_Generic combined with variadic function?

Question

In C11, I could create a function which prototype would look like this:

void myVaFunc(const char* const conv, ...);

I could run it like this:

myVaFunc("ici", 1, "test", 2);

The function would know (after parsing the 1st parameter) that there are 3 additional parameters (4 with the initial one) with types consequently int, string(char pointer) and int. Easy, but not very elegant. Recently I have learned about the _Generic keyword, which allows to derive the type of a variable at the compilation time. I started to wonder either there is a way to combine the variadic functionality (not function anymore, since it always needs the 1st static parameter) and the _Generic functionality. Why? In order to remove the 1st parameter, which tells the function how to parse the others. Can a macro, which would be called like this exist?

MYVAFUNC(1, "test", 2);

And work in a same way as described earlier myVaFunc?

I am thinking about it for a while now, but cannot figure out either it is possible.

Answer 1

That is definitely possible but AFAIK, it requires some nontrivial macro magic (and it's difficult to make it work for an unlimited number of arguments).

In my project I have a BX_foreachc(What,...) macro that allows you to implement it with:

#include <stdio.h>
#define MYVAFUNC(...) /*replace puts with the actual consumer of the generated format string*/ \
    (MYVAFUNC__ptr=MYVAFUNC__buf, \
     BX_foreachc(MYVAFUNC__append,__VA_ARGS__), \
     *MYVAFUNC__ptr=0, \
      puts(MYVAFUNC__buf))
//impl.:
char MYVAFUNC__buf[128]; 
char *MYVAFUNC__ptr = MYVAFUNC__buf;
#define MYVAFUNC__append(X) *MYVAFUNC__ptr++ = _Generic(X,char*:'c',int:'i')

int main(void)
{
    MYVAFUNC(1,"foo",1,2,"bar",1,2,3) ; 
    //generates and consumes "iciiciii" and returns the return value of the consumer
}

The problematic part might be that my implementation of BX_foreachc (with a support for up to 127 arguments) is about 140 lines of cryptic, mostly generated code.

Here's a script that generates it and test-runs it on the above-posted main:

#!/bin/sh -eu
bx_def_BX_argc() #Define an arg-counting macro for Max $1 args (incl) #{{{
{
    local max_args=${1:-128} i
    printf '#define BX_argc(...) BX_argc_(X,##__VA_ARGS__) //{{{\n'
    printf '#define BX_argc_(...) BX_argc__(,##__VA_ARGS__,'
    i=$max_args; while [ $i -gt 0 ]; do printf $i,; i=$((i-1)); done
    printf '0,0)\n'
    printf '#define BX_argc__(_,'
    while [ $i -le $max_args ]; do printf _$i,; i=$((i+1)); done
    printf 'Cnt,...) Cnt //}}}\n'
} #}}}
bx_def_BX_foreach_() #{{{
{
    local Comma="$1" Max="${2:-128}"
    if [ -z "$Comma" ]; then
        echo "#define BX_foreachc_1(What, x, ...) What(x)"
    else
        echo "#define BX_foreach_1(Join, What, x, ...) What(x)"
    fi
    i=2; while [ $i -lt $Max ]; do
        if [ -z "$Comma" ]; then
            printf '#define BX_foreach_%d(Join,What,x,...) What(x) Join BX_paste(BX_foreach_%d(Join, What, __VA_ARGS__))\n' \
              $i $((i-1));
        else
            printf '#define BX_foreachc_%d(What,x,...) What(x) , BX_paste(BX_foreachc_%d(What, __VA_ARGS__))\n' \
              $i $((i-1));
        fi
    i=$((i+1)); done
} #}}}
{
cat <<EOF
#define BX_foreach(Join,What, ...) BX_foreach_(BX_argc(__VA_ARGS__), Join, What, __VA_ARGS__)
#define BX_foreachc(What, ...) BX_foreachc_(BX_argc(__VA_ARGS__), What, __VA_ARGS__)
#define BX_cat(X,...)  BX_cat_(X,__VA_ARGS__) //{{{
#define BX_cat_(X,...) X##__VA_ARGS__ //}}}
#define BX_paste(X) X

///
#define BX_foreach_(N, Join, What, ...) BX_paste(BX_cat(BX_foreach_, N)(Join, What, __VA_ARGS__))
#define BX_foreachc_(N,  What, ...) BX_paste(BX_cat(BX_foreachc_, N)( What, __VA_ARGS__))
EOF

#define BX_argc(...) BX_argc_(X,##__VA_ARGS__)
bx_def_BX_argc
bx_def_BX_foreach_ ''
bx_def_BX_foreach_ 1
} > foreach.h

cat > main.c <<'EOF'
#include "foreach.h" //generated header implementing BX_foreachc
#include <stdio.h>
#define MYVAFUNC(...) /*replace puts with the actual consumer of the generated format string*/ \
    (MYVAFUNC__ptr=MYVAFUNC__buf, \
     BX_foreachc(MYVAFUNC__append,__VA_ARGS__), \
     *MYVAFUNC__ptr=0, \
      puts(MYVAFUNC__buf))
//impl.:
char MYVAFUNC__buf[128]; 
char *MYVAFUNC__ptr = MYVAFUNC__buf;
#define MYVAFUNC__append(X) *MYVAFUNC__ptr++ = _Generic(X,char*:'c',int:'i')

int main(void)
{
    MYVAFUNC(1,"foo",1,2,"bar",1,2,3) ; 
    //generates and consumes "iciiciii" and returns the return value of the consumer
}
EOF

#compile and test-run
gcc main.c
./a.out

If you want to guard against overflowing the 127 maximum argument count, you could replace the above foreach-generated comma expression with an expression statement (nonstandard but common C extension) of the form:

({ 
    char buf[128];
    char *p=buf, *e = buf+sizeof(buf)-1;

    //foreach X:
        if(*p==e) return FAIL; else *p = _Generic(X,char*:'c', int:'i');
    *p = 0;
    puts(buf);
 })

An even better way to tackle this might be to completely forgo the format string an instead generate something like

do{
    //foreach X:
        if(FAILS(_Generic(X,char*: consume_str, int: consume_int)(X))) return FAIL;
}while(0);

Example, working code (no nonstandard C features):

#include <stdio.h>
#include "foreach.h"
#define FAILS(X) (0>(X))
#define FAIL (-1)
int consume_int(int X){ return printf("%d\n", X); }
int consume_str(char const* X){ return puts(X); }

#define MYVAFUNC(...) do{ BX_foreach(;,CONSUME_ARG,__VA_ARGS__); }while(0);
#define CONSUME_ARG(X) if(FAILS(_Generic(X, char*: consume_str, int:consume_int)(X)))
int main(void)
{
    MYVAFUNC(1,"foo",1,2,"bar",1,2,3) ; 
}

(Note that this uses BX_foreach (a macro that uses a custom joiner, in my case it's ;) rather than the BX_foreachc comma-based special case.)