faster alternative to windows.h's Beep()

One way (which would be suitable since you want to target Linux too) would be to generate a WAV file, then play that. (There are simple ways to play WAV files on both Windows and Linux.)

You could use a library to generate the WAV file, or create it yourself, both approaches are pretty simple. There are many examples on the web.

If you do it yourself:

/* WAV header, 44 bytes */struct wav_header {    uint32_t riff packed;    uint32_t len packed;    uint32_t wave packed;    uint32_t fmt packed;    uint32_t flen packed;    uint16_t one packed;    uint16_t chan packed;    uint32_t hz packed;    uint32_t bpsec packed;    uint16_t bpsmp packed;    uint16_t bitpsmp packed;    uint32_t dat packed;    uint32_t dlen packed;};

Initialize with:

void wav_header(wav_header *p, uint32_t dlen){    memcpy(p->riff, "RIFF", 4);    p->len = dlen + 44;    memcpy(p->wave, "WAVE", 4);    memcpy(p->fmt, "fmt ", 4);    p->flen = 0x10;    p->one = 1;    p->chan = 1;    p->hz = 22050;    p->bpsec = hz;    p->bpsmp = 1;    p->bitpsmp = 8;    memcpy(p->dat, "data", 4);    p->dlen = dlen;}