Help me understand a piece of code well

[Harten]

I saw a piece of code from other website. But I don't know what this code does. Please help me to understand this well. Thanks a lot.

Code: Select all

short lc = (short) (a[ISO7816.OFFSET_LC] & 0xff);
short read = apdu.setIncomingAndReceive();
while(read < lc) 
{
read += apdu.receiveBytes(read);
}

[vermont]

Here is the comments version.

Code: Select all

// Get the Lc from a standard length APDU. In JC2.2.2 use the Apdu methods for Lc.
short lc = (short) (a[ISO7816.OFFSET_LC] & 0xff);
// Ask the OS to read the incoming APDU.
short read = apdu.setIncomingAndReceive();
// Check how much was read. The OS may not be able to read the entire APDU in one call.
while(read < lc) {
// Keep reading until the OS has read all of the data in the APDU buffer.
    read += apdu.receiveBytes(read);
}

[pitbar]

Code: Select all

short lc = (short) (a[ISO7816.OFFSET_LC] & 0xff);

It's the standard way to compute Lc byte.

Code: Select all

short read = apdu.setIncomingAndReceive();

This line is to get at least the beginning of the incoming data in the APDU buffer.

Code: Select all

while(read < lc) 
{
    read += apdu.receiveBytes(read);
}

Appends whatever additional bytes receiveBytes returns, update read as this process goes.