Question

I was trying to log data from RS232 into a file with cat:

cat /dev/ttyS0 > rs232.log

The result was that I had everything in my file except for the last line.

By printing to stdout, I was able to discover, that cat only writes the output if it gets a newline character (‘\n’). I discovered the same with:

dd bs=1 if=/dev/ttyS0 of=rs232.log

After reading How can I print text immediately without waiting for a newline in Perl? I was starting to think, if this could be a buffering problem of either the Linux-Kernel or the coreutils package.

According to TJD’s comment, I wrote my own program in C but still had the same problems:

#include <stdio.h>
#include <stdlib.h>
int main(int argc, char* args[])
{
    char buffer;
    FILE* serial;
    serial = fopen(args[1],"r");
    while(1)
    {
        buffer = fgetc(serial);
        printf("%c",buffer);
    }
}

As of the results of my own C-Code this seems to be a Linux-Kernel related issue.

Answer 1

You’re opening a TTY. When that TTY is in cooked (aka canonical) mode, it performs line processing (e.g. backspace removes the previous character from the buffer). You’ll want to put the TTY into raw mode in order to get every single byte when it arrives instead of waiting for the end of line.

From the man page:

Canonical and noncanonical mode

The setting of the ICANON canon flag in c_lflag determines whether
the terminal is operating in canonical mode (ICANON set) or
noncanonical mode (ICANON unset). By default, ICANON set.

In canonical mode:

• Input is made available line by line. An input line is available
  when one
  of the line delimiters is typed (NL, EOL, EOL2; or EOF at the start of
  line). Except in the case of EOF, the line delimiter is included in the
  buffer returned by read(2).

• Line editing is enabled (ERASE, KILL; and if the IEXTEN flag is
  set: WERASE,
  REPRINT, LNEXT). A read(2) returns at most one line of input; if the
  read(2) requested fewer bytes than are available in the current line of
  input, then only as many bytes as requested are read, and the remaining
  characters will be available for a future read(2).

In noncanonical mode input is available immediately (without the
user having to type a line-delimiter character), and line editing
is disabled.

The simplest thing to do is just to call cfmakeraw.