Does a const qualifier inside a struct declaration do anything?

In the first example, bar is constant and you can not change its value once initialized.

In the second case bar itself is not constant, but the data it points to is.

So in the second case you can assign to bar anytime you like, but in the first case it's not possible.

That bar is a member inside a structure doesn't matter, it has the same semantics as for the same declarations anywhere else.

• const int bar aka int const bar means that bar is a constant int. Once initialized, the field can't be modified directly.
• int const bar means that bar is a constant pointer to an int. Once initialized, the field can't be modified directly.
• const int bar means that bar is a pointer to a constant int. bar isn't constant and can be modified. It's that to which bar points that's constant.

#include 
struct Foo {
    const int bar;  // bar is a constant int
};
int main( void ) {
   struct Foo foo = { 123 };
   foo.bar = 456;  // error: assignment of read-only member 'bar'
   printf( "%d\n", foo.bar );
}

#include 
struct Foo {
    int const bar;  // bar is a constant int
};
int main( void ) {
   struct Foo foo = { 123 };
   foo.bar = 456;  // error: assignment of read-only member 'bar'
   printf( "%d\n", foo.bar );
}

#include 
struct Foo {
    int  const bar;  // bar is a constant pointer to an int
};
int main( void ) {
   int i = 123;
   int j = 456;
   struct Foo foo = { &i };
   foo.bar = &j;        // error: assignment of read-only member 'bar'
   ( foo.bar ) = 789;  // ok
   printf( "%d\n", ( foo.bar ) );
}

#include 
struct Foo {
    const int  bar;  // bar is a pointer to a constant int
};
int main( void ) {
   int i = 123;
   int j = 456;
   struct Foo foo = { &i };
   foo.bar = &j;        // ok
   ( foo.bar ) = 789;  // error: assignment of read-only location 'foo.bar'
   printf( "%d\n", ( foo.bar ) );
}