path: root/md/writeup/c_macro_tricks.md

title: C macro tricks
keywords:c,macro,c11,generics

# C macro tricks

## GCC preprocess macros

If you need to run just macro preprocessor without running compiler use

```bash
gcc -E source.c
```

this allows to see resulting source that going to be compiled, macro errors
could be hard to debug, but this is first thing, test them before and then
be sure that everything works. Lets continue with some more deep stuff.

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## __VA_ARGS__ keyword

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### Single argument macros
Writting macros with single argument

```c
#define F(X) X
```

#### Source

So lets code just with one macro

```c
#define F(X) X

F(int main)
F((){)
F(printf("hello world\n");)
F(})
```

any kind of argument can be passed to macro, and that allows to make some tricks

#### Result
```
int main
(){
printf("hello world\n");
}
```

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### Multi argument macro

writting macro with multiple unamed arguments

```c
#define F(...) __VA_ARGS__
```

#### Source
```c
#define F(...) __VA_ARGS__

F(int main)
F((){)
F(printf("hello world\n");)
F(})

F(1,2,3,4,5)
```

Previouse example works just fine, but if add multiple arguments the __VA_ARGS__
just prints them as a whole string

#### Result
```
int main
(){
printf("hello world\n");
}

1,2,3,4,5
```


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### Mixing named arguments and unamed arguments

Mixing together named and unnamed arguments

```c
#define F(X,...) X __VA_ARGS
```

#### Source

```c
#define F1(X,...) __VA_ARGS__
#define F2(X,...) X

F1(1,2,3,4,5)
F2(1,2,3,4,5)

F1(int main,{my code},{more code})
F2(int main,{my code},{more code})
```

#### Result

```
2,3,4,5
1

{my code},{more code}
int main
```

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## Define struct with macros

Lets move to some more practical example lets just define macro that going to
create proper C structure.

C structure have this kind of syntax
```block
struct <name-struct> 
{
<type-name> <variable-name>;
<type-name> <variable-name>;
...
...
};
```

```c
#define N
#define M2(X1,X2,...) X1 X2;
#define M1(X1,X2,...) X1 X2; M2(__VA_ARGS__,N,N)
#define M(X,...) struct X {M1(__VA_ARGS__,N,N)};
```
Here we pass variable arguments first macro M preprocess first arguments and
pass all left-over aruments with __VA_ARGS__ to M1, M1 preprocess 2 arguments 
(2nd,3rd) and pass left-over arguments to next M2 macro.

There is small trict to make support any number of arguments, there is defined
macro N that is just empty macro, so macro M support any number of arguments
and if there is not define needed amount of arguments N is passed, and if there
is not enought arguments N is used.

<!--
Correct amount of arguments is 1,3,5. If need to define more then need to
define deeper structure of M arguments. Dont forget that macro maximum argument
number is 64 so, with this structure you can define just 31 field of structure.
There could be done small fix and it could support 63 arguments.
-->
#### Source
```c
#define N
#define M2(X1,X2,...) X1 X2;
#define M1(X1,X2,...) X1 X2; M2(__VA_ARGS__,N,N)
#define M(X,...) struct X {M1(__VA_ARGS__,N,N)};

M(add,int,a,int,b);

M(dirst,int,c);
```
#### Result
```
struct add {int a; int b;};;

struct dirst {int c; ;};;
```

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## Detect number of arguments

There is one trick that can be used to detect number of arguments passed to 
macro. This common example found in internet. There could be made some 
improvment or can be added some trickery. But for now most important feature
is that it works.

```c
#define PP_NARG(...) \
    PP_NARG_(__VA_ARGS__,PP_RSEQ_N())

#define PP_NARG_(...) \
    PP_ARG_N(__VA_ARGS__)

#define PP_ARG_N( \
     _1, _2, _3, _4, _5, _6, _7, _8, _9,_10, \
    _11,_12,_13,_14,_15,_16,_17,_18,_19,_20, \
    _21,_22,_23,_24,_25,_26,_27,_28,_29,_30, \
    _31,_32,_33,_34,_35,_36,_37,_38,_39,_40, \
    _41,_42,_43,_44,_45,_46,_47,_48,_49,_50, \
    _51,_52,_53,_54,_55,_56,_57,_58,_59,_60, \
    _61,_62,_63,  N, ...) N

#define PP_RSEQ_N() \
    63,62,61,60,                   \
    59,58,57,56,55,54,53,52,51,50, \
    49,48,47,46,45,44,43,42,41,40, \
    39,38,37,36,35,34,33,32,31,30, \
    29,28,27,26,25,24,23,22,21,20, \
    19,18,17,16,15,14,13,12,11,10, \
     9, 8, 7, 6, 5, 4, 3, 2, 1, 0
```

#### Source
```c
#define F(...) PP_NARG(__VA_ARGS__)

F(0)
F()
F(1,2,3,4,5)
```

#### Result
```
1
1
5
```

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## Variable argument macro match macro according number of arguments

Detect number of arguments and match macro according to number of arguments

```
#define FUN3(X1,X2,X3,...) "there is 3"
#define FUN2(X1,X2,...) "there is 2"
#define FUN1(X1,...) "there is 1"
#define FUNN(X,A) X ## A
#define FUNN1(X,A) FUNN(X,A)
#define FUN(X,...) void X  ( FUNN1(FUN,PP_NARG(__VA_ARGS__))(__VA_ARGS__))
```

#### Source
```
FUN(add,int a,int b);
FUN(mul,int a,int b,int c);
FUN(div,int a);
```

#### Result
```
void add ( "there is 2");
void mul ( "there is 3");
void div ( "there is 1");
```