Byte-compiling a file often produces warnings about functions that the compiler doesn’t know about (see Compiler Errors). Sometimes this indicates a real problem, but usually the functions in question are defined in other files which would be loaded if that code is run. For example, byte-compiling fortran.el used to warn:
In end of data: fortran.el:2152:1:Warning: the function ‘gud-find-c-expr’ is not known to be defined.
gud-find-c-expr is only used in the function that
Fortran mode uses for the local value of
gud-find-expr-function, which is a callback from GUD; if it is
called, the GUD functions will be loaded. When you know that such a
warning does not indicate a real problem, it is good to suppress the
warning. That makes new warnings which might mean real problems more
visible. You do that with
All you need to do is add a
declare-function statement before the
first use of the function in question:
(declare-function gud-find-c-expr "gud.el" nil)
This says that
gud-find-c-expr is defined in gud.el (the
‘.el’ can be omitted). The compiler takes for granted that that file
really defines the function, and does not check.
The optional third argument specifies the argument list of
gud-find-c-expr. In this case, it takes no arguments
nil is different from not specifying a value). In other
cases, this might be something like
(file &optional overwrite).
You don’t have to specify the argument list, but if you do the
byte compiler can check that the calls match the declaration.
declare-functionfunction file &optional arglist fileonly
Tell the byte compiler to assume that function is defined, with
arguments arglist, and that the definition should come from the
file file. fileonly non-
nil means only check that
file exists, not that it actually defines function.
To verify that these functions really are declared where
declare-function says they are, use
to check all
declare-function calls in one source file, or use
check-declare-directory check all the files in and under a
These commands find the file that ought to contain a function’s
locate-library; if that finds no file, they
expand the definition file name relative to the directory of the file
that contains the
You can also say that a function is a primitive by specifying a file name ending in ‘.c’ or ‘.m’. This is useful only when you call a primitive that is defined only on certain systems. Most primitives are always defined, so they will never give you a warning.
Sometimes a file will optionally use functions from an external package.
If you prefix the filename in the
declare-function statement with
‘ext:’, then it will be checked if it is found, otherwise skipped
There are some function definitions that ‘check-declare’ does not
defstruct and some other macros). In such cases,
you can pass a non-
nil fileonly argument to
declare-function, meaning to only check that the file exists, not
that it actually defines the function. Note that to do this without
having to specify an argument list, you should set the arglist
argument to t (because
nil means an empty argument list, as
opposed to an unspecified one).