[#bsl-basic_syncbuf-underflow]
= xref:bsl.adoc[bsl]::xref:bsl/basic_syncbuf.adoc[basic_syncbuf]::underflow
:relfileprefix: ../../
:mrdocs:

Fetches more data from the controlled sequence.

== Synopsis

Declared in `<bslstl_syncbuf.h>`

[source,cpp,subs="verbatim,replacements,macros,-callouts"]
----
virtual
int_type
underflow();
----

== Description

Informally, this function is called when the input buffer is exhausted (or does not exist, as buffering need not actually be done). If a buffer exists, it is _refilled._ In either case, the next available character is returned, or `traits::eof()` to indicate a null pending sequence.

For a formal definition of the pending sequence, see a good text such as Langer & Kreft, or [27.5.2.4.3]/7‐14.

A functioning input streambuf can be created by overriding only this function (no buffer area will be used). For an example, see https://gcc.gnu.org/onlinedocs/libstdc++/manual/streambufs.html

[NOTE]
====
Base class version does nothing, returns eof().

====

== Return Value

The first character from the _pending sequence_.


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