| From: Vincent De Groote <vincent.degroote@xxxxxxxxxxx> | I'm a little surprised with that kind of changes. In my knowledge, a | callback to a nested function is a usual way of doing some things ... Nested functions are not part of standard C. They are another GNU C extension. I think that they are implemented with thunks on the stack. So they require the system to allow execution of code on the stack. Just what exec-shield forbids. So there are two good reasons to avoid nested function definitions in C. A function pointer in C is implemented as a simple pointer to the first instruction of the function. But a nested function needs to have access to the local variables of the enclosing function. So (I am guessing, but I am a compiler writer) the declaration of a local function pushes code (machine instructions) on the stack that loads a pointer to the stack frame of the enclosing function (not too hard because the code is actually within the stack frame) and then branches to the body of the nested function. A pointer to the nested function actually points to the generated code on the stack. This kludge is the reinvention of stuff that precursors of C (Algol and its descendants) already had. Dennis Ritchie, the designer of C intentionally omitted this, so I find it odd that GNU re-introduced it (actually, Dennis was continuing a decision make by Martin Richards in the design of BCPL). The old literature is full of mechanisms like the "Dijkstra Display" to implement nested functions.
