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C Language

Let's first start refreshing your memory with a couple of small interesting programs in C.


Because you are always going to be dealing with Executables compiled in C and C++; A better understanding and grip on these languages will help you a lot with solving challenges in CTFs. Also keep in mind that a lot of tools are made with C. A great grip of C will help you contribute to open source tools and maybe build a few yourself. PS : You have a easier time at college. :)

Assignment: C language

If you have not learned C before, don't worry this is your chance to :)

Try taking up a C refresher such that you have:

  • Good idea of Loops(while, for, switch) and control statements.
  • Good grip over functions and functions calls.
  • Basic idea about pointers (Syntax, Usage).
  • Basic knowledge on Structures and objects.
  • Basic File I/O.
  • Recursion.

If you are familiar with all these steps lets just complete a few simple C programming questions. These questions may need you to search around on the internet and also a bit of testing and tweaking.

Programming Questions

  1. Pointers are pretty important in c. A thing clarifying pointers would be nice. So try to implement the python int() function in C. Like atoi(char* ptr, int base) to return the integer stored at the pointer ptr.

    • The function accepts a pointer to a string and an integer mode as arguments and outputs the appropriate number.
    • the format should be Int(char* numstring, int base)
        Int(234234234, 10)
        Output: 234234234
        Int(64, 16)
        Output: 40
    • Banned functions: ato[--anything]
  2. Implement a function “print_this” in C which is the equivalent of the printf in C.

    • The function should accept a format string and the number of arguments as specified by the format string.
    • The function should be able to accept and process:
      • Strings -eg print_this(“%s”,ptr) should print ptr as a string
      • Hex numbers -eg print_this(“%x”, 10) should print 0xa
      • Octal numbers
      • Binary numbers
    • %p, %u, %d should all be handled
    • Numbers before like %02d should be used to specify bytes printed on the screen.
    • %$ should be treated as a illegal format string that cannot be used and print_this should print nothing and return an error. An EACCESS error must be returned.
    • Should print other illegal escape sequences directly
    • The function should not use printf. It can use putc to print to the screen.
    • The function should be available to use by other programs. Write a header file so that other functions can import this function.
    • Macros must be used to denote any integer values used inside thefunction.
    • The function should also return proper values to denote that the execution happened properly
    • Also make sure that the number of arguments passed is same as the number of the format strings present. In case this is not same the function should return some error.
    • Feel free to add any good modifications that you feel will bebeneficial for yourself and the community.
Links to be Referred