# mongoc_handshake_data_append()

## Synopsis

    bool
    mongoc_handshake_data_append (const char *driver_name,
                                  const char *driver_version,
                                  const char *platform);

Appends the given strings to the handshake data for the underlying C Driver.

## Description

This function is intended for use by drivers which wrap the C Driver. Calling this function will store the given strings as handshake data about the system and driver by appending them to the handshake data for the underlying C Driver. These values, along with other handshake data collected during mongoc_init will be sent to the server as part of the initial connection handshake in the “client” document. This function must not be called more than once, or after server monitoring begins. For a single-threaded mongoc_client_t, server monitoring begins on the first operation requiring a server. For a mongoc_client_pool_t, server monitoring begins on the first call to :symbol:`mongoc_client_pool_pop.

The passed in strings are copied, and don’t have to remain valid after the call to mongoc_handshake_data_append(). The driver may store truncated versions of the passed in strings.

Note

This function allocates memory, and therefore caution should be used when using this in conjunction with `bson_mem_set_vtable()`. If this function is called before `bson_mem_set_vtable()`, then `bson_mem_restore_vtable()` must be called before calling mongoc_cleanup(). Failure to do so will result in memory being freed by the wrong allocator.

## Parameters

- `driver_name`: An optional string storing the name of the wrapping driver

- `driver_version`: An optional string storing the version of the wrapping driver.

- `platform`: An optional string storing any information about the current platform, for example configure options or compile flags.

## Returns

`true` if the given fields are set successfully. Otherwise, it returns `false` and logs an error.

The default handshake data the driver sends with “hello” looks something like:

    client: {
      driver: {
        name: "mongoc",
        version: "1.5.0"
      },
      os: {...},
      platform: "CC=gcc CFLAGS=-Wall -pedantic"
    }

If we call mongoc_handshake_data_append() (“phongo”, “1.1.8”, “CC=clang / “) and it returns true, the driver sends handshake data like:

    client: {
      driver: {
        name: "mongoc / phongo",
        version: "1.5.0 / 1.1.8"
      },
      os: {...},
      platform: "CC=clang / gcc CFLAGS=-Wall -pedantic"
    }