MSGSND(2)

💌 MSGSND(2) — bestie edition

okay so this is msgsnd, a system call from the System Calls Manual bestie, and it lives in the Standard C Library (libc, -lc). it sends messages to message queues and honestly? iconic behavior.

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✨ NAME ✨

msgsnd — send a message to a message queue 💬

📚 LIBRARY 📚

Standard C Library (libc, -lc)

🎀 SYNOPSIS 🎀

#include <sys/types.h>
#include <sys/ipc.h>
#include <sys/msg.h>

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

💖 DESCRIPTION 💖

so the way msgsnd() works is it sends a message to the message queue you specified in msqid. and msgp is a pointer to a structure holding the actual message. that structure should have these members, no cap:

long mtype;    /* message type */
char mtext[1]; /* body of message */

mtype is an integer greater than 0 and it's what you use to pick out messages later (peep msgrcv(2) for that one 🫶). and mtext is an array of msgsz bytes. the msgsz argument can go from 0 all the way up to a system-imposed maximum, MSGMAX. lowkey a glow-up range.

now here's the tea: if the number of bytes already chillin' on the message queue plus msgsz is bigger than the maximum number of bytes allowed on the queue (msg_qbytes, see msgctl(2)), or the number of messages on all queues system-wide is already equal to the system limit, then msgflg is the one deciding what msgsnd() does next:

• if msgflg has the IPC_NOWAIT mask set in it, the call just returns immediately. no waiting, main character energy.
• if msgflg does not have IPC_NOWAIT set in it, the call will block until:
  • the condition that made it block no longer exists. the message will be sent ✨
  • the message queue is removed — in which case -1 gets returned and errno is set to EINVAL 😭
  • the caller catches a signal. the call returns with errno set to EINTR.

after a successful call, the data structure associated with the message queue gets updated like this:

• msg_cbytes is incremented by the size of the message.
• msg_qnum is incremented by 1.
• msg_lspid is set to the pid of the calling process.
• msg_stime is set to the current time.

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🌈 RETURN VALUES 🌈

msgsnd() returns the value 0 if it slayed (successful 🎉). otherwise the value -1 is returned and the global variable errno is set to indicate the error. sad girl hours fr.

💀 ERRORS 💀

msgsnd() will fail if:

• [EINVAL] — the msqid argument is not a valid message queue identifier.

the message queue was removed while msgsnd() was waiting for a resource to become available in order to deliver the message.

the msgsz argument is greater than msg_qbytes.

the mtype argument is not greater than 0.

• [EACCES] — the calling process does not have write access to the message queue. 🙈
• [EAGAIN] — there was no space for this message either on the queue, or in the whole system, and IPC_NOWAIT was set in msgflg.
• [EFAULT] — the msgp argument points to an invalid address.
• [EINTR] — the system call was interrupted by the delivery of a signal.

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🦋 HISTORY 🦋

message queues appeared in the first release of AT&T Unix System V. old money behavior honestly 👑

🐛 BUGS 🐛

NetBSD and FreeBSD do not define the EIDRM error value, which should be used in the case of a removed message queue. not them being inconsistent 💀

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and that's the whole page bestie 💜 this one shipped with Debian, dated July 9, 2009, and the man page is literally just called MSGSND(2). go send some messages, you got this 🫶✨