March 25, 2011

Today’s exercise implements the old Unix Sys V R4 sum command. The original sum calculated a checksum as the sum of the bytes in the file, modulo 216−1, as well as the number of 512-byte blocks the file occupied on disk. Called with no arguments, sum read standard input and wrote the checksum and file blocks to standard output; called with one or more filename arguments, sum read each file and wrote for each a line containing the checksum, file blocks, and filename. The GNU version of the program, which implements an additional version of the checksum that was part of BSD Unix, is available at in file sum.c; if you run it, be sure to give the −s argument to get the original Unix Sys V R4 version of the checksum.

Your task is to write a program that implements the original Unix Sys V R4 sum command. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

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