VOP_RDWR(9)

💖 VOP_RDWR — read or write a file 💖

okay bestie, pull up a chair, we're doing the VOP_READ / VOP_WRITE page from the Kernel Developer's Manual (that's section 9, the kernel girlies' section, iykyk) ✨

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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 🦋

so these two entry points read or write the contents of a file. that's it, that's the vibe. 📖✍️

here's the cast of arguments, no cap:

• vp — the vnode of the file.
• uio — the location of the data to be read or written.
• ioflag — various flags.
• cnp — the credentials of the caller.

💀 not the arg list saying cnp while the synopsis says cred… she contains multitudes. keeping both exactly as the page says them, don't @ me.

the ioflag argument is literally 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. the low 16 bits are a bit mask, and that mask can contain these flags:

• IO_UNIT — do I/O as atomic unit.
• IO_APPEND — append write to end.
• IO_SYNC — do I/O synchronously.
• IO_NODELOCKED — underlying node already locked.
• IO_NDELAY — FNDELAY flag set in file table.
• IO_VMIO — data already in VMIO space.

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🔒 LOCKS 🔒

the file should be locked on entry and will still be locked on exit. she arrives locked, she leaves locked, main character consistency. 👑

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⭐ RETURN VALUES ⭐

Zero is returned on success — slay. 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.
• [ENOSPC] — the file system is full.
• [EPERM] — an append-only flag is set on the file, but the caller is attempting to write before the current end of file.

🙈 yes bestie, writing past the end on an append-only file is gonna get you an EPERM, that's just the rules.

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👀 SEE ALSO 👀

• uiomove(9)
• vnode(9)

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💜 AUTHORS 💜

this manual page was written by Doug Rabson. icon behavior, honestly. 🫶

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and that's the whole page, bestie — go read and write some files, but lock your vnodes first. 💅✨

this lil number came from the Debian crew, dated July 24, 1996 — yeah she's vintage, she's still iconic, periodt. 🎀🌈⭐