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In Emacs, a parenthetical grouping at the top level in the buffer is
called a defun. The name derives from the fact that most top-level
lists in a Lisp file are instances of the special form defun, but
any top-level parenthetical grouping counts as a defun in Emacs parlance
regardless of what its contents are, and regardless of the programming
language in use. For example, in C, the body of a function definition is a
defun.
beginning-of-defun).
end-of-defun).
mark-defun).
The commands to move to the beginning and end of the current defun are
C-M-a (beginning-of-defun) and C-M-e (end-of-defun).
If you wish to operate on the current defun, use C-M-h
(mark-defun) which puts point at the beginning and mark at the end
of the current or next defun. For example, this is the easiest way to get
ready to move the defun to a different place in the text. In C mode,
C-M-h runs the function c-mark-function, which is almost the
same as mark-defun; the difference is that it backs up over the
argument declarations, function name and returned data type so that the
entire C function is inside the region. See section Commands to Mark Textual Objects.
Emacs assumes that any open-parenthesis found in the leftmost column is the start of a defun. Therefore, never put an open-parenthesis at the left margin in a Lisp file unless it is the start of a top-level list. Never put an open-brace or other opening delimiter at the beginning of a line of C code unless it starts the body of a function. The most likely problem case is when you want an opening delimiter at the start of a line inside a string. To avoid trouble, put an escape character (`\', in C and Emacs Lisp, `/' in some other Lisp dialects) before the opening delimiter. It will not affect the contents of the string.
In the remotest past, the original Emacs found defuns by moving upward a level of parentheses until there were no more levels to go up. This always required scanning all the way back to the beginning of the buffer, even for a small function. To speed up the operation, Emacs was changed to assume that any `(' (or other character assigned the syntactic class of opening-delimiter) at the left margin is the start of a defun. This heuristic is nearly always right and avoids the costly scan; however, it mandates the convention described above.
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