nbd_set_debug_callback(3)

💖 nbd_set_debug_callback — like, the libnbd page for setting your debug callback ✨

okay bestie, so this is a page from libnbd(3) — the Network Block Device library — and it's all about one li'l function: nbd_set_debug_callback. It's giving "let me spy on my own library" energy and honestly? Iconic.

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🌸 SYNOPSIS 🌸

fr fr, here's the whole shape of it — don't touch, just vibe:

#include <libnbd.h>

typedef struct {
  int (*callback) (void *user_data, const char *context,
                   const char *msg);
  void *user_data;
  void (*free) (void *user_data);
} nbd_debug_callback;

int nbd_set_debug_callback (
      struct nbd_handle *h,
      nbd_debug_callback debug_callback
    );

Like, you pass in a handle h and a whole nbd_debug_callback struct, and it returns an int. Simple, clean, slay. 💅

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🦋 DESCRIPTION 🦋

So what does it do? It sets the debug callback. Groundbreaking, I know. 😭

This callback gets called when the library emits debug messages — specifically when debugging is enabled on a handle. The callback's parameters are:

• "user_data" — the thing you passed to this function, your little personal stash of whatever
• "context" — the name of the libnbd function that's emitting the debug message
• "msg" — the message itself, the actual tea ☕

And listen: if no debug callback is set on a handle, then messages are just printed on "stderr". No callback, no problem — it still tells you stuff, just the default way.

⚠️ okay this part is IMPORTANT bestie, do not skip: the callback should not call nbd_* APIs on the same handle. Why? Because it can be called while holding the handle lock, and that will cause a deadlock. 💀 Like, you'll just sit there forever waiting on yourself. Not the main character energy we want. Do not do it.

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🎀 RETURN VALUE 🎀

If the call is successful, the function returns 0. That's it. Zero. The good zero. 🫶

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🔥 ERRORS 🔥

On error, -1 is returned. 😭 The bad one.

Refer to "ERROR HANDLING" in libnbd(3) for how to get further details of the error — that's where the actual explanation lives, iykyk.

Also, heads up: the following parameters must not be NULL: "h". For more info, see "Non-NULL parameters" in libnbd(3). Don't be passing NULL handles around like it's cute, because it is not.

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⭐ VERSION ⭐

This function first appeared in libnbd 1.0. OG behavior, honestly. 👑

If you need to test whether this function is available at compile time, check if the following macro is defined:

#define LIBNBD_HAVE_NBD_SET_DEBUG_CALLBACK 1

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🌈 SEE ALSO 🌈

• nbd_create(3)
• libnbd(3)

Go read those too, bestie, they're part of the lore. 📚

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👑 AUTHORS 👑

• Eric Blake
• Richard W.M. Jones

Legends. Respect. 💜

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💖 COPYRIGHT 💖

Copyright Red Hat

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🧚 LICENSE 🧚

This library is free software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version.

This library is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public License along with this library; if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA

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aaand that's the whole page!! 🎉 this was libnbd-1.25.8, dated 2026-10-13, and the manual page is nbd_set_debug_callback(3) — go forth and debug, bestie, you've got this 💅✨🦋