Assignment 6 Solution

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  1. (100 points) Programs operate on files with specific format. For example, Linux executes ELF binaries. Microsoft Windows executes PE binaries. ELF is a file format that tells the loader where and what to find inside the file. Similarly, a word processor can open .doc files, Zip application can open .zip, .gz, etc. In order to aid in detecting if a file is of a particular type, a signature is em-bedded within the file to indicate the type of a file. Often, the signature is the first few bytes within a file. This will enable an application to quickly identify if it can operate on the file. In this problem, you will implement a program file recognizer that accepts a file as command line input, and prints the

Table 1: Signature to type mapping. All signatures start from the beginning of the file. This is only a sample for the assignment. For curious minds, more can be found here: of file signatures

Signature (first few bytes in Hex)

File type

Expected output




38 39 61 or 47 49 46 38 37 61

GIF Image

This is a GIF file.

7f 45 4C


ELF file

This is an ELF file.





PDF file

This is a PDF file.



ZIP file

This is a ZIP file.


Java class file

This is a Java class file.



4E 47 0D 0A 1A 0A

PNG file

This is a PNG file.

Anything else

Unknown file

File type unknown.

format of the file and the program that can open it. NOTE: Although file exten-sions can sometimes tell what format a file is, it is an unreliable indicator. The file signature is the true indicator of a file type.

  1. You will implement file recognizer.c and file recognizer.h that contains all your implementation. You will write a Makefile that generates file recognizer executable. Your program must accept exactly one com-mand line argument (i.e., file name). If more or less than expected number of arguments are passed, you are to print ”Invalid input.” and exit.

Useful resources

  1. Examine fopen(), fscanf() and fclose() functions to perform file IO.

  1. Run the find command (e.g., find / -name “*.png”) on to find files of specific types to test.

Testing your code

In order to test your code, we will run the make command to generate the program. We will then call the program in a loop on a set of known file types, and in each case, we will check if the output is as expected in Table 1. We will also provide invalid number of arguments and non-existent files as arguments. NOTE: The testing process may be automated. It is your responsibility to ensure that the files are named correctly. You are advised to test the programs before submitting. Each valid input file is guaranteed to be at least 10 bytes in size.