This function selects a coding system for encoding specified text, asking the user to choose if necessary. Normally the specified text is the text in the current buffer between from and to. If from is a string, the string specifies the text to encode, and to is ignored.
If the specified text includes raw bytes (see Text Representations), select-safe-coding-system suggests raw-text for its encoding.
If default-coding-system is non-nil, that is the first coding system to try; if that can handle the text, select-safe-coding-system returns that coding system. It can also be a list of coding systems; then the function tries each of them one by one. After trying all of them, it next tries the current buffer’s value of buffer-file-coding-system (if it is not undecided), then the default value of buffer-file-coding-system and finally the user’s most preferred coding system, which the user can set using the command prefer-coding-system (see Recognizing Coding Systems in The GNU Emacs Manual).
If one of those coding systems can safely encode all the specified text, select-safe-coding-system chooses it and returns it. 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.
default-coding-system can also be a list whose first element is t and whose other elements are coding systems. Then, if no coding system in the list can handle the text, select-safe-coding-system queries the user immediately, without trying any of the three alternatives described above. This is handy for checking only the coding systems in the list.
The optional argument accept-default-p determines whether a coding system selected without user interaction is acceptable. If it’s omitted or nil, such a silent selection is always acceptable. If it is non-nil, it should be a function; select-safe-coding-system calls this function with one argument, the base coding system of the selected coding system. If the function returns nil, 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.
If the variable select-safe-coding-system-accept-default-p is non-nil, it should be a function taking a single argument. It is used in place of accept-default-p, overriding any value supplied for this argument.
As a final step, before returning the chosen coding system, select-safe-coding-system checks whether that coding system is consistent with what would be selected if the contents of the region were read from a file. (If not, this could lead to data corruption in a file subsequently re-visited and edited.) Normally, select-safe-coding-system uses buffer-file-name as the file for this purpose, but if file is non-nil, it uses that file instead (this can be relevant for write-region and similar functions). If it detects an apparent inconsistency, select-safe-coding-system queries the user before selecting the coding system.
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 select-safe-coding-system. Emacs primitives that write text to files, such as write-region, or send text to other processes, such as process-send-region, normally call the value of this variable, unless coding-system-for-write is bound to a non-nil value (see Specifying Coding Systems).
Here are two functions you can use to let the user specify a coding system, with completion. See Completion.
This function reads a coding system using the minibuffer, prompting with string prompt, and returns the coding system name as a symbol. If the user enters null input, default specifies which coding system to return. It should be a symbol or a string.
This function reads a coding system using the minibuffer, prompting with string prompt, and returns the coding system name as a symbol. If the user tries to enter null input, it asks the user to try again. See Coding Systems.
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https://www.gnu.org/software/emacs/manual/html_node/elisp/User_002dChosen-Coding-Systems.html