Reading NULL character from Serial Port Linux C

Question

I have read many questions and answers but didn't find any solution. May be my question is not right but I need some guidance. I am using serial port in Linux which is reading data from my Arduino device. Whenever I want to send data from Arduino to Linux, I first send two bytes which indicate the total bytes which will come from Arduino. I convert these two bytes to integer value and start reading data from Serial Port. Say, I want to send 300 bytes from Ardiuno to Linux, I will just write {1, 44} first and then convert this 1 and 44 byte into int by the following formula:

char data[] = {1, 44};
int to_read = data[0]
to_read = to_read << 8;
to_read = to_read | data[1];
return to_read;

this will give me 300 int value, this is working like charm. but problem comes when I have to read data less then 255. Say I want to read 100 bytes, then first two bytes will be {0, 100}. 0 is null character, serial port doesn't process it (I manually wrote 0s to serial port, it always give me 0 bytes written), and my all sequence goes wrong. So my question is can I read null characters from serial port OR someone please give me better solution..

thanks in Advance.

Reason #18 why i hate it when people say "null" when talking about characters. "Null" basically means "no value" -- ie: it sits outside the range of allowed values -- while a "null character" is the character with code 0. IE: an allowable, if somewhat odd, value. Please, use NUL instead; that's what's in the ASCII tables. :P — cHao
Point being, character 0 is still a normal character. It just happens to also have special meaning to C, when working with nul-terminated byte strings. The serial port, on the other hand, couldn't care less. — cHao
You might like to show us the code how you setup the serial devices on both sides, and how you then send and receive data via those devices. — alk
Yes - code. Serial ports have no intrinsic problem with the NUL char, or byte value 0, octet 0 or whatever you wish to call it. This is surely an issue with C-style null-terminated strings, as suggested by the other commenters. — Martin James
finally I got my problem solved. Serial port is configured for 1N8 option. actually I was treating Char value as string, and sending data with size computing with function strlen(char*), I just start sending data like char data[] = {0, 0, 0}; write(serial_fd, data, 3); as I entered total byte size as 3, after that I am able to send NULL (:-P) characters also. And besides, 0 is also a character. I hope it will be helpful for some like me :-p — moonzai

moonzai moonzai · Accepted Answer · 2013-12-13T12:43:25

I got my problem solved. When working on bytes in C, don't confuse bytes (char) with string like I was treating bytes array (char data[]) and When I was trying to write these bytes on serial port with write method with length of strlen(data), I was only getting those bytes which were not null. strlen returns the length of data after seeing first null character that is \0, because of this I was not getting my desired output. What I did is that, If I want to write data char data[] = {0, 4} then I will do something like this:

char data[] = {0, 4};
write(serial_port_fd, data, 2);

told the write function to write 2 bytes. This will write 0 and 4, If I write something like this:

char data[] = {0, 4}
write(serial_port_fd, data, strlen(data));

this will write NOTHING.

One more thing, If you want to write non printable characters (which are from byte value 0 to 32) on Serial Port, then make sure that you have configured your Serial Port for raw input and ouput. Have a look on this guide:

http://www.cmrr.umn.edu/~strupp/serial.html#config

Reading NULL character from Serial Port Linux C

1 Answers