux Programmer's Manual | MSGOP(2) |
msgrcv, msgsnd - System V message queue operations
#include <sys/types.h> #include <sys/ipc.h> #include <sys/msg.h>
int msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg);
ssize_t msgrcv(int msqid, void *msgp, size_t msgsz, long msgtyp,
int msgflg);
The msgsnd() and 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.
The msgp argument is a pointer to a caller-defined structure of the following general form:
struct msgbuf {
long mtype; /* message type, must be > 0 */
char mtext[1]; /* message data */
};
The mtext field is an array (or other structure) whose size is specified by msgsz, a nonnegative integer value. Messages of zero length (i.e., no mtext field) are permitted. The 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 msgrcv() below).
The msgsnd() system call appends a copy of the message pointed to by msgp to the message queue whose identifier is specified by msqid.
If sufficient space is available in the queue, msgsnd() succeeds immediately. The queue capacity is governed by the msg_qbytes field in the associated data structure for the message queue. During queue creation this field is initialized to MSGMNB bytes, but this limit can be modified using msgctl(2). A message queue is considered to be full if either of the following conditions is true:
If insufficient space is available in the queue, then the default behavior of msgsnd() is to block until space becomes available. If IPC_NOWAIT is specified in msgflg, then the call instead fails with the error EAGAIN.
A blocked msgsnd() call may also fail if:
Upon successful completion the message queue data structure is updated as follows:
The msgrcv() system call removes a message from the queue specified by msqid and places it in the buffer pointed to by msgp.
The argument msgsz specifies the maximum size in bytes for the member mtext of the structure pointed to by the msgp argument. If the message text has length greater than msgsz, then the behavior depends on whether MSG_NOERROR is specified in msgflg. If MSG_NOERROR is specified, then the message text will be truncated (and the truncated part will be lost); if MSG_NOERROR is not specified, then the message isn't removed from the queue and the system call fails returning -1 with errno set to E2BIG.
Unless MSG_COPY is specified in msgflg (see below), the msgtyp argument specifies the type of message requested, as follows:
The msgflg argument is a bit mask constructed by ORing together zero or more of the following flags:
If no message of the requested type is available and IPC_NOWAIT isn't specified in msgflg, the calling process is blocked until one of the following conditions occurs:
Upon successful completion the message queue data structure is updated as follows:
On failure both functions return -1 with errno indicating the error, otherwise msgsnd() returns 0 and msgrcv() returns the number of bytes actually copied into the mtext array.
When msgsnd() fails, errno will be set to one among the following values:
When msgrcv() fails, errno will be set to one among the following values:
POSIX.1-2001, POSIX.1-2008, SVr4.
The MSG_EXCEPT and MSG_COPY flags are Linux-specific; their definitions can be obtained by defining the _GNU_SOURCE feature test macro.
The inclusion of <sys/types.h> and <sys/ipc.h> isn't required on Linux or by any version of POSIX. However, some old implementations required the inclusion of these header files, and the SVID also documented their inclusion. Applications intended to be portable to such old systems may need to include these header files.
The msgp argument is declared as struct msgbuf * in glibc 2.0 and 2.1. It is declared as void * in glibc 2.2 and later, as required by SUSv2 and SUSv3.
The following limits on message queue resources affect the msgsnd() call:
The implementation has no intrinsic system-wide limits on the number of message headers (MSGTQL) and the number of bytes in the message pool (MSGPOOL).
In Linux 3.13 and earlier, if msgrcv() was called with the MSG_COPY flag, but without IPC_NOWAIT, and the message queue contained less than msgtyp messages, then the call would block until the next message is written to the queue. At that point, the call would return a copy of the message, regardless of whether that message was at the ordinal position msgtyp. This bug is fixed in Linux 3.14.
Specifying both MSG_COPY and MSC_EXCEPT in msgflg is a logical error (since these flags impose different interpretations on msgtyp). In Linux 3.13 and earlier, this error was not diagnosed by msgrcv(). This bug is fixed in Linux 3.14.
The program below demonstrates the use of msgsnd() and msgrcv().
The example program is first run with the -s option to send a message and then run again with the -r option to receive a message.
The following shell session shows a sample run of the program:
$ ./a.out -s sent: a message at Wed Mar 4 16:25:45 2015 $ ./a.out -r message received: a message at Wed Mar 4 16:25:45 2015
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <unistd.h>
#include <errno.h>
#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.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]\n", prog_name);
fprintf(stderr, "Options are:\n");
fprintf(stderr, "-s send message using msgsnd()\n");
fprintf(stderr, "-r read message using msgrcv()\n");
fprintf(stderr, "-t message type (default is 1)\n");
fprintf(stderr, "-k message queue key (default is 1234)\n");
exit(EXIT_FAILURE);
}
static void
send_msg(int qid, int msgtype)
{
struct msgbuf msg;
time_t t;
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\n", 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()\n");
} else
printf("message received: %s\n", 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 's':
mode = 1;
break;
case 'r':
mode = 2;
break;
case 't':
msgtype = atoi(optarg);
if (msgtype <= 0)
usage(argv[0], "-t option must be greater than 0\n");
break;
case 'k':
msgkey = atoi(optarg);
break;
default:
usage(argv[0], "Unrecognized option\n");
}
}
if (mode == 0)
usage(argv[0], "must use either -s or -r option\n");
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);
}
msgctl(2), msgget(2), capabilities(7), mq_overview(7), sysvipc(7)
This page is part of release 5.10 of the Linux man-pages project. A description of the project, information about reporting bugs, and the latest version of this page, can be found at https://www.kernel.org/doc/man-pages/.
| 2020-11-01 | Linux |