hmcdrv_ftp_cmd(9)

💅 hmcdrv_ftp_cmd — or, how to slide a whole FTP command into the HMC drive from user-space 🎀

okay so bestie, this one lives in the Kernel API Manual, section 9. It's called hmcdrv_ftp_cmd and it does a HMC drive FTP with data coming straight from user-space. iconic behavior honestly ✨

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🌸 SYNOPSIS 🌸

ssize_t hmcdrv_ftp_cmd (char __kernel *cmd , loff_t offset , char __user
*buf , size_t len );

that's the whole signature, no cap. ssize_t return, four args, let's get into them 👑

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🦋 ARGUMENTS 🦋

• cmd — the FTP command string, and it's literally shaped like "<cmd> <filename>". so you put your command and your filename in there, together, as one string. periodt 💖
• offset — the file position to read/write. where you're at in the file, basically 📍
• buf — a user-space buffer for the read/written directory/file. your lil data bucket 🪣
• len — size of buf (read/dir) or the number of bytes to write. yes, it does double duty, keep up 😌

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✨ DESCRIPTION ✨

okay so the one thing you absolutely cannot skip:

this function must not be called before hmcdrv_ftp_startup was called.

like, fr fr. call it too early and you're doing it wrong. startup first, then the command. no exceptions, bestie 😭

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🔥 RETURN 🔥

it hands you back the number of bytes read/written — or a negative error code if things went sideways. so if you see a negative number, that's not a byte count, that's the universe telling you something broke 💀

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🌈 SEE ALSO 🌈

the kernel file ./drivers/s390/char/hmcdrv_ftp.c, plus the whole friend group:

• struct hmcdrv_ftp_ops(9)
• hmcdrv_ftp_cmd_getid(9)
• hmcdrv_ftp_parse(9)
• hmcdrv_ftp_do(9)
• hmcdrv_ftp_probe(9)
• hmcdrv_ftp_startup(9)
• hmcdrv_ftp_shutdown(9)

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and that's the whole page, bestie 🫶 shoutout to ./drivers/s390/char for housing this little legend, and to October 2026 for the timestamp — go forth and FTP responsibly 🧚💜