On Sun, 14 Mar 2021, Hal Murray via devel wrote:
[Context is cleaning up ntp_control]
This will probably be simple after somebody gives me a good example and/or
explains things to me.
I want to put a (pointer to a) function in a field of a struct.
The type of that function includes a pointer to the struct.
I need to make several functions of that type.
I assume I want a typedef for the type of the function.
Can anybody give me a pointer to code that does this? Or steer me in the
right direction, or ...
There is a recursive tangle of struct needs function and function needs
struct, but the function only needs a pointer to the struct so I think
that should work.
Typedefs are never mandatory, but they greatly improve readability where
function types are involved.
The main piece you may be missing here is the incomplete structure
definition. This creates a structure type without specifying its content,
which can be used in any context that doesn't require knowing the size of
the structure or accessing any of its fields. In particular, one can
always declare a pointer to an incomplete structure type.
This happens automatically during a normal structure definition, which is
why something like this is legal:
struct foo {
struct foo *next;
};
Here "struct foo" is an incomplete definition at the open brace, and a
complete defition at the closing brace.
Incomplete definitions are most commonly used when structures need to
refer to each other both ways, as in:
struct bar;
struct foo {
struct bar *bar;
};
struct bar {
struct foo *foo;
};
In your case, you probably want something along the lines of:
struct foo;
typedef int foo_func(struct foo *arg);
struct foo {
foo_func *func;
};
Fred Wright
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