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E.8.4 Miscellaneous Convenience Functions for Modules

This subsection describes a few convenience functions provided by the module API. Like the functions described in previous subsections, all of them are actually function pointers, and need to be called via the emacs_env pointer. Description of functions that were introduced after Emacs 25 calls out the first version where they became available.

Function: bool eq (emacs_env *env, emacs_value a, emacs_value b) ¶

This function returns true if the Lisp objects represented by a and b are identical, false otherwise. This is the same as the Lisp function eq (see Equality Predicates), but avoids the need to intern the objects represented by the arguments.

There are no API functions for other equality predicates, so you will need to use intern and funcall, described below, to perform more complex equality tests.

Function: bool is_not_nil (emacs_env *env, emacs_value arg) ¶

This function tests whether the Lisp object represented by arg is non-nil; it returns true or false accordingly.

Note that you could implement an equivalent test by using intern to get an emacs_value representing nil, then use eq, described above, to test for equality. But using this function is more convenient.

Function: emacs_value type_of (emacs_env *env, emacs_value arg) ¶

This function returns the type of arg as a value that represents a symbol: string for a string, integer for an integer, process for a process, etc. See Type Predicates. You can use intern and eq to compare against known type symbols, if your code needs to depend on the object type.

Function: emacs_value intern (emacs_env *env, const char *name) ¶

This function returns an interned Emacs symbol whose name is name, which should be an ASCII null-terminated string. It creates a new symbol if one does not already exist.

Together with funcall, described below, this function provides a means for invoking any Lisp-callable Emacs function, provided that its name is a pure ASCII string. For example, here’s how to intern a symbol whose name name_str is non-ASCII, by calling the more powerful Emacs intern function (see Creating and Interning Symbols):

emacs_value fintern = env->intern (env, "intern");
emacs_value sym_name =
  env->make_string (env, name_str, strlen (name_str));
emacs_value symbol = env->funcall (env, fintern, 1, &sym_name);

Function: emacs_value funcall (emacs_env *env, emacs_value func, ptrdiff_t nargs, emacs_value *args) ¶

This function calls the specified func passing it nargs arguments from the array pointed to by args. The argument func can be a function symbol (e.g., returned by intern described above), a module function returned by make_function (see Writing Module Functions), a subroutine written in C, etc. If nargs is zero, args can be a NULL pointer.

The function returns the value that func returned.

If your module includes potentially long-running code, it is a good idea to check from time to time in that code whether the user wants to quit, e.g., by typing C-g (see Quitting). The following function, which is available since Emacs 26.1, is provided for that purpose.

Function: bool should_quit (emacs_env *env) ¶

This function returns true if the user wants to quit. In that case, we recommend that your module function aborts any on-going processing and returns as soon as possible. In most cases, use process_input instead.

To process input events in addition to checking whether the user wants to quit, use the following function, which is available since Emacs 27.1.

Function: enum emacs_process_input_result process_input (emacs_env *env) ¶

This function processes pending input events. It returns emacs_process_input_quit if the user wants to quit or an error occurred while processing signals. In that case, we recommend that your module function aborts any on-going processing and returns as soon as possible. If the module code may continue running, process_input returns emacs_process_input_continue. The return value is emacs_process_input_continue if and only if there is no pending nonlocal exit in env. If the module continues after calling process_input, global state such as variable values and buffer content may have been modified in arbitrary ways.

Function: int open_channel (emacs_env *env, emacs_value pipe_process) ¶

This function, which is available since Emacs 28, opens a channel to an existing pipe process. pipe_process must refer to an existing pipe process created by make-pipe-process. Pipe Processes. If successful, the return value will be a new file descriptor that you can use to write to the pipe. Unlike all other module functions, you can use the returned file descriptor from arbitrary threads, even if no module environment is active. You can use the write function to write to the file descriptor. Once done, close the file descriptor using close. (libc)Low-Level I/O.