How does a 'const struct' differ from a 'struct'?

The const part really applies to the variable, not the structure itself.

e.g. @Andreas correctly says:

const struct {
    int x;
    int y;
} foo = {10, 20};
foo.x = 5; //Error

But the important thing is that variable foo is constant, not the struct definition itself. You could equally write that as:

struct apoint {
    int x;
    int y;
};

const struct apoint foo = {10, 20};
foo.x = 5; // Error

struct apoint bar = {10, 20};
bar.x = 5; // Okay

It means the struct is constant i.e. you can't edit it's fields after it's been initialized.

const struct {
    int x;
    int y;
} foo = {10, 20};
foo.x = 5; //Error

EDIT: GrahamS correctly points out that the constness is a property of the variable, in this case foo, and not the struct definition:

struct Foo {
    int x;
    int y;
};
const struct Foo foo = {10, 20};
foo.x = 5; //Error
struct Foo baz = {10, 20};
baz.x = 5; //Ok

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