Typecasting an array in C

Question

I was reading FFMPEG source code on extracting audio and I found these macros. What do these macros do?

#define REINTERPRET_CAST(type, variable) C_CAST(type, variable)
#define STATIC_CAST(type, variable) C_CAST(type, variable)
#define C_CAST(type, variable) ((type)variable)

//used like this
int value = 0;
int sampleIndex = 0;
uint8_t* buffer = calloc(50, sizeof(uint8_t));

value = REINTERPRET_CAST(uint8_t*, buffer)[sampleIndex];
value = REINTERPRET_CAST(int16_t*, buffer)[sampleIndex];
value = REINTERPRET_CAST(int32_t*, buffer)[sampleIndex];
value = REINTERPRET_CAST(int64_t*, buffer)[sampleIndex];
int ret = STATIC_CAST(float, *REINTERPRET_CAST(double*, &value));

Answer 1

//used like this

Used like in this code they do nothing meaningful - convert pointer to int and assign to int vartable (abstracting from the wrong syntax)

If they are used like this:

uint64_t value = *REINTERPRET_CAST(int64_t*, buffer + sampleIndex);

BTW the macro is wrong as well

#define C_CAST(type, variable) ((type)(variable))

Then it is called pointer punning and it invokes Undefined Behaviour violating strict aliasing rules.

It should be done this way:

#define pune(var, X) _Generic((var), \
              uint16_t *: pune16, \
              uint32_t *: pune32,  \
              uint64_t*: pune64  \
              )(var, X)


uint16_t pune16(uint16_t *val, const void *ptr)
{
    memcpy(val, ptr, sizeof(*val));
    return *val;
}

uint32_t pune32(uint32_t *val, const void *ptr)
{
    memcpy(val, ptr, sizeof(*val));
    return *val;
}

uint64_t pune64(uint64_t *val, const void *ptr)
{
    memcpy(val, ptr, sizeof(*val));
    return *val;
}

example usage:

void foo(void *v)
{
    uint32_t x;
    x = pune(&x, v);
    printf("%"PRIu32"\n,",  x);
}