MSGSND(3POSIX)

🌸 MSGSND(3POSIX) 🌸

okay bestie, before we even get into it — this page lives in the POSIX Programmer's Manual. The Linux version of this interface might act a little different (so like, go read the matching Linux manual page if that's your situation), or honestly it might just not be implemented on Linux at all. 💅

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✨ NAME ✨

msgsnd — the XSI message send operation. it sends messages. that's the whole vibe. 🦋

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🎀 SYNOPSIS 🎀

#include <sys/msg.h>

int msgsnd(int msqid, const void *msgp, size_t msgsz, int msgflg);

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💖 DESCRIPTION 💖

So msgsnd() operates on XSI message queues — that's the thing defined over in the Base Definitions volume of POSIX.1‐2017, Section 3.226, Message Queue. And lowkey? it is unspecified whether this function interoperates with the realtime interprocess communication facilities from Section 2.8, Realtime. so like, nobody's promising you anything there. 🙈

What msgsnd() actually does: it shall send a message to the queue associated with the message queue identifier you pass in as msqid. periodt.

The application shall ensure that the msgp argument points to a user-defined buffer. And that buffer has to contain, first, a field of type long specifying the type of the message, and then a data portion holding the data bytes of the message. The structure below is an example of what that user-defined buffer might look like:

struct mymsg {
    long   mtype;       /* Message type. */
    char   mtext[1];    /* Message text. */
}

The structure member mtype is a non-zero positive type long that the receiving process can use for message selection. 👑

The structure member mtext is any text of length msgsz bytes. The msgsz argument can range from 0 up to a system-imposed maximum.

The msgflg argument specifies the action to be taken if one or more of the following is true:

• The number of bytes already on the queue is equal to msg_qbytes; see <sys/msg.h>.
• The total number of messages on all queues system-wide is equal to the system-imposed limit.

And these are the actions, bestie:

• If (msgflg & IPC_NOWAIT) is non-zero, the message shall not be sent and the calling thread shall return immediately. 🏃♀️
• If (msgflg & IPC_NOWAIT) is 0, the calling thread shall suspend execution until one of the following occurs:
  • The condition responsible for the suspension no longer exists, in which case the message is sent. ✅
  • The message queue identifier msqid is removed from the system; when this occurs, errno shall be set to [EIDRM] and -1 shall be returned. 💀
  • The calling thread receives a signal that is to be caught; in this case the message is not sent and the calling thread resumes execution in the manner prescribed in sigaction().

Upon successful completion, the following actions are taken with respect to the data structure associated with msqid; see <sys/msg.h>:

• msg_qnum shall be incremented by 1. ⭐
• msg_lspid shall be set to the process ID of the calling process.
• msg_stime shall be set to the current time, as described in Section 2.7.1, IPC General Description.

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🔥 RETURN VALUE 🔥

Upon successful completion, msgsnd() shall return 0. otherwise — and this is the important part — no message shall be sent, msgsnd() shall return -1, and errno shall be set to indicate the error. 😭

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💜 ERRORS 💜

The msgsnd() function shall fail if:

• EACCES — Operation permission is denied to the calling process; see Section 2.7, XSI Interprocess Communication.
• EAGAIN — The message cannot be sent for one of the reasons cited above and (msgflg & IPC_NOWAIT) is non-zero.
• EIDRM — The message queue identifier msqid is removed from the system.
• EINTR — The msgsnd() function was interrupted by a signal.
• EINVAL — The value of msqid is not a valid message queue identifier, or the value of mtype is less than 1; or the value of msgsz is greater than the system-imposed limit.

heads up bestie: the following sections are informative. that means they're the lil bonus chapters, not the strict rules. 🫶

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🌈 EXAMPLES 🌈

🧚 Sending a Message

The following example sends a message to the queue identified by the msqid argument (assuming that value has previously been set). This call specifies that an error should be reported if no message is available. The message size is calculated directly using the sizeof operator.

#include <sys/msg.h>
...
int result;
int msqid;
struct message {
    long type;
    char text[20];
} msg;

msg.type = 1;
strcpy(msg.text, "This is message 1");
...
result = msgsnd(msqid, (void *) &msg, sizeof(msg.text), IPC_NOWAIT);

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🍓 APPLICATION USAGE 🍓

The POSIX Realtime Extension defines alternative interfaces for interprocess communication (IPC). Application developers who need to use IPC should design their applications so that modules using the IPC routines described in Section 2.7, XSI Interprocess Communication can be easily modified to use the alternative interfaces. ✨

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🌟 RATIONALE 🌟

None. literally nothing. moving on. 💀

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🦋 FUTURE DIRECTIONS 🦋

None. also nothing. the future is a blank canvas, babe. 🎀

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👑 SEE ALSO 👑

Section 2.7, XSI Interprocess Communication, Section 2.8, Realtime, mq_close(), mq_getattr(), mq_notify(), mq_open(), mq_receive(), mq_send(), mq_setattr(), mq_unlink(), msgctl(), msgget(), msgrcv(), sigaction()

The Base Definitions volume of POSIX.1‐2017, Section 3.226, Message Queue, <sys_msg.h>

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💅 COPYRIGHT 💅

Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1-2017, Standard for Information Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 7, 2018 Edition, Copyright (C) 2018 by the Institute of Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and the original IEEE and The Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained online at http://www.opengroup.org/unix/online.html .

Any typographical or formatting errors that appear in this page are most likely to have been introduced during the conversion of the source files to man page format. To report such errors, see https://www.kernel.org/doc/man-pages/reporting_bugs.html .

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and that's the whole page, bestie 🫶 this lil masterpiece came from IEEE/The Open Group, dated 2017, and it's filed under MSGSND(3POSIX). slay, stay safe with your message queues, and don't forget IPC_NOWAIT unless you're ready to wait. 💖✨🌈