Linux serial read() extraneous data

Question

I have two applications communicating over serial. The target has a state machine that it navigates. It will receive and throw out data until it gets a start of message byte (0xE3) and then process data until an end of message byte (0x3E). I'm having an issue with some of the data. I believe it's a processing problem and not a communication issue. I also know it's something simple, but struggling to see the problem.

The host is sending data to the target like so. The bfr data is: {0xAA, 0xFF, 0x05, 'H', 'e', 'l', 'l', 'o'};

byte = MSG_START_BYTE; // 0xE3
if (write(fd, &byte, 1) < 0)
    return -1;

if (write(fd, bfr, bfr_len) < 0)
    return -1;

if (write(fd, &crc, sizeof(crc)) < 0)
    return -1;

byte = MSG_END_BYTE;  // 0x3E
if (write(fd, &byte, 1) < 0)
    return -1;

The target is using select() and read() to receive the data.

char rx_bfr[128];
uint8_t max_bytes_to_read = 1;

memset(rx_bfr, 0, strlen(rx_bfr));
...
printf("\tactivity...");
bytes_read = read(fd, rx_bfr, max_bytes_to_read);
if (bytes_read < 0)
    return -1;
else if (bytes_read == 0)
    return -1;

printf(" %d bytes to process...\n", bytes_read);
for (i = 0; i < bytes_read; i++)
{
    printf("byte[%d] = 0x%x\n", i, rx_bfr[i]);
    switch (state)
    {
        case SOM:
            if (rx_bfr[i] == 0xE3)
 ...

And the target console output is:

Waiting for message...
    activity... 1 bytes to process...
byte[0] = 0xffffffe3
    activity... 1 bytes to process...
byte[0] = 0xffffffaa
    activity... 1 bytes to process...
byte[0] = 0xffffffff
    activity... 1 bytes to process...
byte[0] = 0x5
    activity... 1 bytes to process...
byte[0] = 0x48
    activity... 1 bytes to process...
byte[0] = 0x65
    activity... 1 bytes to process...
byte[0] = 0x6c
    activity... 1 bytes to process...
byte[0] = 0x6c
    activity... 1 bytes to process...
byte[0] = 0x6f
    activity... 1 bytes to process...
byte[0] = 0xffffffd4
    activity... 1 bytes to process...
byte[0] = 0xffffff81
    activity... 1 bytes to process...
byte[0] = 0x1a
    activity... 1 bytes to process...
byte[0] = 0x3c
    activity... 1 bytes to process...
byte[0] = 0x3b
    activity... 1 bytes to process...
byte[0] = 0x3e

So in my state machine the if conditional is looking for 0xE3 as the SOM, but the first "byte" in the buffer as seen from the printf is 0xFFFF_FFE3.

TL;DR It's often misconfiguration of the serial port parameters. — Iharob Al Asimi, Jan 08 '16 at 21:45
I'm not sure what kind of configuration you could be referring to. I copied the attrib and blocking setup from this thread (http://stackoverflow.com/questions/6947413/how-to-open-read-and-write-from-serial-port-in-c). Only difference is my applications are setup for blocking instead of non-blocking. — linsek, Jan 08 '16 at 21:49
Please post the code where you setup the serial port, and what is the expected setup, parity bits, baud rate and other settings. — Iharob Al Asimi, Jan 08 '16 at 21:49

David Schwartz · Accepted Answer · 2016-01-08T21:50:13.577

5

char rx_bfr[128];

On your platform char is signed. A signed char with value 0xE3 is negative but an integer with value 0xE3 is positive, so they won't compare as equal. Use unsigned char instead.

edited Jan 08 '16 at 21:50

answered Jan 08 '16 at 21:49

David Schwartz

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Very simple explanation indeed. – Iharob Al Asimi Jan 08 '16 at 21:50
brilliant. thank you, sir. that was precisely the problem. – linsek Jan 08 '16 at 21:53

Linux serial read() extraneous data

1 Answers1