#if #endif preprocessing Directive, PortAudio

Question

I understand the basics to the #if #endif preprocessor directives in C, in that depending which expression evaluates to be true, the proceeding code within the #if will be compiled, however I'm currently learning portaudio(I'm making a VOIP app for school) and I'm looking over some of their examples and I've become confused over a small section

/* Select sample format. */
#if 1
#define PA_SAMPLE_TYPE  paFloat32
typedef float SAMPLE;
#define SAMPLE_SILENCE  (0.0f)
#define PRINTF_S_FORMAT "%.8f"
#elif 1
#define PA_SAMPLE_TYPE  paInt16
typedef short SAMPLE;
#define SAMPLE_SILENCE  (0)
#define PRINTF_S_FORMAT "%d"
#elif 0
#define PA_SAMPLE_TYPE  paInt8
typedef char SAMPLE;
#define SAMPLE_SILENCE  (0)
#define PRINTF_S_FORMAT "%d"
#else
#define PA_SAMPLE_TYPE  paUInt8
typedef unsigned char SAMPLE;
#define SAMPLE_SILENCE  (128)
#define PRINTF_S_FORMAT "%d"
#endif

the first question that comes to mind is

#if 1
#define PA_SAMPLE_TYPE  paFloat32
typedef float SAMPLE;
#define SAMPLE_SILENCE  (0.0f)
#define PRINTF_S_FORMAT "%.8f"
#elif 1
#define PA_SAMPLE_TYPE  paInt16
typedef short SAMPLE;
#define SAMPLE_SILENCE  (0)
#define PRINTF_S_FORMAT "%d"

wont the #elif 1 always be skipped because if somehow #if 1 (#if true) evaluates to false, wont #elif 1 also evaluate to false?

question 2 isn't 1 evaluate to true and 0 to false? so wont #elif 0 always evaluate to false? i.e it doesn't really matter?

question 3 I'm going to be sending these samples over a socket, is skipping this preprocessor directive and just working with the code

#define PA_SAMPLE_TYPE  paInt8
typedef char SAMPLE;
#define SAMPLE_SILENCE  (0)
#define PRINTF_S_FORMAT "%d"

or

#define PA_SAMPLE_TYPE  paUInt8
typedef unsigned char SAMPLE;
#define SAMPLE_SILENCE  (128)
#define PRINTF_S_FORMAT "%d"
#endif

Would that be beter so as my SAMPLE_TYPE/SAMPLE can be treated as an array of characters/ Unsigned Characters (not having to convert floats to chars and then back again) for writing/reading from the socket?

Answer 1

What you need to understand is between a sequence of #if / #elif / #else, only one condition will be selected:

#if is selected here:

#if 1
// only this one will be selected
#elif 1
#else
#endif

#elif is selected here:

#if 0
#elif 1
// only this one will be selected
#else
#endif

#else is selected here:

#if 0
#elif 0
#else
// only this one will be selected
#endif