It doesn't seem like it would be too hard to implement in assembly.
gcc also has a flag (-fnested-functions) to enable their use.
It turns out they're not actually all that easy to implement properly.
Should an internal function have access to the containing scope's variables?
If not, there's no point in nesting it; just make it static (to limit visibility to the translation unit it's in) and add a comment saying "This is a helper function used only by myfunc()".
If you want access to the containing scope's variables, though, you're basically forcing it to generate closures (the alternative is restricting what you can do with nested functions enough to make them useless).
I think GCC actually handles this by generating (at runtime) a unique thunk for every invocation of the containing function, that sets up a context pointer and then calls the nested function. This ends up being a rather Icky hack, and something that some perfectly reasonable implementations can't do (for example, on a system that forbids execution of writable memory - which a lot of modern OSs do for security reasons).
The only reasonable way to make it work in general is to force all function pointers to carry around a hidden context argument, and all functions to accept it (because in the general case you don't know when you call it whether it's a closure or an unclosed function). This is inappropriate to require in C for both technical and cultural reasons, so we're stuck with the option of either using explicit context pointers to fake a closure instead of nesting functions, or using a higher-level language that has the infrastructure needed to do it properly.
I'd like to quote something from the BDFL (Guido van Rossum):
This is because nested function definitions don't have access to the
local variables of the surrounding block -- only to the globals of the
containing module. This is done so that lookup of globals doesn't
have to walk a chain of dictionaries -- as in C, there are just two
nested scopes: locals and globals (and beyond this, built-ins).
Therefore, nested functions have only a limited use. This was a
deliberate decision, based upon experience with languages allowing
arbitraries nesting such as Pascal and both Algols -- code with too
many nested scopes is about as readable as code with too many GOTOs.
Emphasis is mine.
I believe he was referring to nested scope in Python (and as David points out in the comments, this was from 1993, and Python does support fully nested functions now) -- but I think the statement still applies.
The other part of it could have been closures.
If you have a function like this C-like code:
(*int()) foo() {
int x = 5;
int bar() {
x = x + 1;
return x;
}
return &bar;
}
If you use bar in a callback of some sort, what happens with x? This is well-defined in many newer, higher-level languages, but AFAIK there's no well-defined way to track that x in C -- does bar return 6 every time, or do successive calls to bar return incrementing values? That could have potentially added a whole new layer of complication to C's relatively simple definition.
See C FAQ 20.24 and the GCC manual for potential problems:
If you try to call the nested function
through its address after the
containing function has exited, all
hell will break loose. If you try to
call it after a containing scope level
has exited, and if it refers to some
of the variables that are no longer in
scope, you may be lucky, but it's not
wise to take the risk. If, however,
the nested function does not refer to
anything that has gone out of scope,
you should be safe.
This is not really more severe than some other problematic parts of the C standard, so I'd say the reasons are mostly historical (C99 isn't really that different from K&R C feature-wise).
There are some cases where nested functions with lexical scope might be useful (consider a recursive inner function which doesn't need extra stack space for the variables in the outer scope without the need for a static variable), but hopefully you can trust the compiler to correctly inline such functions, ie a solution with a seperate function will just be more verbose.
Nested functions are a very delicate thing. Will you make them closures? If not, then they have no advantage to regular functions, since they can't access any local variables. If they do, then what do you do to stack-allocated variables? You have to put them somewhere else so that if you call the nested function later, the variable is still there. This means they'll take memory, so you have to allocate room for them on the heap. With no GC, this means that the programmer is now in charge of cleaning up the functions. Etc... C# does this, but they have a GC, and it's a considerably newer language than C.
It also wouldn't be too hard to add members functions to structs but they are not in the standard either.
Features are not added to C standard based on soley whether or not they are easy to implement. It's a combination of many other factors including the point in time in which the standard was written and what was common / practical then.
One more reason: it is not at all clear that nested functions are valuable. Twenty-odd years ago I used to do large scale programming and maintenance in (VAX) Pascal. We had lots of old code that made heavy use of nested functions. At first, I thought this was way cool (compared to K&R C, which I had been working in before) and started doing it myself. After awhile, I decided it was a disaster, and stopped.
The problem was that a function could have a great many variables in scope, counting the variables of all the functions in which it was nested. (Some old code had ten levels of nesting; five was quite common, and until I changed my mind I coded a few of the latter myself.) Variables in the nesting stack could have the same names, so that "inner" function local variables could mask variables of the same name in more "outer" functions. A local variable of a function, that in C-like languages is totally private to it, could be modified by a call to a nested function. The set of possible combinations of this jazz was near infinite, and a nightmare to comprehend when reading code.
So, I started calling this programming construct "semi-global variables" instead of "nested functions", and telling other people working on the code that the only thing worse than a global variable was a semi-global variable, and please do not create any more. I would have banned it from the language, if I could. Sadly, there was no such option for the compiler...
ANSI C has been established for 20 years. Perhaps between 1983 and 1989 the committee may have discussed it in the light of the state of compiler technology at the time but if they did their reasoning is lost in dim and distant past.
I disagree with Dave Vandervies.
Defining a nested function is much better coding style than defining it in global scope, making it static and adding a comment saying "This is a helper function used only by myfunc()".
What if you needed a helper function for this helper function? Would you add a comment "This is a helper function for the first helper function used only by myfunc"? Where do you take the names from needed for all those functions without polluting the namespace completely?
How confusing can code be written?
But of course, there is the problem with how to deal with closuring, i.e. returning a pointer to a function that has access to variables defined in the function from which it is returned.
Either you don't allow references to local variables of the containing function in the contained one, and the nesting is just a scoping feature without much use, or you do. If you do, it is not a so simple feature: you have to be able to call a nested function from another one while accessing the correct data, and you also have to take into account recursive calls. That's not impossible -- techniques are well known for that and where well mastered when C was designed (Algol 60 had already the feature). But it complicates the run-time organization and the compiler and prevent a simple mapping to assembly language (a function pointer must carry on information about that; well there are alternatives such as the one gcc use). It was out of scope for the system implementation language C was designed to be.
Related
Can a function tell what's calling it, through the use of memory addresses maybe? For example, function foo(); gets data on whether it is being called in main(); rather than some other function?
If so, is it possible to change the content of foo(); based on what is calling it?
Example:
int foo()
{
if (being called from main())
printf("Hello\n");
if (being called from some other function)
printf("Goodbye\n");
}
This question might be kind of out there, but is there some sort of C trickery that can make this possible?
For highly optimized C it doesn't really make sense. The harder the compiler tries to optimize the less the final executable resembles the source code (especially for link-time code generation where the old "separate compilation units" problem no longer prevents lots of optimizations). At least in theory (but often in practice for some compilers) functions that existed in the source code may not exist in the final executable (e.g. may have been inlined into their caller); functions that didn't exist in the source code may be generated (e.g. compiler detects common sequences in many functions and "out-lines" them into a new function to avoid code duplication); and functions may be replaced by data (e.g. an "int abcd(uint8_t a, uint8_t b)" replaced by a abcd_table[a][b] lookup table).
For strict C (no extensions or hacks), no. It simply can't support anything like this because it can't expect that (for any compiler including future compilers that don't exist yet) the final output/executable resembles the source code.
An implementation defined extension, or even just a hack involving inline assembly, may be "technically possible" (especially if the compiler doesn't optimize the code well). The most likely approach would be to (ab)use debugging information to determine the caller from "what the function should return to when it returns".
A better way for a compiler to support a hypothetical extension like this may be for the compiler to use some of the optimizations I mentioned - specifically, split the original foo() into 2 separate versions where one version is only ever called from main() and the other version is used for other callers. This has the bonus of letting the compiler optimize out the branches too - it could become like int foo_when_called_from_main() { printf("Hello\n"); }, which could be inlined directly into the caller, so that neither version of foo exists in the final executable. Of course if foo() had other code that's used by all callers then that common code could be lifted out into a new function rather than duplicating it (e.g. so it might become like int foo_when_called_from_main() { printf("Hello\n"); foo_common_code(); }).
There probably isn't any hypothetical compiler that works like that, but there's no real reason you can't do these same optimizations yourself (and have it work on all compilers).
Note: Yes, this was just a crafty way of suggesting that you can/should refactor the code so that it doesn't need to know which function is calling it.
Knowing who called a specific function is essentially what a stack trace is visualizing. There are no general standard way of extracting that though. In theory one could write code that targeted each system type the software would run on, and implement a stack trace function for each of them. In that case you could examine the stack and see what is before the current function.
But with all that said and done, the question you should probably ask is why? Writing a function that functions in a specific way when called from a specific function is not well isolated logic. Instead you could consider passing in a parameter to the function that caused the change in logic. That would also make the result more testable and reliable.
How to actually extract a stack trace has already received many answers here: How can one grab a stack trace in C?
I think if loop in C cannot have a condition as you have mentioned.
If you want to check whether this function is called from main(), you have to do the printf statement in the main() and also at the other function.
I don't really know what you are trying to achieve but according to what I understood, what you can do is each function will pass an additional argument that would uniquely identify that function in form of a character array, integer or enumeration.
for example:
enum function{main, add, sub, div, mul};
and call functions like:
add(3,5,main);//adds 3 and 5. called from main
changes to the code would be typical like if you are adding more functions. but it's an easier way to do it.
No. The C language does not support obtaining the name or other information of who called a function.
As all other answers show, this can only be obtained using external tools, for example that use stack traces and compiler/linker emitted symbol tables.
everyone. I actually have two questions, somewhat related.
Question #1: Why is gcc letting me declare variables after action statements? I thought the C89 standard did not allow this. (GCC Version: 4.4.3) It even happens when I explicitly use --std=c89 on the compile line. I know that most compilers implement things that are non-standard, i.e. C compilers allowing // comments, when the standard does not specify that. I'd like to learn just the standard, so that if I ever need to use just the standard, I don't snag on things like this.
Question #2: How do you cope without objects in C? I program as a hobby, and I have not yet used a language that does not have Objects (a.k.a. OO concepts?) -- I already know some C++, and I'd like to learn how to use C on it's own. Supposedly, one way is to make a POD struct and make functions similar to StructName_constructor(), StructName_doSomething(), etc. and pass the struct instance to each function - is this the 'proper' way, or am I totally off?
EDIT: Due to some minor confusion, I am defining what my second question is more clearly: I am not asking How do I use Objects in C? I am asking How do you manage without objects in C?, a.k.a. how do you accomplish things without objects, where you'd normally use objects?
In advance, thanks a lot. I've never used a language without OOP! :)
EDIT: As per request, here is an example of the variable declaration issue:
/* includes, or whatever */
int main(int argc, char *argv[]) {
int myInt = 5;
printf("myInt is %d\n", myInt);
int test = 4; /* This does not result in a compile error */
printf("Test is %d\n", test);
return 0;
}
c89 doesn't allow this, but c99 does. Although it's taken a long time to catch on, some compilers (including gcc) are finally starting to implement c99 features.
IMO, if you want to use OOP, you should probably stick to C++ or try out Objective C. Trying to reinvent OOP built on top of C again just doesn't make much sense.
If you insist on doing it anyway, yes, you can pass a pointer to a struct as an imitation of this -- but it's still not a good idea.
It does often make sense to pass (pointers to) structs around when you need to operate on a data structure. I would not, however, advise working very hard at grouping functions together and having them all take a pointer to a struct as their first parameter, just because that's how other languages happen to implement things.
If you happen to have a number of functions that all operate on/with a particular struct, and it really makes sense for them to all receive a pointer to that struct as their first parameter, that's great -- but don't feel obliged to force it just because C++ happens to do things that way.
Edit: As far as how you manage without objects: well, at least when I'm writing C, I tend to operate on individual characters more often. For what it's worth, in C++ I typically end up with a few relatively long lines of code; in C, I tend toward a lot of short lines instead.
There is more separation between the code and data, but to some extent they're still coupled anyway -- a binary tree (for example) still needs code to insert nodes, delete nodes, walk the tree, etc. Likewise, the code for those operations needs to know about the layout of the structure, and the names given to the pointers and such.
Personally, I tend more toward using a common naming convention in my C code, so (for a few examples) the pointers to subtrees in a binary tree are always just named left and right. If I use a linked list (rare) the pointer to the next node is always named next (and if it's doubly-linked, the other is prev). This helps a lot with being able to write code without having to spend a lot of time looking up a structure definition to figure out what name I used for something this time.
#Question #1: I don't know why there is no error, but you are right, variables have to be declared at the beginning of a block. Good thing is you can declare blocks anywhere you like :). E.g:
{
int some_local_var;
}
#Question #2: actually programming C without inheritance is sometimes quite annoying. but there are possibilities to have OOP to some degree. For example, look at the GTK source code and you will find some examples.
You are right, functions like the ones you have shown are common, but the constructor is commonly devided into an allocation function and an initialization function. E.G:
someStruct* someStruct_alloc() { return (someStruct*)malloc(sizeof(someStruct)); }
void someStruct_init(someStruct* this, int arg1, arg2) {...}
In some libraries, I have even seen some sort of polymorphism, where function pointers are stored within the struct (which have to be set in the initializing function, of course). This results in a C++ like API:
someStruct* str = someStruct_alloc();
someStruct_init(str);
str->someFunc(10, 20, 30);
Regarding OOP in C, have you looked at some of the topics on SO? For instance, Can you write object oriented code in C?.
I can't put my finger on an example, but I think they enforce an OO like discipline in Linux kernel programming as well.
In terms of learning how C works, as opposed to OO in C++, you might find it easier to take a short course in some other language that doesn't have an OO derivative -- say, Modula-2 (one of my favorites) or even BASIC (if you can still find a real BASIC implementation -- last time I wrote BASIC code it was with the QBASIC that came with DOS 5.0, later compiled in full Quick BASIC).
The methods you use to get things done in Modula-2 or Pascal (barring the strong typing, which protects against certain types of errors but makes it more complicated to do certain things) are exactly those used in non-OO C, and working in a language with different syntax might (probably will, IMO) make it easier to learn the concepts without your "programming reflexes" kicking in and trying to do OO operations in a nearly-familiar language.
Why is still C99 mixed declarations and code not used in open source C projects like the Linux kernel or GNOME?
I really like mixed declarations and code since it makes the code more readable and prevents hard to see bugs by restricting the scope of the variables to the narrowest possible. This is recommended by Google for C++.
For example, Linux requires at least GCC 3.2 and GCC 3.1 has support for C99 mixed declarations and code
You don't need mixed declaration and code to limit scope. You can do:
{
int c;
c = 1;
{
int d = c + 1;
}
}
in C89. As for why these projects haven't used mixed declarations (assuming this is true), it's most likely a case of "If it ain't broke don't fix it."
This is an old question but I'm going to suggest that inertia is the reason that most of these projects still use ANSI C declarations rules.
However there are a number of other possibilities, ranging from valid to ridiculous:
Portability. Many open source projects work under the assumption that pedantic ANSI C is the most portable way to write software.
Age. Many of these projects predate the C99 spec and the authors may prefer a consistent coding style.
Ignorance. The programmers submitting predate C99 and are unaware of the benefits of mixed declarations and code. (Alternate interpretation: Developers are fully aware of the potential tradeoffs and decide that mixed declarations and statements are not worth the effort. I highly disagree, but it's rare that two programmers will agree on anything.)
FUD. Programmers view mixed declarations and code as a "C++ism" and dislike it for that reason.
There is little reason to rewrite the Linux kernel to make cosmetic changes that offer no performance gains.
If the code base is working, so why change it for cosmetic reasons?
There is no benefit. Declaring all variables at the beginning of the function (pascal like) is much more clear, in C89 you can also declare variables at the beginning of each scope (inside loops example) which is both practical and concise.
I don't remember any interdictions against this in the style guide for kernel code. However, it does say that functions should be as small as possible, and only do one thing. This would explain why a mixture of declarations and code is rare.
In a small function, declaring variables at the start of scope acts as a sort of Introit, telling you something about what's coming soon after. In this case the movement of the variable declaration is so limited that it would likely either have no effect, or serve to hide some information about the functionality by pushing the barker into the crowd, so to speak. There is a reason that the arrival of a king was declared before he entered a room.
OTOH, a function which must mix variables and code to be readable is probably too big. This is one of the signs (along with too-nested blocks, inline comments and other things) that some sections of a function need to be abstracted into separate functions (and declared static, so the optimizer can inline them).
Another reason to keep declarations at the beginning of the functions: should you need to reorder the execution of statements in the code, you may move a variable out of its scope without realizing it, since the scope of a variable declared in the middle of code is not evident in the indentation (unless you use a block to show the scope). This is easily fixed, so it's just an annoyance, but new code often undergoes this kind of transformation, and annoyance can be cumulative.
And another reason: you might be tempted to declare a variable to take the error return code from a function, like so:
void_func();
int ret = func_may_fail();
if (ret) { handle_fail(ret) }
Perfectly reasonable thing to do. But:
void_func();
int ret = func_may_fail();
if (ret) { handle_fail(ret) }
....
int ret = another_func_may_fail();
if (ret) { handle_other_fail(ret); }
Ooops! ret is defined twice. "So? Remove the second declaration." you say. But this makes the code asymmetric, and you end up with more refactoring limitations.
Of course, I mix declarations and code myself; no reason to be dogmatic about it (or else your karma may run over your dogma :-). But you should know what the concomitant problems are.
Maybe it's not needed, maybe the separation is good? I do it in C++, which has this feature as well.
There is no reason to change the code away like this, and C99 is still not widely supported by compilers. It is mostly about portability.
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I know that nested functions are not part of the standard C, but since they're present in gcc (and the fact that gcc is the only compiler i care about), i tend to use them quite often.
Is this a bad thing ? If so, could you show me some nasty examples ?
What's the status of nested functions in gcc ? Are they going to be removed ?
Nested functions really don't do anything that you can't do with non-nested ones (which is why neither C nor C++ provide them). You say you are not interested in other compilers - well this may be atrue at this moment, but who knows what the future will bring? I would avoid them, along with all other GCC "enhancements".
A small story to illustrate this - I used to work for a UK Polytechinc which mostly used DEC boxes - specifically a DEC-10 and some VAXen. All the engineering faculty used the many DEC extensions to FORTRAN in their code - they were certain that we would remain a DEC shop forever. And then we replaced the DEC-10 with an IBM mainframe, the FORTRAN compiler of which didn't support any of the extensions. There was much wailing and gnashing of teeth on that day, I can tell you. My own FORTRAN code (an 8080 simulator) ported over to the IBM in a couple of hours (almost all taken up with learning how to drive the IBM compiler), because I had written it in bog-standard FORTRAN-77.
There are times nested functions can be useful, particularly with algorithms that shuffle around lots of variables. Something like a written-out 4-way merge sort could need to keep a lot of local variables, and have a number of pieces of repeated code which use many of them. Calling those bits of repeated code as an outside helper routine would require passing a large number of parameters and/or having the helper routine access them through another level of pointer indirection.
Under such circumstances, I could imagine that nested routines might allow for more efficient program execution than other means of writing the code, at least if the compiler optimizes for the situation where there any recursion that exists is done via re-calling the outermost function; inline functions, space permitting, might be better on non-cached CPUs, but the more compact code offered by having separate routines might be helpful. If inner functions cannot call themselves or each other recursively, they can share a stack frame with the outer function and would thus be able to access its variables without the time penalty of an extra pointer dereference.
All that being said, I would avoid using any compiler-specific features except in circumstances where the immediate benefit outweighs any future cost that might result from having to rewrite the code some other way.
Like most programming techniques, nested functions should be used when and only when they are appropriate.
You aren't forced to use this aspect, but if you want, nested functions reduce the need to pass parameters by directly accessing their containing function's local variables. That's convenient. Careful use of "invisible" parameters can improve readability. Careless use can make code much more opaque.
Avoiding some or all parameters makes it harder to reuse a nested function elsewhere because any new containing function would have to declare those same variables. Reuse is usually good, but many functions will never be reused so it often doesn't matter.
Since a variable's type is inherited along with its name, reusing nested functions can give you inexpensive polymorphism, like a limited and primitive version of templates.
Using nested functions also introduces the danger of bugs if a function unintentionally accesses or changes one of its container's variables. Imagine a for loop containing a call to a nested function containing a for loop using the same index without a local declaration. If I were designing a language, I would include nested functions but require an "inherit x" or "inherit const x" declaration to make it more obvious what's happening and to avoid unintended inheritance and modification.
There are several other uses, but maybe the most important thing nested functions do is allow internal helper functions that are not visible externally, an extension to C's and C++'s static not extern functions or to C++'s private not public functions. Having two levels of encapsulation is better than one. It also allows local overloading of function names, so you don't need long names describing what type each one works on.
There are internal complications when a containing function stores a pointer to a contained function, and when multiple levels of nesting are allowed, but compiler writers have been dealing with those issues for over half a century. There are no technical issues making it harder to add to C++ than to C, but the benefits are less.
Portability is important, but gcc is available in many environments, and at least one other family of compilers supports nested functions - IBM's xlc available on AIX, Linux on PowerPC, Linux on BlueGene, Linux on Cell, and z/OS. See
http://publib.boulder.ibm.com/infocenter/comphelp/v8v101index.jsp?topic=%2Fcom.ibm.xlcpp8a.doc%2Flanguage%2Fref%2Fnested_functions.htm
Nested functions are available in some new (eg, Python) and many more traditional languages, including Ada, Pascal, Fortran, PL/I, PL/IX, Algol and COBOL. C++ even has two restricted versions - methods in a local class can access its containing function's static (but not auto) variables, and methods in any class can access static class data members and methods. The upcoming C++ standard has lamda functions, which are really anonymous nested functions. So the programming world has lots of experience pro and con with them.
Nested functions are useful but take care. Always use any features and tools where they help, not where they hurt.
As you said, they are a bad thing in the sense that they are not part of the C standard, and as such are not implemented by many (any?) other C compilers.
Also keep in mind that g++ does not implement nested functions, so you will need to remove them if you ever need to take some of that code and dump it into a C++ program.
Nested functions can be bad, because under specific conditions the NX (no-execute) security bit will be disabled. Those conditions are:
GCC and nested functions are used
a pointer to the nested function is used
the nested function accesses variables from the parent function
the architecture offers NX (no-execute) bit protection, for instance 64-bit linux.
When the above conditions are met, GCC will create a trampoline https://gcc.gnu.org/onlinedocs/gccint/Trampolines.html. To support trampolines, the stack will be marked executable. see: https://www.win.tue.nl/~aeb/linux/hh/protection.html
Disabling the NX security bit creates several security issues, with the notable one being buffer overrun protection is disabled. Specifically, if an attacker placed some code on the stack (say as part of a user settable image, array or string), and a buffer overrun occurred, then the attackers code could be executed.
update
I'm voting to delete my own post because it's incorrect. Specifically, the compiler must insert a trampoline function to take advantage of the nested functions, so any savings in stack space are lost.
If some compiler guru wants to correct me, please do so!
original answer:
Late to the party, but I disagree with the accepted answer's assertion that
Nested functions really don't do anything that you can't do with
non-nested ones.
Specifically:
TL;DR: Nested Functions Can Reduce Stack Usage in Embedded Environments
Nested functions give you access to lexically scoped variables as "local" variables without needing to push them onto the call stack. This can be really useful when working on a system with limited resource, e.g. embedded systems. Consider this contrived example:
void do_something(my_obj *obj) {
double times2() {
return obj->value * 2.0;
}
double times4() {
return times2() * times2();
}
...
}
Note that once you're inside do_something(), because of nested functions, the calls to times2() and times4() don't need to push any parameters onto the stack, just return addresses (and smart compilers even optimize them out when possible).
Imagine if there was a lot of state that the internal functions needed to access. Without nested functions, all that state would have to be passed on the stack to each of the functions. Nested functions let you access the state like local variables.
I agree with Stefan's example, and the only time I used nested functions (and then I am declaring them inline) is in a similar occasion.
I would also suggest that you should rarely use nested inline functions rarely, and the few times you use them you should have (in your mind and in some comment) a strategy to get rid of them (perhaps even implement it with conditional #ifdef __GCC__ compilation).
But GCC being a free (like in speech) compiler, it makes some difference... And some GCC extensions tend to become de facto standards and are implemented by other compilers.
Another GCC extension I think is very useful is the computed goto, i.e. label as values. When coding automatons or bytecode interpreters it is very handy.
Nested functions can be used to make a program easier to read and understand, by cutting down on the amount of explicit parameter passing without introducing lots of global state.
On the other hand, they're not portable to other compilers. (Note compilers, not devices. There aren't many places where gcc doesn't run).
So if you see a place where you can make your program clearer by using a nested function, you have to ask yourself 'Am I optimising for portability or readability'.
I'm just exploring a bit different kind of use of nested functions. As an approach for 'lazy evaluation' in C.
Imagine such code:
void vars()
{
bool b0 = code0; // do something expensive or to ugly to put into if statement
bool b1 = code1;
if (b0) do_something0();
else if (b1) do_something1();
}
versus
void funcs()
{
bool b0() { return code0; }
bool b1() { return code1; }
if (b0()) do_something0();
else if (b1()) do_something1();
}
This way you get clarity (well, it might be a little confusing when you see such code for the first time) while code is still executed when and only if needed.
At the same time it's pretty simple to convert it back to original version.
One problem arises here if same 'value' is used multiple times. GCC was able to optimize to single 'call' when all the values are known at compile time, but I guess that wouldn't work for non trivial function calls or so. In this case 'caching' could be used, but this adds to non readability.
I need nested functions to allow me to use utility code outside an object.
I have objects which look after various hardware devices. They are structures which are passed by pointer as parameters to member functions, rather as happens automagically in c++.
So I might have
static int ThisDeviceTestBram( ThisDeviceType *pdev )
{
int read( int addr ) { return( ThisDevice->read( pdev, addr ); }
void write( int addr, int data ) ( ThisDevice->write( pdev, addr, data ); }
GenericTestBram( read, write, pdev->BramSize( pdev ) );
}
GenericTestBram doesn't and cannot know about ThisDevice, which has multiple instantiations. But all it needs is a means of reading and writing, and a size. ThisDevice->read( ... ) and ThisDevice->Write( ... ) need the pointer to a ThisDeviceType to obtain info about how to read and write the block memory (Bram) of this particular instantiation. The pointer, pdev, cannot have global scobe, since multiple instantiations exist, and these might run concurrently. Since access occurs across an FPGA interface, it is not a simple question of passing an address, and varies from device to device.
The GenericTestBram code is a utility function:
int GenericTestBram( int ( * read )( int addr ), void ( * write )( int addr, int data ), int size )
{
// Do the test
}
The test code, therefore, need be written only once and need not be aware of the details of the structure of the calling device.
Even wih GCC, however, you cannot do this. The problem is the out of scope pointer, the very problem needed to be solved. The only way I know of to make f(x, ... ) implicitly aware of its parent is to pass a parameter with a value out of range:
static int f( int x )
{
static ThisType *p = NULL;
if ( x < 0 ) {
p = ( ThisType* -x );
}
else
{
return( p->field );
}
}
return( whatever );
Function f can be initialised by something which has the pointer, then be called from anywhere. Not ideal though.
Nested functions are a MUST-HAVE in any serious programming language.
Without them, the actual sense of functions isn't usable.
It's called lexical scoping.
Being a developer born and raised on OO, I was curious to hear how it's possible to avoid global state in a procedural program.
You can also write object-oriented code in C. You don't get all the C++ goodies and it's ugly, and you have to manually pass the this pointer (I've seen self used for this, in order to make it compatible with C++), but it works. So technically, you don't need global state in pure procedural languages for the very same reasons you don't need it in object-oriented languages. You just have to pass the state around explicitly, rather than implicitly like in OO languages.
As an example, look at how the file I/O functions in the C standard library work with pointer to FILE objects that are (largely) opaque. Or look at how OS APIs deal with handles and such to encapsulate information. A program creates objects, uses APIs that act on those objects and closes/deletes the objects - all using straight C.
A global variable is nothing but an implicit procedure argument. Make it explicit and the global variable goes away.
Note: the fact that you no longer use a global variable does not mean that you no longer use global state! What we did above was just a purely syntactical transformation, the semantics of the program haven't changed at all. It's just as non-composable, non-modular, non-threadsafe, non-parallelizable as it was before.
All OO is a mindset and a whole bunch of compiler support.
You can achieve much the same by discipline, coding conventions, and passing around structures in most languages.
For example I used to have functions/procedures prefixed with their module identity, taking the first parameter as being the related module struct.
// System.h
typedef struct _System
{
struct _System *owner;
LinkedList *elements;
} System;
// System.c
int System_FindName ( System * system, char *name)
{
..
}
etc..
I'd really seriously not like to have to go back to coding like this though. I'm very happy that I haven't had to write and debug a linked list for at least 18 years. It was hard back then without the internet and sitting there isolated in the corner of a cold brightly lit room with green phosphors burning into your retina...
Of course. Just declare a struct somewhere, allocate some memory for it, pass the pointer to the allocated memory to an initialization function, and off you go. Just pass the pointer to all the functions that require using the struct.
Though the question arises as to where you store the pointer to the data you don't want to be global, and then you may end up with a global pointer ;-)
You can have variables on stack or in heap that will exist during all the program life.
Passing object style structure pointers to every function is a good way to have OO C coding style.
(I would suggest to have a look in linux sources)
You could try, as an example, create with dia (the diagramming tool), a simple class (for example, a square).
http://projects.gnome.org/dia/
http://dia-installer.de/index_en.html
Then, you can transform that class in C code using dia2code:
http://dia2code.sourceforge.net/
Specifically, say you created the class square inside the square.dia diagram. Then, you type:
$ dia2code -t c square.dia
... and you will see that it is possible to convert any object-oriented programming in a C program without global variables. Explore the created files square.c and square.h
NOTE: in Windows, you'll need a workaround in order to make dia2code work. Before using dia2code, change square.dia to square.zip, unzip it, and rename the result as square.dia
Simple. Whenever a procedure accesses a global variable, then give this variable as an argument to the procedure instead, either by value or by reference or by pointer, or by whatever your programming language provides. After that there is no more need for the variable to be global.