Function Overload C multiple arguments

Question

I like to voverload a C function. I found following: Function overloading in C using GCC - functions with mutiple arguments. This works very fine, but now i want to sellect the function based on the type of two arguments.

#define dataioWriteBin(file, data, len)                                                                                                                                                 \
    (__builtin_choose_expr(__builtin_types_compatible_p(typeof(*data), uint8_t )    && __builtin_types_compatible_p(typeof(len), uint64_t ), dataioWriteBin_uint8Data_uint64Len,    \
     __builtin_choose_expr(__builtin_types_compatible_p(typeof(*data), uint8_t )    && __builtin_types_compatible_p(typeof(len), uint32_t ), dataioWriteBin_uint8Data_uint32Len,    \
     __builtin_choose_expr(__builtin_types_compatible_p(typeof(*data), uint32_t )   && __builtin_types_compatible_p(typeof(len), uint64_t ), dataioWriteBin_uint32Data_uint64Len,   \
     __builtin_choose_expr(__builtin_types_compatible_p(typeof(*data), uint32_t )   && __builtin_types_compatible_p(typeof(len), uint32_t ), dataioWriteBin_uint32Data_uint32Len,   \
     (void)0))))(file, data, len))

But with this code i got a compile following error:

error: called object is not a function or function pointer (__builtin_choose_expr(__builtin_types_compatible_p(typeof(*data), uint8_t )

Anybody a Idea, is it not possibel to dispatch on two argumnents. The construct itself does also work. It Fails wenn i call the function with uint32_t data and uint32_t length. The Function calls with uint8_t are working.

Thanks for your help.

"I like to overload a C function". Why? There might be valid cases, but you better make yours a good one before you even consider this. — Lundin
– Lundin, Commented Jun 5, 2018 at 9:10
There are better tools than C for this. "Birmingham screwdriver" seems apropos. — Andrew Henle
– Andrew Henle, Commented Jun 5, 2018 at 10:04

jxh · Accepted Answer · 2018-06-05 08:12:02Z

You can achieve this with C.11's _Generic:

#define uint8Data_(file, data, len)                          \
_Generic((len),                                              \
    uint64_t : dataioWriteBin_uint8Data_uint64Len,           \
    uint32_t : dataioWriteBin_uint8Data_uint32Len)

#define uint32Data_(file, data, len)                         \
_Generic((len),                                              \
    uint64_t : dataioWriteBin_uint32Data_uint64Len,          \
    uint32_t : dataioWriteBin_uint32Data_uint32Len)

#define dataioWriteBin(file, data, len)                      \
_Generic(                                                    \
    (data),                                                  \
    uint8_t * :        uint8Data_(file, data, len),          \
    const uint8_t * :  uint8Data_(file, data, len),          \
    uint32_t * :       uint32Data_(file, data, len),         \
    const uint32_t * : uint32Data_(file, data, len)          \
    )(file, data, len)

The expression selection syntax using GCC's __builtin_choose_expr is described in the accepted answer to the question you cited in your post.

Petr Skocik · Accepted Answer · 2018-06-05 09:34:47Z

This might not be the best use of type-based overloading (whether based on __builtin_choose_expr or the standardized _Generic) but here's an example that shows that your code does work with uint32_t *data and uint32_t len. The code compiles:

#include <stdint.h>
#include <stdio.h>
//prototypes
void dataioWriteBin_uint8Data_uint64Len(FILE *f, uint8_t const *data, uint64_t len);
void dataioWriteBin_uint8Data_uint32Len(FILE *f, uint8_t const *data, uint32_t len);
void dataioWriteBin_uint32Data_uint64Len(FILE *f, uint32_t const *data, uint64_t len);
void dataioWriteBin_uint32Data_uint32Len(FILE *f, uint32_t const *data, uint32_t len);

//
#define dataioWriteBin(file, data, len)                                                                                                                                                 \
    (__builtin_choose_expr(__builtin_types_compatible_p(typeof(*data), uint8_t )    && __builtin_types_compatible_p(typeof(len), uint64_t ), dataioWriteBin_uint8Data_uint64Len,    \
     __builtin_choose_expr(__builtin_types_compatible_p(typeof(*data), uint8_t )    && __builtin_types_compatible_p(typeof(len), uint32_t ), dataioWriteBin_uint8Data_uint32Len,    \
     __builtin_choose_expr(__builtin_types_compatible_p(typeof(*data), uint32_t )   && __builtin_types_compatible_p(typeof(len), uint64_t ), dataioWriteBin_uint32Data_uint64Len,   \
     __builtin_choose_expr(__builtin_types_compatible_p(typeof(*data), uint32_t )   && __builtin_types_compatible_p(typeof(len), uint32_t ), dataioWriteBin_uint32Data_uint32Len,   \
     (void)0))))(file, data, len))

//
void use_it(void)
{
    uint32_t data[4]={};
    dataioWriteBin(stdout,data,(uint32_t)sizeof(data));
}

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