---
name: "CURLOPT_PREREQFUNCTION"
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_PREREQFUNCTION"
---

# CURLOPT\_PREREQFUNCTION(3)

user callback called when a connection has been established, but before a request has been made.

## NAME

CURLOPT\_PREREQFUNCTION - user callback called when a connection has been established, but before a request has been made.

## SYNOPSIS

```
#include <curl/curl.h>
/* These are the return codes for the pre-request callback. */
#define CURL_PREREQFUNC_OK 0
#define CURL_PREREQFUNC_ABORT 1 /* fail the entire transfer */
int prereq_callback(void *clientp,
                    char *conn_primary_ip,
                    char *conn_local_ip,
                    int conn_primary_port,
                    int conn_local_port);
CURLcode curl_easy_setopt(CURL *handle, CURLOPT_PREREQFUNCTION, prereq_callback);
```

## DESCRIPTION

Pass a pointer to your callback function, which should match the prototype shown above.

This function gets called by libcurl after a connection has been established or a connection has been reused (including any SSL handshaking), but before any request is actually made on the connection. For example, for HTTP, this callback is called once a connection has been established to the server, but before a GET/HEAD/POST/etc request has been sent.

This function may be called multiple times if redirections are enabled and are being followed (see *[CURLOPT\_FOLLOWLOCATION(3)](/CURLOPT_FOLLOWLOCATION/3.md)*).

The callback function must return *CURL\_PREREQFUNC\_OK* on success, or *CURL\_PREREQFUNC\_ABORT* to cause the transfer to fail with result *CURLE\_ABORTED\_BY\_CALLBACK*.

This function is passed the following arguments:

- conn\_primary\_ip

  A pointer to a null-terminated C string containing the primary IP of the remote server established with this connection. For FTP, this is the IP for the control connection. IPv6 addresses are represented without surrounding brackets.

- conn\_local\_ip

  A pointer to a null-terminated C string containing the originating IP for this connection. IPv6 addresses are represented without surrounding brackets.

- conn\_primary\_port

  The primary port number on the remote server established with this connection. For FTP, this is the port for the control connection. This can be a TCP or a UDP port number depending on the protocol.

- conn\_local\_port

  The originating port number for this connection. This can be a TCP or a UDP port number depending on the protocol.

- clientp

  The pointer you set with *[CURLOPT\_PREREQDATA(3)](/CURLOPT_PREREQDATA/3.md)*.

## DEFAULT

NULL

## PROTOCOLS

This functionality affects all supported protocols

## EXAMPLE

```
struct priv {
  void *custom;
};
static int prereq_callback(void *clientp,
                           char *conn_primary_ip,
                           char *conn_local_ip,
                           int conn_primary_port,
                           int conn_local_port)
{
  printf("Connection made to %s:%d\n", conn_primary_ip, conn_primary_port);
  return CURL_PREREQFUNC_OK;
}
int main(void)
{
  struct priv prereq_data;
  CURL *curl = curl_easy_init();
  if(curl) {
    CURLcode result;
    curl_easy_setopt(curl, CURLOPT_PREREQFUNCTION, prereq_callback);
    curl_easy_setopt(curl, CURLOPT_PREREQDATA, &prereq_data);
    result = curl_easy_perform(curl);
    curl_easy_cleanup(curl);
  }
}
```

## AVAILABILITY

Added in curl 7.80.0

## RETURN VALUE

*[curl\_easy\_setopt(3)](/curl_easy_setopt/3.md)* returns a CURLcode indicating success or error.

CURLE\_OK (0) means everything was OK, non-zero means an error occurred, see *[libcurl-errors(3)](/libcurl-errors/3.md)*.

## SEE ALSO

[**CURLINFO\_PRIMARY\_IP**(3)](/CURLINFO_PRIMARY_IP/3.md), [**CURLINFO\_PRIMARY\_PORT**(3)](/CURLINFO_PRIMARY_PORT/3.md), [**CURLOPT\_PREREQDATA**(3)](/CURLOPT_PREREQDATA/3.md)