EXPORT int jitter_
buffer_get(Jitter
Buffer *jitter, Jitter
BufferPacket *packet, spx_int32_t desired_span, spx_int32_t *start_offset)
{
spx_uint32_t i;
unsigned int j;
spx_int16_t opt;
if (start_offset != NULL)
*start_offset = 0;
/* Syncing on the first call */
if (jitter
->reset_state)
{
int found = 0;
/* Find the oldest packet */
spx_uint32_t oldest=0;
for (i=0;i<jitter
->
buffer_size;i++)
{
if (jitter
->packets[i].data && (!found || LT32(jitter
->packets[i].timestamp,oldest)))
{
oldest = jitter
->packets[i].timestamp;
found = 1;
}
}
if (found)
{
jitter
->reset_state=0;
jitter
->pointer_timestamp = oldest;
jitter
->next_stop = oldest;
} else {
packet
->timestamp = 0;
packet
->span = jitter
->interp_requested;
return JITTER_
BUFFER_MISSING;
}
}
jitter
->last_returned_timestamp = jitter
->pointer_timestamp;
if (jitter
->interp_requested != 0)
{
packet
->timestamp = jitter
->pointer_timestamp;
packet
->span = jitter
->interp_requested;
/* Increment the pointer because it got decremented in the delay update */
jitter
->pointer_timestamp += jitter
->interp_requested;
packet
->len = 0;
/*fprintf (stderr, "Deferred interpolate\n");*/
jitter
->interp_requested = 0;
jitter
->
buffered = packet
->span
- desired_span;
return JITTER_
BUFFER_INSERTION;
}
/* Searching for the packet that fits best */
/* Search the
buffer for a packet with the right timestamp
and spanning the whole current chunk */
for (i=0;i<jitter
->
buffer_size;i++)
{
if (jitter
->packets[i].data && jitter
->packets[i].timestamp==jitter
->pointer_timestamp && GE32(jitter
->packets[i].timestamp+jitter
->packets[i].span,jitter
->pointer_timestamp+desired_span))
break;
}
/* If no match, try for an "older" packet that still spans (fully) the current chunk */
if (i==jitter
->
buffer_size)
{
for (i=0;i<jitter
->
buffer_size;i++)
{
if (jitter
->packets[i].data && LE32(jitter
->packets[i].timestamp, jitter
->pointer_timestamp) && GE32(jitter
->packets[i].timestamp+jitter
->packets[i].span,jitter
->pointer_timestamp+desired_span))
break;
}
}
/* If still no match, try for an "older" packet that spans part of the current chunk */
if (i==jitter
->
buffer_size)
{
for (i=0;i<jitter
->
buffer_size;i++)
{
if (jitter
->packets[i].data && LE32(jitter
->packets[i].timestamp, jitter
->pointer_timestamp) && GT32(jitter
->packets[i].timestamp+jitter
->packets[i].span,jitter
->pointer_timestamp))
break;
}
}
/* If still no match, try for earliest packet possible */
if (i==jitter
->
buffer_size)
{
int found = 0;
spx_uint32_t best_time=0;
int best_span=0;
int besti=0;
for (i=0;i<jitter
->
buffer_size;i++)
{
/* check if packet starts within current chunk */
if (jitter
->packets[i].data && LT32(jitter
->packets[i].timestamp,jitter
->pointer_timestamp+desired_span) && GE32(jitter
->packets[i].timestamp,jitter
->pointer_timestamp))
{
if (!found || LT32(jitter
->packets[i].timestamp,best_time) || (jitter
->packets[i].timestamp==best_time && GT32(jitter
->packets[i].span,best_span)))
{
best_time = jitter
->packets[i].timestamp;
best_span = jitter
->packets[i].span;
besti = i;
found = 1;
}
}
}
if (found)
{
i=besti;
/*fprintf (stderr, "incomplete: %d %d %d %d\n", jitter
->packets[i].timestamp, jitter
->pointer_timestamp, chunk_size, jitter
->packets[i].span);*/
}
}
/* If we find something */
if (i!=jitter
->
buffer_size)
{
spx_int32_t offset;
/* We (obviously) haven't lost this packet */
jitter
->lost_count = 0;
/* In this case, 0 isn't as a valid timestamp */
if (jitter
->arrival[i] != 0)
{
update_timings(jitter, ((spx_int32_t)jitter
->packets[i].timestamp)
- ((spx_int32_t)jitter
->arrival[i])
- jitter
->
buffer_margin);
}
/* Copy packet */
if (jitter
->destroy)
{
packet
->data = jitter
->packets[i].data;
packet
->len = jitter
->packets[i].len;
} else {
if (jitter
->packets[i].len > packet
->len)
{
speex_warning_int("jitter_
buffer_get(): packet too large to fit. Size is", jitter
->packets[i].len);
} else {
packet
->len = jitter
->packets[i].len;
}
for (j=0;j<packet
->len;j++)
packet
->data[j] = jitter
->packets[i].data[j];
/* Remove packet */
speex_free(jitter
->packets[i].data);
}
jitter
->packets[i].data = NULL;
/* Set timestamp
and span (if requested) */
offset = (spx_int32_t)jitter
->packets[i].timestamp
-(spx_int32_t)jitter
->pointer_timestamp;
if (start_offset != NULL)
*start_offset = offset;
else if (offset != 0)
speex_warning_int("jitter_
buffer_get() discarding non
-zero start_offset", offset);
packet
->timestamp = jitter
->packets[i].timestamp;
jitter
->last_returned_timestamp = packet
->timestamp;
packet
->span = jitter
->packets[i].span;
packet
->sequence = jitter
->packets[i].sequence;
packet
->user_data = jitter
->packets[i].user_data;
/* Point to the end of the current packet */
jitter
->pointer_timestamp = jitter
->packets[i].timestamp+jitter
->packets[i].span;
jitter
->
buffered = packet
->span
- desired_span;
if (start_offset != NULL)
jitter
->
buffered += *start_offset;
return JITTER_
BUFFER_OK;
}
/* If we haven't found anything worth returning */
/*fprintf (stderr, "not found\n");*/
jitter
->lost_count++;
/*fprintf (stderr, "m");*/
/*fprintf (stderr, "lost_count = %d\n", jitter
->lost_count);*/
opt = compute_opt_delay(jitter);
/* Should we force an increase in the
buffer or just do normal interpolation? */
if (opt < 0)
{
/* Need to increase
buffering */
/* Shift histogram to compensate */
shift_timings(jitter,
-opt);
packet
->timestamp = jitter
->pointer_timestamp;
packet
->span =
-opt;
/* Don't move the pointer_timestamp forward */
packet
->len = 0;
jitter
->
buffered = packet
->span
- desired_span;
return JITTER_
BUFFER_INSERTION;
/*jitter
->pointer_timestamp
-= jitter
->delay_step;*/
/*fprintf (stderr, "Forced to interpolate\n");*/
} else {
/* Normal packet loss */
packet
->timestamp = jitter
->pointer_timestamp;
desired_span = ROUND_DOWN(desired_span, jitter
->concealment_size);
packet
->span = desired_span;
jitter
->pointer_timestamp += desired_span;
packet
->len = 0;
jitter
->
buffered = packet
->span
- desired_span;
return JITTER_
BUFFER_MISSING;
/*fprintf (stderr, "Normal loss\n");*/
}
}
本文探讨了Linux系统中的I/O操作模型，包括标准库提供的buffered I/O与系统调用实现的non-buffered I/O。重点介绍了如何通过设置O_DIRECT标志实现Direct I/O，以绕过操作系统缓存直接访问设备。
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