CURLOPT_SSH_HOSTKEYDATA(3)

💅 CURLOPT_SSH_HOSTKEYDATA — the manual page, but make it make sense ✨

okay so real talk: this is the CURLOPT_SSH_HOSTKEYDATA(3) page from the libcurl Library Functions Manual, and it's genuinely short and sweet. It's about handing a little pointer to your SSH host key callback so your callback has somewhere to stash its vibe. Let's get into it 🫶

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✨ NAME ✨

CURLOPT_SSH_HOSTKEYDATA — literally just a pointer you pass along to the SSH host key callback 💅

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🦋 SYNOPSIS 🦋

#include <curl/curl.h>

CURLcode curl_easy_setopt(CURL *handle, CURLOPT_SSH_HOSTKEYDATA, void *pointer);

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🌸 DESCRIPTION 🌸

So here's the tea ☕: you pass a void * as the parameter. That's it. That pointer gets passed along untouched — like, zero meddling, not even a little — to the callback you set with CURLOPT_SSH_HOSTKEYFUNCTION(3).

Think of it as the little bag your callback carries around. It's your stuff, you put it in there, and curl just hands the whole bag over. No cap. 💖

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🎀 DEFAULT 🎀

NULL

(yeah, empty bag by default, bestie 🙈)

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🌈 PROTOCOLS 🌈

This functionality affects scp and sftp 🧚

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⭐ EXAMPLE ⭐

ok bestie here's the whole thing in action, copy it exactly, don't be weird about it:

struct mine {
  void *custom;
};

static int hostkeycb(void *clientp,   /* CURLOPT_SSH_HOSTKEYDATA */
                     int keytype,     /* CURLKHTYPE */
                     const char *key, /* host key to check */
                     size_t keylen)   /* length of the key */
{
  /* 'clientp' points to the callback_data struct */
  /* investigate the situation and return the correct value */
  return CURLKHMATCH_OK;
}

int main(void)
{
  CURL *curl = curl_easy_init();
  if(curl) {
    struct mine callback_data;
    curl_easy_setopt(curl, CURLOPT_URL, "sftp://example.com/thisfile.txt");
    curl_easy_setopt(curl, CURLOPT_SSH_HOSTKEYFUNCTION, hostkeycb);
    curl_easy_setopt(curl, CURLOPT_SSH_HOSTKEYDATA, &callback_data);

    curl_easy_perform(curl);
  }
}

see how &callback_data gets yeeted straight into clientp inside the callback? that's the whole trick. iconic behavior honestly 👑

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💀 NOTES 💀

Works only with the libssh2 backend. not negotiable, not flexible, that's just how it is 😭

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👑 AVAILABILITY 👑

Added in curl 7.84.0 ⭐ (so if you're on something ancient, this option is gonna look at you like 👁️👄👁️)

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🔥 RETURN VALUE 🔥

curl_easy_setopt(3) returns a CURLcode indicating success or error.

CURLE_OK (0) means everything was OK — slay 💖 — and non-zero means an error occurred. go see libcurl-errors(3) for the full drama.

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🫶 SEE ALSO 🫶

• CURLOPT_SSH_HOSTKEYFUNCTION(3) — the callback that actually receives your precious pointer, duh ✨

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and that's the page, bestie 🎀 huge shoutout to the libcurl girlies — this one's from the libcurl source, dated 2026-10-12, and it says CURLOPT_SSH_HOSTKEYDATA(3) right there in the footnotes. go forth and pass your pointers responsibly 💜🦋