How to write a while loop with the C preprocessor? - c

Question

I am asking this question from an educational/hacking point of view, (I wouldn't really want to code like this).
Is it possible to implement a while loop only using C preprocessor directives. I understand that macros cannot be expanded recursively, so how would this be accomplished?

Answer 1

If you want to implement a while loop, you will need to use recursion in the preprocessor. The easiest way to do recursion is to use a deferred expression. A deferred expression is an expression that requires more scans to fully expand:
#define EMPTY()
#define DEFER(id) id EMPTY()
#define OBSTRUCT(id) id DEFER(EMPTY)()
#define EXPAND(...) __VA_ARGS__
#define A() 123
A() // Expands to 123
DEFER(A)() // Expands to A () because it requires one more scan to fully expand
EXPAND(DEFER(A)()) // Expands to 123, because the EXPAND macro forces another scan
Why is this important? Well when a macro is scanned and expanding, it creates a disabling context. This disabling context will cause a token, that refers to the currently expanding macro, to be painted blue. Thus, once its painted blue, the macro will no longer expand. This is why macros don't expand recursively. However, a disabling context only exists during one scan, so by deferring an expansion we can prevent our macros from becoming painted blue. We will just need to apply more scans to the expression. We can do that using this EVAL macro:
#define EVAL(...) EVAL1(EVAL1(EVAL1(__VA_ARGS__)))
#define EVAL1(...) EVAL2(EVAL2(EVAL2(__VA_ARGS__)))
#define EVAL2(...) EVAL3(EVAL3(EVAL3(__VA_ARGS__)))
#define EVAL3(...) EVAL4(EVAL4(EVAL4(__VA_ARGS__)))
#define EVAL4(...) EVAL5(EVAL5(EVAL5(__VA_ARGS__)))
#define EVAL5(...) __VA_ARGS__
Next, we define some operators for doing some logic(such as if, etc):
#define CAT(a, ...) PRIMITIVE_CAT(a, __VA_ARGS__)
#define PRIMITIVE_CAT(a, ...) a ## __VA_ARGS__
#define CHECK_N(x, n, ...) n
#define CHECK(...) CHECK_N(__VA_ARGS__, 0,)
#define NOT(x) CHECK(PRIMITIVE_CAT(NOT_, x))
#define NOT_0 ~, 1,
#define COMPL(b) PRIMITIVE_CAT(COMPL_, b)
#define COMPL_0 1
#define COMPL_1 0
#define BOOL(x) COMPL(NOT(x))
#define IIF(c) PRIMITIVE_CAT(IIF_, c)
#define IIF_0(t, ...) __VA_ARGS__
#define IIF_1(t, ...) t
#define IF(c) IIF(BOOL(c))
Now with all these macros we can write a recursive WHILE macro. We use a WHILE_INDIRECT macro to refer back to itself recursively. This prevents the macro from being painted blue, since it will expand on a different scan(and using a different disabling context). The WHILE macro takes a predicate macro, an operator macro, and a state(which is the variadic arguments). It keeps applying this operator macro to the state until the predicate macro returns false(which is 0).
#define WHILE(pred, op, ...) \
IF(pred(__VA_ARGS__)) \
( \
OBSTRUCT(WHILE_INDIRECT) () \
( \
pred, op, op(__VA_ARGS__) \
), \
__VA_ARGS__ \
)
#define WHILE_INDIRECT() WHILE
For demonstration purposes, we are just going to create a predicate that checks when number of arguments are 1:
#define NARGS_SEQ(_1,_2,_3,_4,_5,_6,_7,_8,N,...) N
#define NARGS(...) NARGS_SEQ(__VA_ARGS__, 8, 7, 6, 5, 4, 3, 2, 1)
#define IS_1(x) CHECK(PRIMITIVE_CAT(IS_1_, x))
#define IS_1_1 ~, 1,
#define PRED(x, ...) COMPL(IS_1(NARGS(__VA_ARGS__)))
Next we create an operator, which we will just concat two tokens. We also create a final operator(called M) that will process the final output:
#define OP(x, y, ...) CAT(x, y), __VA_ARGS__
#define M(...) CAT(__VA_ARGS__)
Then using the WHILE macro:
M(EVAL(WHILE(PRED, OP, x, y, z))) //Expands to xyz
Of course, any kind of predicate or operator can be passed to it.

Answer 2

Take a look at the Boost preprocessor library, which allows you to write loops in the preprocessor, and much more.

Answer 3

You use recursive include files. Unfortunately, you can't iterate the loop more than the maximum depth that the preprocessor allows.
It turns out that C++ templates are Turing Complete and can be used in similar ways. Check out Generative Programming

Answer 4

I use meta-template programming for this purpose, its fun once you get a hang of it. And very useful at times when used with discretion. Because as mentioned its turing complete, to the point where you can even cause the compiler to get into an infinite loop, or stack-overflow! There is nothing like going to get some coffee just to find your compilation is using up 30+ gigabytes of memory and all the CPU to compile your infinite loop code!

Answer 5

well, not that it's a while loop, but a counter loop, nonetheless the loop is possible in clean CPP (no templates and no C++)
#ifdef pad_always
#define pad(p,f) p##0
#else
#define pad0(p,not_used) p
#define pad1(p,not_used) p##0
#define pad(p,f) pad##f(p,)
#endif
// f - padding flag
// p - prefix so far
// a,b,c - digits
// x - action to invoke
#define n0(p,x)
#define n1(p,x) x(p##1)
#define n2(p,x) n1(p,x) x(p##2)
#define n3(p,x) n2(p,x) x(p##3)
#define n4(p,x) n3(p,x) x(p##4)
#define n5(p,x) n4(p,x) x(p##5)
#define n6(p,x) n5(p,x) x(p##6)
#define n7(p,x) n6(p,x) x(p##7)
#define n8(p,x) n7(p,x) x(p##8)
#define n9(p,x) n8(p,x) x(p##9)
#define n00(f,p,a,x) n##a(pad(p,f),x)
#define n10(f,p,a,x) n00(f,p,9,x) x(p##10) n##a(p##1,x)
#define n20(f,p,a,x) n10(f,p,9,x) x(p##20) n##a(p##2,x)
#define n30(f,p,a,x) n20(f,p,9,x) x(p##30) n##a(p##3,x)
#define n40(f,p,a,x) n30(f,p,9,x) x(p##40) n##a(p##4,x)
#define n50(f,p,a,x) n40(f,p,9,x) x(p##50) n##a(p##5,x)
#define n60(f,p,a,x) n50(f,p,9,x) x(p##60) n##a(p##6,x)
#define n70(f,p,a,x) n60(f,p,9,x) x(p##70) n##a(p##7,x)
#define n80(f,p,a,x) n70(f,p,9,x) x(p##80) n##a(p##8,x)
#define n90(f,p,a,x) n80(f,p,9,x) x(p##90) n##a(p##9,x)
#define n000(f,p,a,b,x) n##a##0(f,pad(p,f),b,x)
#define n100(f,p,a,b,x) n000(f,p,9,9,x) x(p##100) n##a##0(1,p##1,b,x)
#define n200(f,p,a,b,x) n100(f,p,9,9,x) x(p##200) n##a##0(1,p##2,b,x)
#define n300(f,p,a,b,x) n200(f,p,9,9,x) x(p##300) n##a##0(1,p##3,b,x)
#define n400(f,p,a,b,x) n300(f,p,9,9,x) x(p##400) n##a##0(1,p##4,b,x)
#define n500(f,p,a,b,x) n400(f,p,9,9,x) x(p##500) n##a##0(1,p##5,b,x)
#define n600(f,p,a,b,x) n500(f,p,9,9,x) x(p##600) n##a##0(1,p##6,b,x)
#define n700(f,p,a,b,x) n600(f,p,9,9,x) x(p##700) n##a##0(1,p##7,b,x)
#define n800(f,p,a,b,x) n700(f,p,9,9,x) x(p##800) n##a##0(1,p##8,b,x)
#define n900(f,p,a,b,x) n800(f,p,9,9,x) x(p##900) n##a##0(1,p##9,b,x)
#define n0000(f,p,a,b,c,x) n##a##00(f,pad(p,f),b,c,x)
#define n1000(f,p,a,b,c,x) n0000(f,p,9,9,9,x) x(p##1000) n##a##00(1,p##1,b,c,x)
#define n2000(f,p,a,b,c,x) n1000(f,p,9,9,9,x) x(p##2000) n##a##00(1,p##2,b,c,x)
#define n3000(f,p,a,b,c,x) n2000(f,p,9,9,9,x) x(p##3000) n##a##00(1,p##3,b,c,x)
#define n4000(f,p,a,b,c,x) n3000(f,p,9,9,9,x) x(p##4000) n##a##00(1,p##4,b,c,x)
#define n5000(f,p,a,b,c,x) n4000(f,p,9,9,9,x) x(p##5000) n##a##00(1,p##5,b,c,x)
#define n6000(f,p,a,b,c,x) n5000(f,p,9,9,9,x) x(p##6000) n##a##00(1,p##6,b,c,x)
#define n7000(f,p,a,b,c,x) n6000(f,p,9,9,9,x) x(p##7000) n##a##00(1,p##7,b,c,x)
#define n8000(f,p,a,b,c,x) n7000(f,p,9,9,9,x) x(p##8000) n##a##00(1,p##8,b,c,x)
#define n9000(f,p,a,b,c,x) n8000(f,p,9,9,9,x) x(p##9000) n##a##00(1,p##9,b,c,x)
#define n00000(f,p,a,b,c,d,x) n##a##000(f,pad(p,f),b,c,d,x)
#define n10000(f,p,a,b,c,d,x) n00000(f,p,9,9,9,9,x) x(p##10000) n##a##000(1,p##1,b,c,d,x)
#define n20000(f,p,a,b,c,d,x) n10000(f,p,9,9,9,9,x) x(p##20000) n##a##000(1,p##2,b,c,d,x)
#define n30000(f,p,a,b,c,d,x) n20000(f,p,9,9,9,9,x) x(p##30000) n##a##000(1,p##3,b,c,d,x)
#define n40000(f,p,a,b,c,d,x) n30000(f,p,9,9,9,9,x) x(p##40000) n##a##000(1,p##4,b,c,d,x)
#define n50000(f,p,a,b,c,d,x) n40000(f,p,9,9,9,9,x) x(p##50000) n##a##000(1,p##5,b,c,d,x)
#define n60000(f,p,a,b,c,d,x) n50000(f,p,9,9,9,9,x) x(p##60000) n##a##000(1,p##6,b,c,d,x)
#define n70000(f,p,a,b,c,d,x) n60000(f,p,9,9,9,9,x) x(p##70000) n##a##000(1,p##7,b,c,d,x)
#define n80000(f,p,a,b,c,d,x) n70000(f,p,9,9,9,9,x) x(p##80000) n##a##000(1,p##8,b,c,d,x)
#define n90000(f,p,a,b,c,d,x) n80000(f,p,9,9,9,9,x) x(p##90000) n##a##000(1,p##9,b,c,d,x)
#define cycle5(c1,c2,c3,c4,c5,x) n##c1##0000(0,,c2,c3,c4,c5,x)
#define cycle4(c1,c2,c3,c4,x) n##c1##000(0,,c2,c3,c4,x)
#define cycle3(c1,c2,c3,x) n##c1##00(0,,c2,c3,x)
#define cycle2(c1,c2,x) n##c1##0(0,,c2,x)
#define cycle1(c1,x) n##c1(,x)
#define concat(a,b,c) a##b##c
#define ck(arg) a[concat(,arg,-1)]++;
#define SIZEOF(x) (sizeof(x) / sizeof((x)[0]))
void check5(void)
{
int i, a[32769];
for (i = 0; i < SIZEOF(a); i++) a[i]=0;
cycle5(3,2,7,6,9,ck);
for (i = 0; i < SIZEOF(a); i++) if (a[i] != 1) printf("5: [%d] = %d\n", i+1, a[i]);
}

Answer 6

Here's an abuse of the rules that would get it done legally. Write your own C preprocessor. Make it interpret some #pragma directives the way you want.

Answer 7

I found this scheme useful when the compiler got cranky and wouldn't unroll certain loops for me
#define REPEAT20(x) { x;x;x;x;x;x;x;x;x;x;x;x;x;x;x;x;x;x;x;x;}
REPEAT20( val = pleaseconverge(val) );
But IMHO, if you need something much more complicated than that, then you should write your own pre-preprocessor. Your pre-preprocessor could for instance generate an appropriate header file for you, and it is easy enough to include this step in a Makefile to have everything compile smoothly by a single command. I've done it.