Connecting two serial ports in C - c

Question

I want to connect/bridge two serial ports in C.
I have 2 threads reading the ports, and writing to the other port.
Here is one example:
void *rfid_to_uart_thread(void *) {
char rfid_read_buffer[100];
int writeCounter;
do {
writeCounter = read(rfidCom, rfid_read_buffer, sizeof(rfid_read_buffer)-1);
if (writeCounter > 0) {
write(uartCom, rfid_read_buffer, writeCounter);
} else
usleep(25);
} while (!bKillBridgeThreads);
return NULL;}
The problem is, it seems that the writes are too slow. I often receive only half of the String on the other side. It seems like the write is asynchronously and thus the buffer is overwritten again in the next loop and overwrites the last 'write', so that the data is crippled?!
Is that right?
The ports are opened NON_BLOCKING and RW, Baudrate is and has to be 9600.

Answer 1

Looking at the man:
read() attempts to read up to count bytes from file descriptor fd into the buffer starting at buf.
In other wordsread does not grants to return all bytes send by other task, can give you a single byte up to sizeof(rfid_read_buffer)-1
What you can do is:
loop reading from rfidCom until the number of chars matches the number of chars sent.
You can use a specific terminator of messages and check for it to validate received message
encapsulate chars into a protocol message with an header that embed the message length, so the receiver can count the received chars and stop reading when last char is received.
For example:
void *rfid_to_uart_thread(void *)
{
char rfid_read_buffer[100] = {0};
int writeCounter;
char RXchar;
ssize_t retVal;
bool send = false;
do
{
memset(rfid_read_buffer, 0x00, sizeof(rfid_read_buffer));
send = true;
do
{
retVal = read(rfidCom, &RXchar, 1);
if (retVal > 0)
{
rfid_read_buffer[writeCounter] = RXchar;
writeCounter++;
}
else if (retVal < 0)
{
send = false;
RXchar = '\r'
break;
}
else
{
usleep(25);
}
}
while(RXchar != '\r');
if (send)
{
write(uartCom, rfid_read_buffer, writeCounter);
}
}
while (!bKillBridgeThreads);
return NULL;
}

Answer 2

OK, I've found a solution to my problem I think.
void *rfid_to_uart_thread(void *) {
char rfid_read_buffer[10];
ssize_t writeCounter = -1;
do {
writeCounter = read(rfidCom, &rfid_read_buffer, sizeof(rfid_read_buffer)-1);
if (writeCounter>0){
rfid_read_buffer[writeCounter] = 0;
LOGE("RFID -> UART: %s", rfid_read_buffer);
write(uartCom, rfid_read_buffer, writeCounter);
}else{
usleep(25);
}
tcdrain(uartCom);
} while (!bKillBridgeThreads);
return NULL;}
I've created my own define for a tcdrain, because the Android NDK I am using is not offering it in termios.h
Now, all the values seem to get transmitted to the UART port.
tcdrain is now defined as:
#define tcdrain(fd) ioctl(fd, TCSBRK, 1)