MSGCTL(3POSIX)

💌 msgctl — XSI message control operations 💌

okay bestie, this one's from the POSIX Programmer's Manual (that's the 3POSIX section, iykyk). quick lil heads up before we dive in:

PROLOG: this page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (so go check the corresponding Linux manual page for the real tea on Linux behavior), or honestly the interface might not even be implemented on Linux at all. 🙈

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✨ NAME ✨

msgctl — XSI message control operations 🎀

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🦋 SYNOPSIS 🦋

#include <sys/msg.h>

int msgctl(int msqid, int cmd, struct msqid_ds *buf);

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💅 DESCRIPTION 💅

so the msgctl() function does its thing on XSI message queues — like, the whole concept is defined in the Base Definitions volume of POSIX.1‐2017, Section 3.226, Message Queue. and bestie, important caveat: it is unspecified whether this function interoperates with the realtime interprocess communication facilities defined in Section 2.8, Realtime. so like, don't assume, okay? 😭

the msgctl() function shall provide message control operations as specified by cmd. and the values for cmd — plus what each one actually does — are giving:

• IPC_STAT — places the current value of each member of the msqid_ds data structure associated with msqid into the structure pointed to by buf. the contents of this structure are defined in <sys/msg.h>. 💖
• IPC_SET — sets the value of these members of the msqid_ds data structure associated with msqid to the corresponding value found in the structure pointed to by buf:
  • msg_perm.uid
  • msg_perm.gid
  • msg_perm.mode
  • msg_qbytes

also, the msg_ctime timestamp shall be set to the current time, as described in Section 2.7.1, IPC General Description. ✨

so IPC_SET can only be executed by a process with appropriate privileges or one that has an effective user ID equal to the value of msg_perm.cuid or msg_perm.uid in the msqid_ds data structure associated with msqid. and only a process with appropriate privileges can raise the value of msg_qbytes. no cap. 👑

• IPC_RMID — removes the message queue identifier specified by msqid from the system and destroys the message queue and msqid_ds data structure associated with it. IPC_RMD can only be executed by a process with appropriate privileges or one that has an effective user ID equal to the value of msg_perm.cuid or msg_perm.uid in the msqid_ds data structure associated with msqid.

yes i typed IPC_RMD exactly like that because the original says IPC_RMD. character for character, bestie, that's the rule. 💀

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🌈 RETURN VALUE 🌈

upon successful completion, msgctl() shall return 0 ✨ — otherwise it shall return -1 and set errno to indicate the error. 😭

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🔥 ERRORS 🔥

the msgctl() function shall fail if:

• EACCES — the argument cmd is IPC_STAT and the calling process does not have read permission; see Section 2.7, XSI Interprocess Communication.
• EINVAL — the value of msqid is not a valid message queue identifier; or the value of cmd is not a valid command.
• EPERM — the argument cmd is IPC_RMID or IPC_SET and the effective user ID of the calling process is not equal to that of a process with appropriate privileges and it is not equal to the value of msg_perm.cuid or msg_perm.uid in the data structure associated with msqid.
• EPERM — the argument cmd is IPC_SET, an attempt is being made to increase to the value of msg_qbytes, and the effective user ID of the calling process does not have appropriate privileges.

the following sections are informative. 🫶

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🍓 EXAMPLES 🍓

None. 💀 (literally just none, that's the whole section, i'm not gonna invent examples, that's not the vibe)

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🧚 APPLICATION USAGE 🧚

so 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. future-proofing, bestie, it's giving main character energy. 💅

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🌸 RATIONALE 🌸

None. (again — none. iconic of it honestly.)

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⭐ FUTURE DIRECTIONS ⭐

None.

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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()
• msgget()
• msgrcv()
• msgsnd()

and also, from the Base Definitions volume of POSIX.1‐2017: Section 3.226, Message Queue, and <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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okay so that's the whole page fr fr 💖 huge shoutout to IEEE/The Open Group for this 2017 release — this is the MSGCTL(3POSIX) page, POSIX Programmer's Manual, and that's a wrap bestie, slay, periodt. 🎀✨🫶