# Final Release Note A pre-allocation size may be specified for input-output (IO) string parameters in the [external call-out table](https://docs.yottadb.com/ProgrammersGuide/extrout.html#using-external-calls-call-outs), e.g. `IO:ydb_buffer_t*[100]`. Pre-allocation for `ydb_char_t *`, `ydb_string_t *`, and `ydb_buffer_t *` types was previously available for output (O) parameters but with this enhancement may also be specified for input-output (IO) parameters. Previously, an external function could only populate IO parameter strings up to the size of the input string. With pre-allocation, a maximum size may optionally be specified in the external call table to allocate more return space than the input string size. Note that IO `ydb_char_t*` and `ydb_string_t*` parameters do not allow querying the pre-allocation size at runtime; For robust applications, we recommend using `ydb_buffer_t*` for external calls. - Pre-allocation for non-string parameters in the external call-out table produces a [ZCPREALLVALSTR](https://docs.yottadb.com/MessageRecovery/errors.html#zcpreallvalstr) error. Previously, specifying a pre-allocation size for non-string parameters was silently ignored by YDB. Note that specifying the allocated space for non-string parameters is meaningless, as sizes are determined by the platform. - For IO parameters of type `ydb_string_t*` and `ydb_buffer_t*`, YDB distinguishes the empty string from skipped arguments in external callouts, treating the empty string consistently with non-empty strings. Previously, passing an empty string would behave as if the parameter were omitted altogether. - Note that `ydb_char_t*` still represents skipped strings as a 0-length null-terminated string. We recommend using `ydb_buffer_t*` for IO parameters to avoid ambiguity. - For O parameters, YDB distinguishes skipped arguments from other kinds of arguments in callouts. Previously, there would be no indication that the argument had been skipped and the C code was forced to assume an M actual value was present. - YDB passes skipped `ydb_string_t*` parameters as a `ydb_string_t { .length = 0, .address = NULL }` pair, for both IO and O parameters. Previously it only did so for skipped IO parameters, whereas skipped O parameters were passed as an empty string, which did not match the documentation. See [Using External Calls: Call-Outs](https://docs.yottadb.com/ProgrammersGuide/extrout.html#using-external-calls-call-outs) for more detail about how pre-allocated IO parameters are represented in C. [#1056] # Description Let users specify a pre-allcoation size for IO parameters in the external call-out table. See draft release note below for details of intended functionality. The new feature will be more intuitive for C developers who will otherwise be confused by buffer overruns. # Draft Release Note - A pre-allocation size may now be specified for input-output string parameters in the external call-out table, e.g. `IO:ydb_buffer_t*[100]`. Pre-allocation for `ydb_char_t *`, `ydb_string_t *`, and `ydb_buffer_t *` types was previously available for output (O) parameters but may now also be specified for input-output (IO). Previously, the external function could only populate IO parameter strings up to the size of the input string. Now with pre-allocation, a maximum size may optionally be specified in the external call table to allocate more return space than the input string size. Note that IO `ydb_char_t*` and `ydb_string_t*` parameters do not allow querying the pre-allocation size at runtime; to avoid mistakes, we recommend using `ydb_buffer_t*` for external call-outs. - Pre-allocation is no longer allowed for non-string parameters in the external call-out table. Previously, specifying a pre-allocation for non-string parameters was silently ignored by YDB. Now, it gives a ZCPREALLVALSTR error. - For IO parameters of type `ydb_string_t*` and `ydb_buffer_t*`, YDB now distinguishes the empty string from skipped arguments in external callouts. Previously, passing an empty string would behave as if the parameter were omitted altogether. Now, it is treated consistently with other non-empty strings. - Note that `ydb_char_t*` still represents skipped strings as a 0-length nul-terminated string. We recommend using `ydb_buffer_t*` for IO parameters to avoid ambiguity in this case. - For O parameters, YDB now distinguishes skipped arguments from other kinds of arguments in external callouts. Previously, there would be no indication that the argument had been skipped and the C code was forced to assume an M actual was present. - YDB now passes skipped `ydb_string_t*` parameters as a `ydb_string_t { .length = 0, .address = NULL }` pair, as documented. Previously, the documentation stated that skipped parameters would be passed as `{ .length = PRE_ALLOC_SIZE, .address = NULL }`, and additionally the behavior did not match the documentation. See https://docs.yottadb.com/ProgrammersGuide/extrout.html#using-external-calls-call-outs for more detail about how pre-allocated IO parameters are represented in C.