VOP_RDWR(9)

💅 VOP_RDWR — the kernel dev manual, section 9, bestie

okay so this is VOP_RDWR(9) from the Kernel Developer's Manual (section 9, the kernel-dev one, not the user-command one — iykyk) and it's literally the whole tea on reading and writing file contents at the vnode layer. pull up a chair. 🎀

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✨ NAME ✨

VOP_READ, VOP_WRITE — read or write a file

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✨ SYNOPSIS ✨

#include <sys/param.h>
#include <sys/vnode.h>
#include <sys/uio.h>

int
VOP_READ(struct vnode *vp, struct uio *uio, int ioflag,
    struct ucred *cred);

int
VOP_WRITE(struct vnode *vp, struct uio *uio, int ioflag,
    struct ucred *cred);

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✨ DESCRIPTION ✨

these are the entry points that read or write the actual contents of a file. main character energy, honestly. 🌟

the arguments are:

• vp — the vnode of the file. the whole file's little identity card. 🪪
• uio — the location of the data to be read or written. where the goods live.
• ioflag — various flags. (we get into these, don't worry)
• cnp — the credentials of the caller. yes bestie, the arg rundown literally says cnp while the synopsis says cred — that's just how the page is written, no cap, we're keeping it exactly as-is. 🙈

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🌸 the ioflag situation (this is the good part)

so the ioflag argument is how you give directives and hints to the file system. when you're attempting a read, the high 16 bits are used to provide a read-ahead hint (in units of file system blocks) that the file system should attempt. cute, right? ✨

the low 16 bits are a bit mask which can contain the following flags:

• IO_UNIT — do I/O as atomic unit. all-or-nothing vibes. 💅
• IO_APPEND — append write to end. no cutting the line.
• IO_SYNC — do I/O synchronously. we wait. we're patient. 🧚
• IO_NODELOCKED — underlying node already locked. somebody got here first.
• IO_NDELAY — FNDELAY flag set in file table.
• IO_VMIO — data already in VMIO space. been there, done that. 👑

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🔒 LOCKS 🔒

the file should be locked on entry and will still be locked on exit. like, you hold the lock the whole time, you don't drop it, periodt. 🔥

also: a rangelock covering the whole i/o range should be owned around the call.

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🫶 RETURN VALUES 🫶

zero is returned on success 🎉 — otherwise an error code is returned. 😭

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💀 ERRORS 💀

• [EFBIG] — an attempt was made to write a file that exceeds the process's file size limit or the maximum file size. too big, babe.
• [ENOSPC] — the file system is full. it's giving crowded elevator. 🍓
• [EPERM] — an append-only flag is set on the file, but the caller is attempting to write before the current end of file. rude, honestly.

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🌈 SEE ALSO 🌈

uiomove(9), vnode(9)

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👑 AUTHORS 👑

this manual page was written by Doug Rabson. iconic of him, fr fr. 🫶

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quick vibe check before you go: this page is a kernel developer's page, so nothing here is deprecated or undefined-behaviour-flagged in the original — it's just the straight-up contract. read the flags carefully, hold your locks, and you're slaying. 💖

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that's the whole page, bestie — originally dated July 24, 1996, shipped under the Debian banner, and still living rent-free in VOP_RDWR(9) to this day. mwah. 💋