arduino low i2c read speed; - c++

Question

I'm currently working on a project using the genuino 101 where i need to read large amounts of data trough i2c, to fill an arbitrarily sized buffer.from the following image i can see that the read requests themselves only take about 3 milliseconds and the write request about 200 nanoseconds.
however there is a very large time (750+ ms) between read transactions in the same block
#define RD_BUF_SIZE 32
void i2cRead(unsigned char device, unsigned char memory, int len, unsigned char * rdBuf)
{
ushort bytesRead = 0;
ushort _memstart = memory;
while (bytesRead < len)
{
Wire.beginTransmission((int)device);
Wire.write(_memstart);
Wire.endTransmission();
Wire.requestFrom((int)device, BLCK_SIZE);
int i = 0;
while (Wire.available())
{
rdBuf[bytesRead+i] = Wire.read();
i++;
}
bytesRead += BLCK_SIZE;
_memstart += BLCK_SIZE;
}
}
from my understanding this shouldn't take that long, unless adding to memstart and bytesRead is taking extremely long. by my, arguably limited, understanding of time complexity this function has a time complexity of O(n) and should, in the best case only take about 12 ms for a 128 byte query
Am i missing something?

Answer 1

Those 700ms are not caused by the execution time of the few instructions in your function. Those should be done in microseconds. You may have a buffer overflow, or the other device might be delaying transfers, or there's another bug not related to buffer overflow.
This is about how I'd do it:
void i2cRead(unsigned char device, unsigned char memory, int len, unsigned char * rdBuf, int bufLen)
{
ushort _memstart = memory;
if ( bufLen < len ) {
len = bufLen;
}
while (len > 0)
{
Wire.beginTransmission((int)device);
Wire.write(_memstart);
Wire.endTransmission();
int reqSize = 32;
if ( len < reqSize ) {
reqSize = len;
}
Wire.requestFrom((int)device, reqSize);
while (Wire.available() && (len != 0))
{
*(rdBuf++) = Wire.read();
_memstart++;
len--;
}
}
}