Lecture 27 – Nov 13th, 2019


  1. Log in to clyde.


  1. Last time, you wrote two versions of a program to compute a 1-byte checksum. If you completed them, you may use those programs. Otherwise, you can copy ~steve/ex/checksum-v2 to your home directory and use my solutions.

    If you use your own, edit the Makefile to change -fsanitize=address,undefined to -fsanitize=undefined in both the CFLAGS and LDFLAGS. Run $ make clean and then $ make to rebuild without the Address Sanitizer.

  2. You can time how long a Bash command takes to run but prefixing the command with time. Time how long checksum and checksum2 take to run on themselves:
    $ time ./checksum checksum checksum2
    $ time ./checksum2 checksum checksum2

    Time will print out three numbers, the total (real) time it took to run the command and how much of that was spent in your code (user) versus the kernel (sys).

    You probably saw that checksum spent significantly more time in the kernel than checksum2.

    Let’s figure out why that is.

  3. You’re going to use strace(1) to trace the system calls made by a program. We’re interested in file operations since that’s what the difference between the two programs are. We can use the -e option to limit the set of system calls strace(1) prints out. To make things shorter, let’s run the checksum programs on the Makefile.
    $ strace -e trace=open,openat,close,read,write ./checksum Makefile
    $ strace -e trace=open,openat,close,read,write ./checksum2 Makefile

    In both cases, it will open, read from, and close files you don’t care about. You can ignore those lines of output and just focus on the ones where it’s opening and reading from the Makefile. I.e., focus on the lines starting with

    openat(AT_FDCWD, "Makefile", O_RDONLY)  = 3

    (openat(2) is a system call like open(2) except it takes the path to be relative to a directory. You can read about its operation in its man page, if you like.)

    Why did checksum take so much more time than checksum2? Next time you find yourself writing code to read and write from files, think carefully about whether you want to use a FILE * or a file descriptor.

  4. In Bash, we can redirect output of an arbitrary command to a file like this $ cmd >output.txt.

    Let’s figure out how Bash does that using strace(1). Bash’s -c option takes a string to run as input. E.g., if we run

    $ bash -c '/bin/echo hello >output.txt'

    in Bash, then this will start a new instance of Bash which will run the command in quotes and then exit.

    We can use strace’s -f option to trace children processes (remember, Bash will first fork(2) and then use execve(2) to transform the child process into /bin/echo.

    Run this command.

    $ strace -o syscalls.txt -f bash -c '/bin/echo hello >output.txt'

    Passing -o file to strace(1) causes it to write its output to file rather than stderr.

  5. Open syscalls.txt in an editor (Vim will highlight the strace(1) output by default. There’s an Emacs mode that will do it too, but it doesn’t appear to be installed on Clyde.) There’s a lot of output here. The number preceding each line is the process id (PID) of the process that made the system call.

    Let’s find the fork(2) in the output. Interestingly, Linux doesn’t use a fork system call. Instead, it uses clone which is more general. When I did this, I got the line

    8038  clone(child_stack=NULL, flags=CLONE_CHILD_CLEARTID|CLONE_CHILD_SETTID|SIGCHLD, child_tidptr=0x7f76f75c2a10) = 8039

    which tells me that the Bash with PID 8038 forked and the child had PID 8039. The next few lines of rt_sig... system calls are changing signal handling masks, we’ll talk about this next time.

    There’s also a getpid(2) and wait4(2). The latter is just like wait(2) but with more options.

    Finally, there’s an openat(2) which opens output.txt for writing, creating it if it doesn’t already exist, and truncating the file if it does.

    Starting with the openat(2) and going up to the system call just before the execve("/bin/echo", ...) is the sequence of system calls that Bash made to perform the redirection. There’s only one new one there. Read its corresponding man page.

  6. Copy ~steve/ex/redir to your directory. Modify redir.c such that when you run $ ./redir -o file command arguments it runs command arguments with stdout redirected to file. You only need to add some lines inside the if statement to open the file and perform the redirection. You can make a similar set of system calls that Bash did to perform the redirection.