Mercado Mercado - 1 year ago 77
Linux Question

Why does the sys_read system call end when it detects a new line?

I'm a beginner in assembly (using nasm). I'm learning assembly through a college course.

I'm trying to understand the behavior of the sys_read linux system call when it's invoked. Specifically, sys_read stops when it reads a new line or line feed. According to what I've been taught, this is true. This online tutorial article also affirms the fact/claim.

When sys_read detects a linefeed, control returns to the program and the users input is located at the memory address you passed in ECX.

I checked the linux programmer's manual for the sys_read call (via "man 2 read"). It does not mention the behavior when it's supposed to, right?

read() attempts to read up to count bytes from file descriptor fd
into the buffer starting at buf.

On files that support seeking, the read operation commences at the
file offset, and the file offset is incremented by the number of bytes
read. If the file offset is at or past the end of file, no bytes are
read, and read() returns zero.

If count is zero, read() may detect the errors described below. In
the absence of any errors, or if read() does not check for errors, a
read() with a count of 0 returns zero and has no other effects.

If count is greater than SSIZE_MAX, the result is unspecified.

So my question really is, why does the behavior happen? Is it a specification in the linux kernel that this should happen or is it a consequence of something else?

Answer Source

It's because you're reading from a POSIX tty in canonical mode (where backspace works before you press return to "submit" the line; that's all handled by the kernel's tty driver). Look up POSIX tty semantics / stty / ioctl. If you ran ./a.out < input.txt, you wouldn't see this behaviour.

Note that read() on a TTY will return without a newline if you hit control-d (the EOF tty control-sequence).

Assuming that read() reads whole lines is ok for a toy program, but don't start assuming that in anything that needs to be robust, even if you've checked that you're reading from a TTY. I forget what happens if the user pastes multiple lines of text into a terminal emulator. Quite probably they all end up in a single read() buffer.

See also my answer on a question about small read()s leaving unread data on the terminal: if you type more characters on one line than the read() buffer size, you'll need at least one more read system call to clear out the input.

As you noted, the read(2) libc function is just a thin wrapper around sys_read. The answer to this question really has nothing to do with assembly language, and is the same for systems programming in C (or any other language).

Further reading:

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