POSIX_MEM_OFFSET(3P)

✨ PROLOG ✨

okay bestie, before we even start — heads up 🫶 this page is part of the POSIX Programmer's Manual. The Linux implementation of this interface may differ (so like, go check the corresponding Linux manual page for the deets on Linux behavior), or honestly the interface might just not be implemented on Linux at all. not me giving you the disclaimer first 💀

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💖 NAME 💖

posix_mem_offset — find the offset and length of a mapped typed memory block ✨ (ADVANCED REALTIME) — yes that's a whole vibe, we're in the advanced realtime section, iykyk

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🎀 SYNOPSIS 🎀

#include <sys/mman.h>

int posix_mem_offset(const void *restrict addr, size_t len,
    off_t *restrict off, size_t *restrict contig_len,
    int *restrict fildes);

look at all those restrict qualifiers, so cute, so demure 🌸

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🦋 DESCRIPTION 🦋

okay so here's the tea. The posix_mem_offset() function shall return in the variable pointed to by off a value that identifies the offset (or location), within a memory object, of the memory block currently mapped at addr. slay, it's literally telling you where in the memory object you're standing 💅

The function shall also return, in the variable pointed to by fildes, the descriptor that was used (via mmap()) to establish the mapping which contains addr. And here's the plot twist: if that descriptor was closed since the mapping was established, the returned value of fildes shall be -1. 😭 iconic but also devastating.

The len argument specifies the length of the block of the memory object the user wishes the offset for. Upon return, the value pointed to by contig_len shall equal either len, or the length of the largest contiguous block of the memory object that is currently mapped to the calling process starting at addr — whichever is smaller. it's giving "we'll give you what you asked for, or the best we can do" 🫶

Now for the advanced realtime lore ✨ — if the memory object mapped at addr is a typed memory object, then if the off and contig_len values obtained by calling posix_mem_offset() are used in a call to mmap() with a file descriptor that refers to the same memory pool as fildes (either through the same port or through a different port), and that was opened with neither the POSIX_TYPED_MEM_ALLOCATE nor the POSIX_TYPED_MEM_ALLOCATE_CONTIG flag, then the typed memory area that is mapped shall be exactly the same area that was mapped at addr in the address space of the process that called posix_mem_offset(). periodt, same area, no cap 🔥

And if the memory object specified by fildes is not a typed memory object? then the behavior of this function is implementation-defined. so like, good luck, it depends 🙈

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🌈 RETURN VALUE 🌈

Upon successful completion, posix_mem_offset() shall return zero. otherwise, the corresponding error status value shall be returned. no in-between, bestie — it's zero or it's a whole problem 💀

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🔥 ERRORS 🔥

The posix_mem_offset() function shall fail if:

• EACCES — the process has not mapped a memory object supported by this function at the given address addr. basically you're asking about an address that isn't playing by the rules, and she said "not today" 💅

also important: this function shall not return an error code of [EINTR]. noted, logged, filed away 📝

The following sections are informative. (translation: the next few bits are the bonus content, not the main character energy of the spec ✨)

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💜 EXAMPLES 💜

None. (she's mysterious like that 🙈)

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⭐ APPLICATION USAGE ⭐

None. (nothing to add, we're keeping it minimal, we're keeping it chic 🎀)

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🌸 RATIONALE 🌸

None. (the reasoning is a mystery, we simply respect it 💖)

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🧚 FUTURE DIRECTIONS 🧚

None. (no glow-up planned for this one, she's staying exactly as she is 👑)

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🌈 SEE ALSO 🌈

• mmap()
• posix_typed_mem_open()
• The Base Definitions volume of POSIX.1‐2017, <sys_mman.h>

go read those next, bestie, it's a whole connected universe 🦋

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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 one came to you from IEEE/The Open Group, dated 2017, under the name POSIX_MEM_OFFSET(3P) — a true icon of the POSIX Programmer's Manual, no cap 👑✨💖