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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, respectively, 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 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 defined 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).) 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:

  • the queue is removed, in which case the system call fails with errno set to EIDRM; or
  • a signal is caught, in which case the system call fails with errno set to EINTR;see signal(7). (msgsnd() is never automatically restarted after being interrupted by a signal handler, regardless of the setting of the SA_RESTART flag when establishing a signal handler.)

Upon successful completion the message queue data structure is updated as follows:

msg_lspid is set to the process ID of the calling process.
msg_qnum is incremented by 1.
msg_stime is set to the current time.

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.

The argument msgtyp specifies the type of message requested as follows:

  • If msgtyp is 0, then the first message in the queue is read.
  • If msgtyp is greater than 0, then the first message in the queue of type msgtyp is read, unless MSG_EXCEPT was specified in msgflg, in which case the first message in the queue of type not equal to msgtyp will be read.
  • If 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 msgtyp will be read.

The msgflg argument is a bit mask constructed by ORing together zero or more of the following flags:

Return immediately if no message of the requested type is in the queue. The system call fails with errno set to ENOMSG.
Used with msgtyp greater than 0 to read the first message in the queue with message type that differs from msgtyp.
To truncate the message text if longer than msgsz bytes.

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:

  • A message of the desired type is placed in the queue.
  • The message queue is removed from the system. In this case the system call fails with errno set to EIDRM.
  • The calling process catches a signal. In this case the system call fails with errno set to EINTR. (msgrcv() is never automatically restarted after being interrupted by a signal handler, regardless of the setting of the SA_RESTART flag when establishing a signal handler.)

Upon successful completion the message queue data structure is updated as follows:

msg_lrpid is set to the process ID of the calling process.
msg_qnum is decremented by 1.
msg_rtime is set to the current time.

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:

The calling process does not have write permission on the message queue, and does not have the CAP_IPC_OWNER capability.
The message can't be sent due to the msg_qbytes limit for the queue and IPC_NOWAIT was specified in msgflg.
The address pointed to by msgp isn't accessible.
The message queue was removed.
Sleeping on a full message queue condition, the process caught a signal.
Invalid msqid value, or nonpositive mtype value, or invalid msgsz value (less than 0 or greater than the system value MSGMAX).
The system does not have enough memory to make a copy of the message pointed to by msgp.

When msgrcv() fails, errno will be set to one among the following values:

The message text length is greater than msgsz and MSG_NOERROR isn't specified in msgflg.
The calling process does not have read permission on the message queue, and does not have the CAP_IPC_OWNER capability.
No message was available in the queue and IPC_NOWAIT was specified in msgflg.
The address pointed to by msgp isn't accessible.
While the process was sleeping to receive a message, the message queue was removed.
While the process was sleeping to receive a message, the process caught a signal; see signal(7).
msgqid was invalid, or msgsz was less than 0.
IPC_NOWAIT was specified in msgflg and no message of the requested type existed on the message queue.

SVr4, POSIX.1-2001.

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 * with libc4, libc5, glibc 2.0, glibc 2.1. It is declared as void * with glibc 2.2 and later, as required by SUSv2 and SUSv3.

The following limits on message queue resources affect the msgsnd() call:

Maximum size for a message text: 8192 bytes (on Linux, this limit can be read and modified via /proc/sys/kernel/msgmax).
Default maximum size in bytes of a message queue: 16384 bytes (on Linux, this limit can be read and modified via /proc/sys/kernel/msgmnb). The superuser can increase the size of a message queue beyond MSGMNB by a msgctl(2) system call.

The implementation has no intrinsic limits for the system wide maximum number of message headers (MSGTQL) and for the system wide maximum size in bytes of the message pool (MSGPOOL).

msgctl(2), msgget(2), capabilities(7), mq_overview(7), svipc(7)

This page is part of release 3.54 of the Linux man-pages project. A description of the project, and information about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.