---
name: "CURLOPT_HSTSWRITEFUNCTION"
section: "3"
platform: "linux"
package: "curl"
version: "8.23.0-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux", "macos"]
url: "https://new.carta.tech/CURLOPT_HSTSWRITEFUNCTION"
---

# CURLOPT\_HSTSWRITEFUNCTION(3)

write callback for HSTS hosts

## NAME

CURLOPT\_HSTSWRITEFUNCTION - write callback for HSTS hosts

## SYNOPSIS

```
#include <curl/curl.h>
struct curl_hstsentry {
  char *name;
  size_t namelen;
  unsigned int includeSubDomains:1;
  char expire[18]; /* YYYYMMDD HH:MM:SS [null-terminated] */
};
struct curl_index {
  size_t index; /* the provided entry's "index" or count */
  size_t total; /* total number of entries to save */
};
CURLSTScode hstswrite(CURL *easy, struct curl_hstsentry *sts,
                      struct curl_index *count, void *clientp);
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HSTSWRITEFUNCTION, hstswrite);
```

## DESCRIPTION

Pass a pointer to your callback function, as the prototype shows above.

This callback function gets called by libcurl repeatedly to allow the application to store the in-memory HSTS cache when libcurl is about to discard it.

Set the *clientp* argument with the *[CURLOPT\_HSTSWRITEDATA(3)](/CURLOPT_HSTSWRITEDATA/3.md)* option or it is NULL. When the callback is invoked, the *sts* pointer points to a populated struct: Read the hostname to 'name' (it is *namelen* bytes long and null-terminated. The *includeSubDomains* field is non-zero if the entry matches subdomains. The *expire* string is a date stamp null-terminated string using the syntax YYYYMMDD HH:MM:SS.

The callback should return *CURLSTS\_OK* if it succeeded and is prepared to be called again (for another host) or *CURLSTS\_DONE* if there is nothing more to do. It can also return *CURLSTS\_FAIL* to signal error.

This option does not enable HSTS, you need to use *[CURLOPT\_HSTS\_CTRL(3)](/CURLOPT_HSTS_CTRL/3.md)* to do that.

## DEFAULT

NULL - no callback.

## PROTOCOLS

This functionality affects http only

## EXAMPLE

```
struct priv {
  void *custom;
};
static CURLSTScode hswr_cb(CURL *easy, struct curl_hstsentry *sts,
                           struct curl_index *count, void *clientp)
{
  /* save the passed in HSTS data somewhere */
  return CURLSTS_OK;
}
int main(void)
{
  CURL *curl = curl_easy_init();
  if(curl) {
    struct priv my_stuff;
    CURLcode result;
    /* set HSTS read callback */
    curl_easy_setopt(curl, CURLOPT_HSTSWRITEFUNCTION, hswr_cb);
    /* pass in suitable argument to the callback */
    curl_easy_setopt(curl, CURLOPT_HSTSWRITEDATA, &my_stuff);
    result = curl_easy_perform(curl);
  }
}
```

## AVAILABILITY

Added in curl 7.74.0

## RETURN VALUE

This returns CURLE\_OK.

## SEE ALSO

[**CURLOPT\_HSTS**(3)](/CURLOPT_HSTS/3.md), [**CURLOPT\_HSTSWRITEDATA**(3)](/CURLOPT_HSTSWRITEDATA/3.md), [**CURLOPT\_HSTSWRITEFUNCTION**(3)](/CURLOPT_HSTSWRITEFUNCTION/3.md), [**CURLOPT\_HSTS\_CTRL**(3)](/CURLOPT_HSTS_CTRL/3.md)