Lecture 26 – Nov 11th, 2019

Setup

1. Log in to clyde.
2. Copy ~steve/ex/checksum to your home directory

Task

1. Write a function in C to compute a 1-byte checksum of a file. Look in checksum.c. You’ll need to modify the function to
   1. Open the file with open(2);
   2. In a loop, read the file one byte at a time using read(2) into the variable byte and add that value to cs;
   3. Close the file descriptor using close(2); and
   4. Return 256 - cs.

   If step 1 fails, return -1. If step 2 fails, save errno into a local variable, call close(2) on the file descriptor, set errno back to the value it had before calling close(2), and return -1. (If close(2) fails, there’s nothing we can do anyway, so don’t bother check its return value. But after class, you should read this about a case where failing to check the return of close(2) can actually lead to data corruption!)

   Note that the return value of read(2) is a ssize_t which is like a size_t but is signed. So read(2) returns -1 on error and 0 on EOF.

   You should be able to run checksum on various files to get a simple 1-byte checksum.

   $ ./checksum checksum checksum.c Makefile
   47  checksum
   f5  checksum.c
   f8  Makefile
   

   (You will almost certainly get different values for checksum and checksum.c, but the value for Makefile should be the same, unless you have modified it.)

2. In checksum2.c, you’re going to write the same program except you’re going to use fopen(3), fread(3), and fclose(3) instead of open(2), read(2), and close(2).

   One thing to notice is the file descriptor is the first argument to read(2) whereas the file pointer (FILE *) is the last argument to fread(3).

   In addition, fread(3) returns an unsigned size_t rather than a signed ssize_t. On failure, fread(3) returns 0, just as it does for EOF. You should use ferror(3) to check if the fread(3) failed. If it did fail, save errno, call fclose(3), restore errno, and return -1.

   $ ./checksum checksum checksum.c checksum2 checksum2.c Makefile
   47  checksum
   f5  checksum.c
   bb  checksum2
   a3  checksum2.c
   f8  Makefile
   

   Again, your code and binaries will probably be different from mine, but you should get the same results with either checksum or checksum2.

3. You probably noticed that checksum took several seconds where as checksum2 finished almost immediately.

   Think about why that is and try to come up with an explanation. We’ll figure out why this happened next class.