---
name: "CURLOPT_HSTS_CTRL"
section: "3"
platform: "macos"
package: "curl"
version: "166.0.0.0.1"
release: "macOS 26.5"
platforms: ["linux", "macos"]
url: "https://new.carta.tech/CURLOPT_HSTS_CTRL"
---

# CURLOPT\_HSTS\_CTRL(3)

control HSTS behavior

## NAME

CURLOPT\_HSTS\_CTRL - control HSTS behavior

## SYNOPSIS

```
#include <curl/curl.h>
#define CURLHSTS_ENABLE       (1<<0)
#define CURLHSTS_READONLYFILE (1<<1)
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_HSTS_CTRL, long bitmask);
```

## DESCRIPTION

HSTS (HTTP Strict Transport Security) means that an HTTPS server can instruct the client to not contact it again over clear-text HTTP for a certain period into the future. libcurl then automatically redirects HTTP attempts to such hosts to instead use HTTPS. This is done by libcurl retaining this knowledge in an in-memory cache.

Populate the long *bitmask* with the correct set of features to instruct libcurl how to handle HSTS for the transfers using this handle.

## BITS

- CURLHSTS\_ENABLE

  Enable the in-memory HSTS cache for this handle.

- CURLHSTS\_READONLYFILE

  Make the HSTS file (if specified) read-only - makes libcurl not save the cache to the file when closing the handle.

## DEFAULT

0\. HSTS is disabled by default.

## PROTOCOLS

HTTPS and HTTP

## EXAMPLE

```
int main(void)
{
  CURL *curl = curl_easy_init();
  if(curl) {
    curl_easy_setopt(curl, CURLOPT_HSTS_CTRL, (long)CURLHSTS_ENABLE);
    curl_easy_perform(curl);
  }
}
```

## AVAILABILITY

Added in 7.74.0

## RETURN VALUE

Returns CURLE\_OK if the option is supported, and CURLE\_UNKNOWN\_OPTION if not.

## SEE ALSO

[**CURLOPT\_ALTSVC**(3)](/CURLOPT_ALTSVC/3.md), [**CURLOPT\_CONNECT\_TO**(3)](/CURLOPT_CONNECT_TO/3.md), [**CURLOPT\_HSTS**(3)](/CURLOPT_HSTS/3.md), [**CURLOPT\_RESOLVE**(3)](/CURLOPT_RESOLVE/3.md)