POSIX_MADVISE(3POSIX)

📦 version 2017a, in debian bookworm, debian trixie, ubuntu jammy, ubuntu noble, ubuntu resolute, ubuntu stonking

👑 POSIX_MADVISE 👑

okay bestie, gather round, because we're reading the POSIX Programmer's Manual together and honestly? it's giving iconic. 💅 this page lives in section 3POSIX and it's about posix_madvise, so let's get into it fr fr. 🦋

🌸 PROLOG 🌸

so like, real talk first: this manual page is part of the POSIX Programmer's Manual. the Linux implementation of this interface may differ (go consult the corresponding Linux manual page for the deets on Linux behavior, no cap), or the interface may not be implemented on Linux at all. 💀

✨ NAME ✨

posix_madvise — memory advisory information and alignment control (ADVANCED REALTIME)

🎀 SYNOPSIS 🎀

#include <sys/mman.h>

int posix_madvise(void *addr, size_t len, int advice);

💖 DESCRIPTION 💖

so posix_madvise() is basically you sliding into the implementation's DMs to be like "hey bestie, here's what I'm expecting to do with this memory." specifically: it shall advise the implementation on the expected behavior of the application with respect to the data in the memory starting at address addr, and continuing for len bytes. the implementation may use this information to optimize handling of the specified data. 🫶

and here's the part that matters: the posix_madvise() function shall have no effect on the semantics of access to memory in the specified range, although it may affect the performance of access. it's a vibe check, not a rewrite. 🔥

also, the implementation may require that addr be a multiple of the page size, which is the value returned by sysconf() when the name value _SC_PAGESIZE is used. lowkey important. ✨

the advice to be applied to the memory range is specified by the advice parameter and may be one of the following values:

  • POSIX_MADV_NORMAL — says the application has no advice to give on its behavior with respect to the specified range. it's the default characteristic if no advice is given for a range of memory. the "no thoughts, head empty" option, and honestly? valid. 💜
  • POSIX_MADV_SEQUENTIAL — says the application expects to access the specified range sequentially, from lower addresses to higher addresses. main character walking a straight line. 👑
  • POSIX_MADV_RANDOM — says the application expects to access the specified range in a random order. chaotic energy, we love her. 🌀
  • POSIX_MADV_WILLNEED — says the application expects to access the specified range in the near future. manifesting it. ⭐
  • POSIX_MADV_DONTNEED — says the application expects that it will not access the specified range in the near future. it's giving "we're not gonna need that." 🙈

these values are defined in the <sys/mman.h> header. periodt.

🌈 RETURN VALUE 🌈

upon successful completion, posix_madvise() shall return zero 🎉; otherwise, an error number shall be returned to indicate the error. not me getting a whole error number instead of a zero 😭

💅 ERRORS 💅

the posix_madvise() function shall fail if:

  • EINVAL — the value of advice is invalid. bestie that's not a real advice value. 🚫
  • ENOMEM — addresses in the range starting at addr and continuing for len bytes are partly or completely outside the range allowed for the address space of the calling process. that memory is not yours, respectfully. 💀

the posix_madvise() function may fail if:

  • EINVAL — the value of addr is not a multiple of the value returned by sysconf() when the name value _SC_PAGESIZE is used.
  • EINVAL — the value of len is zero.
heads up: the following sections are informative. so like, they're the bonus content, not the rules. 🎀

🦋 EXAMPLES 🦋

None.

(yeah. none. the examples section said "not today." iconic behavior honestly 😌)

🍓 APPLICATION USAGE 🍓

the posix_madvise() function is part of the Advisory Information option and need not be provided on all implementations. so if you can't find her, she might just not be invited to that platform's party. 💔

🧚 RATIONALE 🧚

None.

🌟 FUTURE DIRECTIONS 🌟

None.

💜 SEE ALSO 💜

  • mmap()
  • posix_fadvise()
  • sysconf()

and from The Base Definitions volume of POSIX.1‐2017: <sys_mman.h> 📚

legal moment, bestie — copying this exactly because the lawyers said so, and we respect the lawyers. 🫡

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 .

and that's the whole page, babe 💖 this one came to you from IEEE/The Open Group, dated 2017, under the name POSIX_MADVISE(3POSIX) — thanks for reading with me, mwah, go optimize something responsibly 💋✨

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