CURLOPT\_PREREQFUNCTION(3) Library Functions Manual CURLOPT\_PREREQFUNCTION(3)

# [NAME](#NAME)

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

# [SYNOPSIS](#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](#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)*](/man/os/curl/CURLOPT_FOLLOWLOCATION.3)).

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](#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](#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](#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](#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](#clientp)

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

# [DEFAULT](#DEFAULT)

NULL

# [PROTOCOLS](#PROTOCOLS)

This functionality affects all supported protocols

# [EXAMPLE](#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](#AVAILABILITY)

Added in curl 7.80.0

# [RETURN VALUE](#RETURN_VALUE)

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

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

# [SEE ALSO](#SEE_ALSO)

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

2026-10-22 libcurl

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Document: CURLOPT_PREREQFUNCTION(3) from curl 8.23.0-r2 (Chainguard OS)
Source: https://github.com/curl/curl.git