internal-complete-buffer

internal-complete-buffer is a built-in function in `C source code'.

(internal-complete-buffer STRING PREDICATE FLAG)

Perform completion on buffer names.
STRING and PREDICATE have the same meanings as in `try-completion',
`all-completions', and `test-completion'.

If FLAG is nil, invoke `try-completion'; if it is t, invoke
`all-completions'; otherwise invoke `test-completion'.