---
c: Copyright (C) The libssh2 project and its contributors.
SPDX-License-Identifier: BSD-3-Clause
Title: libssh2_session_callback_set2
Section: 3
Source: libssh2
See-also:
  - libssh2_agent_sign(3)
  - libssh2_session_init_ex(3)
---

# NAME

libssh2_session_callback_set2 - set a callback function

# SYNOPSIS

~~~c
#include <libssh2.h>

libssh2_cb_generic *
libssh2_session_callback_set2(LIBSSH2_SESSION *session, int cbtype,
                              libssh2_cb_generic *callback);
~~~

# DESCRIPTION

Sets a custom callback handler for a previously initialized session
object. Callbacks are triggered by the receipt of special packets at the
Transport layer. To disable a callback, set it to NULL.

*session* - Session instance as returned by libssh2_session_init_ex(3)

*cbtype* - Callback type. One of the types listed in Callback Types.

*callback* - Pointer to custom callback function. The prototype for
this function must match the associated callback declaration macro.

# CALLBACK TYPES

## LIBSSH2_CALLBACK_IGNORE

Called when a SSH_MSG_IGNORE message is received

## LIBSSH2_CALLBACK_DEBUG

Called when a SSH_MSG_DEBUG message is received

## LIBSSH2_CALLBACK_DISCONNECT

Called when a SSH_MSG_DISCONNECT message is received

## LIBSSH2_CALLBACK_MACERROR

Called when a mismatched MAC has been detected in the transport layer. If the
function returns 0, the packet will be accepted nonetheless.

## LIBSSH2_CALLBACK_X11

Called when an X11 connection has been accepted

## LIBSSH2_CALLBACK_SEND

Called when libssh2 wants to send data on the connection. Can be set to a
custom function to handle I/O your own way.

The prototype of the callback:

~~~c
ssize_t sendcb(libssh2_socket_t sockfd, const void *buffer,
               size_t length, int flags, void **abstract);
~~~

**sockfd** is the socket to write to, **buffer** points to the data to
send, **length** is the size of the data, **flags** is the flags that
would have been used to a *send()* call and **abstract** is a pointer
to the abstract pointer set in the *libssh2_session_init_ex(3)* call.

The callback returns the number of bytes sent, or -1 for error. The special
return code **-EAGAIN** can be returned to signal that the send was aborted
to prevent getting blocked and it needs to be called again.

## LIBSSH2_CALLBACK_RECV

Called when libssh2 wants to read data from the connection. Can be set to a
custom function to handle I/O your own way.

The prototype of the callback:

~~~c
ssize_t recvcb(libssh2_socket_t sockfd, void *buffer,
               size_t length, int flags, void **abstract);
~~~

**sockfd** is the socket to read from, **buffer** where to store received
data into, **length** is the size of the buffer, **flags** is the flags
that would have been used to a *recv()* call and **abstract** is a pointer
to the abstract pointer set in the *libssh2_session_init_ex(3)* call.

The callback returns the number of bytes read, or -1 for error. The special
return code **-EAGAIN** can be returned to signal that the read was aborted
to prevent getting blocked and it needs to be called again.

## LIBSSH2_CALLBACK_AUTHAGENT

Called during authentication process to allow the client to connect to the
ssh-agent and perform any setup, such as configuring the agent or adding keys.

The prototype of the callback:

~~~c
void authagent(LIBSSH2_SESSION* session, LIBSSH2_CHANNEL *channel,
               void **abstract);
~~~

## LIBSSH2_CALLBACK_AUTHAGENT_IDENTITIES

Not called by libssh2. The client is responsible for calling this method when
a SSH2_AGENTC_REQUEST_IDENTITIES message has been received.

The prototype of the callback:

~~~c
void identities(LIBSSH2_SESSION* session, void *buffer,
                const char *agent_path,
                void **abstract)
~~~

**buffer** must be filled in by the callback. Different clients may implement
this differently. For example, one client may pass in an unsigned char ** for
this parameter, while another may pass in a pointer to a struct.

Regardless of the type of buffer used, the client will need to send back a list
of identities in the following format.

uint32 buffer length
uint32 number of entries
entries

Where each entry in the entries list is of the format:

string data
cstring comment

**agent_path** The path to a running ssh-agent on the client machine, from
which identities can be listed.

## LIBSSH2_CALLBACK_AUTHAGENT_SIGN

Not called by libssh2. The client is responsible for calling this method when
a SSH2_AGENTC_SIGN_REQUEST message has been received.

The prototype of the callback:

~~~c
void sign(LIBSSH2_SESSION* session,
          unsigned char *blob, unsigned int blen,
          const unsigned char *data, unsigned int dlen,
          unsigned char **sig, unsigned int *sig_len,
          const char *agent_path,
          void **abstract);
~~~

When interfacing with an ssh-agent installed on the client system, this method
can call libssh2_agent_sign(3) to perform signing.

# RETURN VALUE

Pointer to previous callback handler. Returns NULL if no prior callback
handler was set or the callback type was unknown.