C++ || Multi-Hash Interprocess Communication Using Fork, Popen, & Pipes

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The following is another homework assignment which was presented in an Operating Systems Concepts class. Using two pipes, the following is a program which implements the computing of hash values on a file using the MD5, SHA1, SHA224, SHA256, SHA384, and SHA512 hashing algorithms provided on Unix based systems.

REQUIRED KNOWLEDGE FOR THIS PROGRAM

How To Use Fork
How To Use Pipe
How To Use Popen

==== 1. OVERVIEW ====

Hash algorithms map large data sets of variable length (e.g. files), to data sets of a fixed length. For example, the contents of a 1GB file may be hashed into a single 128-bit integer. Many hash algorithms exhibit an important property called an avalanche effect – slight changes in the input data trigger significant changes in the hash value.

Hash algorithms are often used for verifying the integrity of files downloaded from the WEB. For example, websites hosting a file usually post the hash value of the file using the MD5 hash algorithm. By doing this, the user can then verify the integrity of the downloaded file by computing the MD5 algorithm on their own, and compare their hash value against the hash value posted on the website. The user will know if the download was valid only if the two hash values match.

==== 2. TECHNICAL DETAILS ====

The following implements a program for computing the hash value of a file using the MD5, SHA1, SHA224, SHA256, SHA384, and SHA512 hashing algorithms provided on Unix based systems.

This program takes the name of the target file being analyzed as a command line argument, and does the following:

1. Check to make sure the file exists.
2. Create two pipes.
3. Create a child process.
4. The parent transmits the name of the file to the child (over the first pipe).
5. The child receives the name of the file and computes the hash of the file using the MD5 algorithm (using Linux program md5sum).
6. The child transmits the computed hash to the parent (over the second pipe) and terminates.
7. The parent receives the hash, prints it, and calls wait().
8. Repeat the same process starting with step 3, but using algorithms SHA1...SHA512.
9. The parent terminates after all hashes have been computed.

The use of the popen function is used in order to launch the above programs and capture their output into a character array buffer.

This program also uses two pipes. The two pipes created are the following:

(1) Parent to child pipe: Used by the parent to transfer the name of the file to the child. The parent writes to this pipe and the child reads it.

(2) Child to parent pipe: Used by the child to transfer the computed hashes to the parent. The child writes to this pipe and the parent reads it.


QUICK NOTES:
The highlighted lines are sections of interest to look out for.

The code is heavily commented, so no further insight is necessary. If you have any questions, feel free to leave a comment below.

Using the following example input file located here, the following is sample output:

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