Libgcrypt provides a way to install a different log handler to be used instead of the internal. Only few programs make use of it and thus it has been deprecated. If a log handler is not installed and since version 1.11 Libgcrypt uses the logging facility of GpgRT (aka Libgpg-error). They are more flexible that Libgcrypt’s old logging functions and given that GpgRT is anyway a dependency of Libgcrypt it is better to always use the GpgRT logging facilities.
This type is defined as: void (*gcry_handler_log_t) (void *, int, const char *, va_list)
void gcry_set_log_handler (gcry_handler_log_t func_log, void *cb_data) ¶This deprecated function registers func_log as ‘logging handler’, which means
that it will be called in case Libgcrypt wants to log a message. This
function may and should be used prior to calling
gcry_check_version.