From: Eli Zaretskii Date: Tue, 10 Jan 2017 18:02:37 +0000 (+0200) Subject: Improve documentation of coding-systems X-Git-Tag: emacs-25.2-rc1~32 X-Git-Url: http://git.eshelyaron.com/gitweb/?a=commitdiff_plain;h=0e35405a92be502ab3851890fc4eb660842fd3a9;p=emacs.git Improve documentation of coding-systems * doc/lispref/nonascii.texi (Coding System Basics): Mention 'prefer-utf-8'. Index it and 'undecided'. (Encoding and I/O): Fix a typo. (User-Chosen Coding Systems): Improve the documentation of ACCEPT-DEFAULT-P argument to select-safe-coding-system. Document select-safe-coding-system-function. (Specifying Coding Systems): Document coding-system-require-warning. --- diff --git a/doc/lispref/nonascii.texi b/doc/lispref/nonascii.texi index ab8f2fc94fb..e3cb5d0d369 100644 --- a/doc/lispref/nonascii.texi +++ b/doc/lispref/nonascii.texi @@ -1002,10 +1002,14 @@ alternative encodings for the same characters; for example, there are three coding systems for the Cyrillic (Russian) alphabet: ISO, Alternativnyj, and KOI8. +@vindex undecided@r{ coding system} +@vindex prefer-utf-8@r{ coding system} Every coding system specifies a particular set of character code conversions, but the coding system @code{undecided} is special: it leaves the choice unspecified, to be chosen heuristically for each -file, based on the file's data. +file, based on the file's data. The coding system @code{prefer-utf-8} +is like @code{undecided}, but it prefers to choose @code{utf-8} when +possible. In general, a coding system doesn't guarantee roundtrip identity: decoding a byte sequence using coding system, then encoding the @@ -1109,7 +1113,7 @@ encode the buffer contents. (@pxref{Specifying Coding Systems}), or implicitly using a default mechanism (@pxref{Default Coding Systems}). But these methods may not completely specify what to do. For example, they may choose a coding -system such as @code{undefined} which leaves the character code +system such as @code{undecided} which leaves the character code conversion to be determined from the data. In these cases, the I/O operation finishes the job of choosing a coding system. Very often you will want to find out afterwards which coding system was chosen. @@ -1411,19 +1415,21 @@ Otherwise, it asks the user to choose from a list of coding systems which can encode all the text, and returns the user's choice. @var{default-coding-system} can also be a list whose first element is -t and whose other elements are coding systems. Then, if no coding +@code{t} and whose other elements are coding systems. Then, if no coding system in the list can handle the text, @code{select-safe-coding-system} queries the user immediately, without trying any of the three -alternatives described above. - -The optional argument @var{accept-default-p}, if non-@code{nil}, -should be a function to determine whether a coding system selected -without user interaction is acceptable. @code{select-safe-coding-system} -calls this function with one argument, the base coding system of the -selected coding system. If @var{accept-default-p} returns @code{nil}, -@code{select-safe-coding-system} rejects the silently selected coding -system, and asks the user to select a coding system from a list of -possible candidates. +alternatives described above. This is handy for checking only the +coding systems in the list. + +The optional argument @var{accept-default-p} determines whether a +coding system selected without user interaction is acceptable. If +it's omitted or @code{nil}, such a silent selection is always +acceptable. If it is non-@code{nil}, it should be a function; +@code{select-safe-coding-system} calls this function with one +argument, the base coding system of the selected coding system. If +the function returns @code{nil}, @code{select-safe-coding-system} +rejects the silently selected coding system, and asks the user to +select a coding system from a list of possible candidates. @vindex select-safe-coding-system-accept-default-p If the variable @code{select-safe-coding-system-accept-default-p} is @@ -1444,6 +1450,18 @@ similar functions). If it detects an apparent inconsistency, coding system. @end defun +@defvar select-safe-coding-system-function +This variable names the function to be called to request the user to +select a proper coding system for encoding text when the default +coding system for an output operation cannot safely encode that text. +The default value of this variable is @code{select-safe-coding-system}. +Emacs primitives that write text to files, such as +@code{write-region}, or send text to other processes, such as +@code{process-send-region}, normally call the value of this variable, +unless @code{coding-system-for-write} is bound to a non-@code{nil} +value (@pxref{Specifying Coding Systems}). +@end defvar + Here are two functions you can use to let the user specify a coding system, with completion. @xref{Completion}. @@ -1704,7 +1722,9 @@ including @code{file-coding-system-alist}, @defvar coding-system-for-write This works much like @code{coding-system-for-read}, except that it applies to output rather than input. It affects writing to files, -as well as sending output to subprocesses and net connections. +as well as sending output to subprocesses and net connections. It +also applies to encoding command-line arguments with which Emacs +invokes subprocesses. When a single operation does both input and output, as do @code{call-process-region} and @code{start-process}, both @@ -1712,6 +1732,23 @@ When a single operation does both input and output, as do affect it. @end defvar +@defvar coding-system-require-warning +Binding @code{coding-system-for-write} to a non-@code{nil} value +prevents output primitives from calling the function specified by +@code{select-safe-coding-system-function} (@pxref{User-Chosen Coding +Systems}). This is because @kbd{C-x RET c} +(@code{universal-coding-system-argument}) works by binding +@code{coding-system-for-write}, and Emacs should obey user selection. +If a Lisp program binds @code{coding-system-for-write} to a value that +might not be safe for encoding the text to be written, it can also bind +@code{coding-system-require-warning} to a non-@code{nil} value, which +will force the output primitives to check the encoding by calling the +value of @code{select-safe-coding-system-function} even though +@code{coding-system-for-write} is non-@code{nil}. Alternatively, call +@code{select-safe-coding-system} explicitly before using the specified +encoding. +@end defvar + @defopt inhibit-eol-conversion When this variable is non-@code{nil}, no end-of-line conversion is done, no matter which coding system is specified. This applies to all the