A demonstration of the hack needed to pull a (not-always-going-to-be-valid) pointer out of P_SRCTRACK

GetTrackSendInfo_Value.c

#include <stdio.h>
#include <stdlib.h>

typedef struct {
    int foo;
    int bar;
} MediaTrack;

typedef union {
    long l;
    double d;
    void *p;
} ReaperReturn;

double what_reaper_actually_does()
{
    MediaTrack *track = malloc(sizeof(MediaTrack));
    return (long) track;
}

ReaperReturn what_reaper_should_do()
{
    ReaperReturn ret;
    MediaTrack *track = malloc(sizeof(MediaTrack));
    ret.p = track;
    return ret;
}

int main(int argc, const char *argv[])
{
    double d = what_reaper_actually_does();
    ReaperReturn r = what_reaper_should_do();
    
    printf("what_reaper_actually_does: %ld, %f, %p\n"
           "what_reaper_should_do: %ld, %f, %p\n",
           (long) d, d, *(void **)(&d),
           r.l, r.d, r.p);
    return 0;
}

There's the undocumented function GetSetTrackSendInfo which returns a void * which should absolutely be used instead of this insanity.